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European	Journal	of	Chemistry	3	(3)	(2012)	367‐394	

European	Journal	of	Chemistry	
ISSN	2153‐2249	(Print)	/	ISSN	2153‐2257	(Online)		2012	EURJCHEM	

DOI:10.5155/eurjchem.3.3.367‐394.604	

European	Journal	of	Chemistry	
Journal	homepage:	www.eurjchem.com	

The	chemistry	of	group‐VIb	metal	carbonyls	

Manish	Kaushika,*,	Ayodhya	Singhb	and	Munesh	Kumara	

a	Department	of	Chemistry,	Durga	Prasad	Baljeet	Singh	Post	Graduate	College,	Anoopshahr,	Bulandshahr,	Uttar	Pradesh,	202390,	India	
b	Department	of	Chemistry,	Mahanand	Mission	Harijan	College,	Ghaziabad,	Uttar	Pradesh,	201001,	India	

*Corresponding	author	at:	Department	of	Chemistry,	Durga	Prasad	Baljeet	Singh	Post	Graduate	College,	Anoopshahr,	Bulandshahr,	Uttar	Pradesh,	202390,	India.	
Tel.:	+91.120.2328234;	fax:	+91.120.2320355.	E‐mail	address:	manish.kaushik@sify.com	(M.	Kaushik).	

REVIEW	INFORMATION	 ABSTRACT

Received:	28	February	2012	
Received	in	revised	form:	17	April	2012	
Accepted:	19	May	2012	
Online:	30	September	2012	

KEYWORDS	

The	special	interest	attached	to	the	chemistry	of	metal	carbonyls	arises	from	several	causes.
While	quite	distinct	from	the	metal	carbonyls	in	the	organometallic	compounds,	they	differ	in
physical	properties	 (e.g.,	 their	volatility)	 from	all	 other	 compounds	of	 the	 transition	metals.
Chemically,	 they	 constitute	 a	group	of	 compounds	 in	which	 the	 formal	valency	of	 the	metal
atoms	is	zero,	and	in	this	respect	(apart,	perhaps,	from	the	ammoniates	of	the	alkali	metals)
they	are	comparable	only	with	the	recently	discovered	compounds.	As	a	class,	 the	carbonyls
are	 reactive	 compounds,	 and	 a	 number	 of	 new	 types	 of	 inorganic	 compounds	 have	 been
discovered.	 In	 the	 concepts	 for	 new	 products,	 performance,	 product	 safety,	 and	 product
economy	 criteria	 are	 equally	 important.	 They	 are	 taken	 into	 account	 already	when	 the	 raw
material	 base	 for	 a	 new	 industrial	 product	 development	 is	 defined.	 Since	 the	 discovery	 of
nickel	 carbonyl	 by	 Mond	 and	 Langer	 in	 1888,	 the	 carbonyls	 of	 the	 iron	 group	 and	 of
chromium,	molybdenum	and	tungsten	have	found	important	technical	applications,	e.g.,	in	the
Mond	nickel	 process,	 and	 for	 the	 preparation	 of	 the	metals	 in	 a	 state	 of	 subdivision	 and	 of
purity	 suitable	 for	 powder	 metallurgy,	 for	 catalysts,	 etc.	 The	 reaction	 mechanism	 of	 the
processes	 developed	 for	 producing	 the	 carbonyls	 technically	 has	 only	 recently	 received	 its
interpretations.	Within	the	space	of	review	it	is	necessary	to	limit	discussion	to	a	few	topics.
Particular	 stress	has	accordingly	 laid	upon	(a)	 the	chemical	bonding	 in	metal	carbonyls,	 (b)
importance	 of	 IR	 and	 NMR	 spectroscopy	 in	 characterization	 of	 metal	 carbonyls,	 (c)
substitution	 reactions	 of	G‐VIb	metal	 carbonyls,	 (d)	 kinetics	 and	mechanism	of	 substitution
reactions	 in	metal	 carbonyls,	 (e)	 substituted	 complexes	 of	 G‐VIb	metal	 carbonyl,	 (f)	 chelate
complexes	 of	 G‐VIb	 metal	 carbonyls,	 (g)	 uses	 of	 G‐VIb	 metal	 carbonyl	 complexes	 and	 (h)
studies	done	on	G‐VIb	metal	carbonyls.	

Complexes	
Metal	carbonyl	
Complexation	agents	
Homonuclear	clusters	
Substitution	reactions	
Heteronuclear	clusters	

1.	Introduction	

Binary	 metal	 carbonyl	 compounds	 that	 consist	 only	 of	 a	
metal	and	carbon	monoxide	(CO)	ligands	are	usually	prepared	
by	direct	reaction	of	the	powder	of	a	highly	reactive	metal	and	
carbon	monoxide,	 or	 by	 the	 reduction	 of	 a	metal	 salt	 to	 zero	
valance	 followed	 by	 reaction	 with	 high‐pressure	 carbon	
monoxide.	However,	tetracarbonyl	nickel	first	discovered	at	the	
end	 of	 the	 19thcentury,	 forms	 by	 the	 reaction	 of	 nickel	metal	
and	carbon	monoxide	under	atmospheric	pressure	and	at	room	
temperature.	 The	 preparation	 of	 other	 metal	 carbonyl	
compounds,	on	the	other	hand,	requires	high	temperatures	and	
high	pressures.		

Carbonyl	 groups	 are	 the	 functional	 group	 consisting	 of	 a	
carbon	atom	bonded	to	an	oxygen	atom	through	a	double	bond.	
In	certain	other	carbonyl	compounds,	where	carbon	monoxide	
functions	 as	 a	 ligand	 in	 some	 inorganic	 and	 organometallic	
complexes,	 carbon	 is	bonded	 to	Oxygen	 through	a	 triple	bond	
(C≡O).		

In	 metal	 carbonyl	 a	 transition	 metal	 is	 bonded	 to	 CO.	
Elements	from	d‐block	of	the	periodic	table	are	usually	referred	
to	 as	 transition	 metals	 as	 they	 contain	 an	 incomplete	 d	 sub‐
shell.	CO	forms	bonds	with	metal	ions	through	π‐backbonding.	
Metal	 carbonyls	 are	 most	 of	 the	 times	 non‐polar	 and	
electrically	 neutral	 compounds	 and	 demonstrates	 physical	
properties	of	organic	compounds.	The	general	formula	of	metal	

carbonyls	is	Mx(CO)y.	These	complexes	may	be	homoleptic,	that	
is	containing	only	CO	ligands,	such	as	nickel	carbonyl	(Ni(CO)4),	
but	more	commonly	metal	 carbonyls	contain	a	mix	of	 ligands,	
such	 as	 Re(CO)3(2,2'‐bipyridine)Cl.	 Carbon	 monoxide	 is	 an	
important	building	block	for	the	synthesis	of	many	compounds,	
for	 example	 hydro	 formylation,	 and	 metal	 carbonyl	 catalysts	
are	 central	 to	 its	utilization.	Metal	 carbonyls	are	 toxic,	 in	part	
because	 of	 their	 ability	 to	 carbonylate	 hemoglobin	 to	 give	
carboxy	hemoglobin,	which	will	not	bind	O2	[1].	Most	of	metal	
carbonyls	 are	 solid	 at	 ordinary	 temperatures,	 but	 nickel	
carbonyl,	 iron	 pentacarbonyl	 and	 ruthenium	 pantacarbonyls	
are	 liquid,	 and	 cobalt	 hydrocarbonyl	 is	 gas.	 Most	 metal	
carbonyl	 reacts	 vigorously	 with	 oxygen	 and	 oxidizing	
substances,	 and	 some	 ignite	 spontaneously.	 Metal	 carbonyls	
include	 solubility	 in	 only	 organic	 solvent	 but	 have	 poor	
solubility	with	water,	highly	toxic	nature,	volatile	solid	or	liquid	
appearance	at	room	temperature.		

1.1.	Bonding	in	metal	carbonyls	

A	simple	molecular	orbital	 (MO)	diagram	 for	CO	 is	shown	
below	 (Figure	 1).	 The	 highest	 occupied	 molecular	 orbital	
(HOMO)	is	indicated	by	the	pair	of	electrons.	This	is	the	5σ	lone	
pair	 orbital	 mainly	 centered	 on	 the	 carbon	 and	 weakly	 anti	
bonding	with	respect	to	the	C‐O	bond.		

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Figure	1.	Molecular	orbital	diagram	of	CO.		
	
	
	

	
	

Figure	2.	π*‐Acceptor	orbitals	on	carbonyl.		
	
	
The	weak	C‐O	 antibonding	 nature	 of	 this	MO,	 however,	 is	

clearly	 seen	 in	 the	 experimental	 data	 presented	 below.	 The	
LUMO	is	strongly	π*	antibonding	and	 is	 low	enough	 in	energy	
to	 act	 as	 a	 good	 acceptor	 orbital	 for	 interacting	with	 filled	d‐
orbitals	on	metals.		

CO	 forms	 bonds	with	metal	 ions	 through	π‐back	 bonding.	
As	 a	 result	 of	 π‐back	 bonding,	 the	 transition	 metal	 attains	
nearer	 inert	gas	electron	configuration.	CO	contains	 lone	pairs	
of	electrons	on	carbon	atom	and	forms	sigma	bond	to	the	metal.	
The	 metal	 atom	 inturn	 donates	 electrons	 from	 its	 valence	 d‐
shell	to	π	orbitals	of	CO	(Figure	2)	and	metal	atom	attains	inert	
gas	 configuration	 due	 to	 this	 donation	 of	 valence	 electrons	
(Figure	3).		

	

	
Figure	3.	π‐Back	bonding	between	metal	and	CO	group.		

	
For	 an	 example,	 in	 chromium	 hexacarbonyl	 [Cr(CO)6],	

elemental	 chromium	 contains	 six	 valence	 electrons	 hence	
combines	 with	 six	 CO	 molecules	 and	 attains	 isoelectronic	
configuration	with	nearer	inert	gas	krypton	(Kr).		

 
1.2.	Bonding	modes	in	clusters	
	

The	 carbonyl	 ligand	 engages	 in	 three	 types	 of	 bonding	
modes	 in	 metal	 carbonyl	 cluster	 chemistry	 [1,2].	 Most	
frequently,	 CO	 binds	 in	 the	 familiar	 terminal	mode,	 but	 CO	 is	
often	bridges	between	two	(μ2)	or	three	(μ3)	metals	(Figure	4).	
Much	 less	common	are	bonding	modes	 in	which	both	C	and	O	
bond	to	the	metal,	e.	g.	μ3‐η2.		

The	 increased	 π‐bonding	 due	 to	 back‐donation	 from	
multiple	metal	centers	results	in	 further	weakening	of	the	C‐O	
bond.		

	
	

	
	

Figure	4.	Standard	CO	bonding	modes	in	metal	carbonyl	clusters.		
	

1.3.	Chromium	hexacarbonyl	
	

Chromium	 carbonyl,	 also	 known	 as	 chromium	
hexacarbonyl,	 is	 the	 chemical	 compound	 with	 the	 formula	
Cr(CO)6	(Scheme	1).	At	room	temperature	the	solid	is	stable	to	
air,	although	 it	does	have	a	high	vapor	pressure	and	sublimes	
readily.	 Cr(CO)6	 is	 zero	 valent,	 meaning	 that	 Cr	 has	 a	 formal	
charge	 of	 zero,	 and	 it	 is	 called	 a	 homoleptic	 complex,	 which	
means	that	all	the	ligands	are	the	same.		

	

	
	

Scheme	1	
 
1.3.1.	Structure	
	

The	 complex	 is	 octahedral	with	Cr‐C	 and	C‐O	distances	 of	
1.92	and	1.17	Å,	respectively	(Scheme	1)	[3].		

 
1.3.2.	Properties	
	

Chromium	hexacarbonyl	 is	white	 crystals	 or	 powder.	 It	 is	
white	 crystalline	 or	 granular	 solid	 which	 sublimes	 at	 room	
temperature	and	burns	with	a	luminous	flame.		

	

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Chromium	hexacarbonyl	 is	 insoluble	 in	water	 and	 alcohol	
but	 soluble	 in	 most	 of	 organic	 solvents.	 Chromium	
hexacarbonyl	 decomposes	 violently	 at	 210	 oC.	 Chromium	
hexacarbonyl	is	decomposed	by	chlorine	and	fuming	nitric	acid,	
Chromium	hexacarbonyl	is	incompatible	with	oxidizing	agents.	
Flash	Point	for	chromium	hexacarbonyl	is	210	oC,	however,	it	is	
probably	combustible	[4].		

	
1.3.3.	Preparation	
	

The	chromium	hexacarbonyls	can	be	prepared	by	any	one	
of	 the	 following	 methods.	 Chromium	 hexacarbonyl	 has	 been	
prepared	by	reacting	chromic	chloride	with	phenyl	magnesium	
bromide	 (Grignard	 reagent)	 and	 carbon	 monoxide	 in	 an	
ethereal	 solution.	 A	 carbon	 monoxide	 pressure	 of	 one	
atmosphere	 or	 somewhat	 higher	 is	 required.	 This	 process	
requires	 a	 large	 amount	 of	 phenyl	 magnesium	 bromide	
(Grignard	 reagent)	 therefore	 is	 impractical	 for	 technical	
preparation	of	chromium	hexacarbonyl.		

Brimm	 et	al.	 found	 a	 alternate	method	 for	 preparation	 of	
Chromium	 hexacarbonyls	 by	 reacting	 anhydrous	 chromium	
chloride	with	carbon	monoxide	in	the	presence	of	an	aromatic	
solvent	 compound,	 a	 Friedel‐Craft	 catalyst	 AlCl3	 and	 Al	 as	
reducing	 agent	 under	 pressure	 at	 140oC	 temperature	 [5]	
(Scheme	2).		

	

CrCl3 + Al + 6CO Cr(CO)6 + AlCl3
AlCl3

C6H6  
	

Scheme	2	
	

1.3.4.	Applications	in	inorganic	and	organometallic	
chemistry	
	

Chromium	 hexacarbonyl	 is	 useful	 in	 serving	 as	 a	
decomposable	material	 in	 the	production	of	metallic	 coatings,	
such	 as	 for	 mirrors,	 as	 a	 catalyst	 and	 as	 a	 chemical	
intermediate.	 It	 is	 used	 as	 a	 catalyst	 for	 polymerization	 and	
isomerization	 of	 olefins.	 When	 heated	 or	 photolyzed	 in	
tetrahydrofuran	 (THF)	 solution,	 Cr(CO)6	 converts	 to	 Cr(CO)5	
(THF)	with	loss	of	one	CO	ligand.	Similarly,	heating	a	solution	of	
Cr(CO)6	 in	an	aromatic	solvent	results	in	replacement	of	three	
CO	ligands	(Scheme	3).		
	

 
	

Scheme	3	
	

Such	reactions	proceed	particularly	well	with	electron‐rich	
arenes	 such	 as	 anisole,	 either	 as	 the	 neat	 reagent	 or	 using	 a	
mixture	 of	 THF	 and	 dibutyl	 ether,	 the	 latter	 to	 increase	 the	
boiling	point	of	the	mixture.	The	products	adopt	a	"piano‐stool"	
structure.	 These	 species	 are	 typically	 yellow	 solids,	 which	
dissolve	 well	 in	 common	 organic	 solvents.	 The	 arene	 can	 be	
liberated	 from	 the	 chromium	with	 iodine	 or	 by	 photolysis	 in	
air.	 In	 general,	 substituted	 derivatives	 of	 Cr(CO)6	 decompose	
upon	exposure	to	air.		

Alkyl	and	aryl	organolithium	reagents	RLi	add	to	a	carbonyl	
ligand	 to	 give	 anionic	 acyl	 complexes	 [1].	 These	 species	 react	
with	 alkylating	 agents	 such	 as	 Me3O+	 to	 form	
(OC)5Cr=C(OMe)R,	 an	 example	 of	 a	 Fischer	 carbene.	 If	 the	 R	
group	 is	 a	 vinyl	 or	 an	 aryl	 group,	 then	 the	 resulting	 carbene	
complex	 can	 react	 with	 an	 acetylene	 to	 form	 a	 new	 benzene	
ring	 to	 which	 is	 bonded	 the	 chromium	 tricarbonyl	 fragment.	
The	two	acetylene	carbon	atoms	become	part	of	the	new	ring,	
as	 does	 a	 carbon	 from	 one	 of	 the	 carbonyl	 ligands.	 Also	 the	
three	carbons	 from	 the	vinyl	carbene	become	part	of	 the	new	
benzene	ring.		

 
 

1.3.5.	Safety	and	handling	
	

In	 common	 with	 many	 of	 the	 other	 homoleptic	 metal	
carbonyls	 (e.	 g.	nickel	 carbonyl	and	 iron	 carbonyl),	 chromium	
hexacarbonyl	is	toxic	and	thought	to	be	carcinogenic.	Its	vapor	
pressure	 is	 relatively	high	 for	 a	metal	 complex,	 1	mm	Hg	 (36	
°C)	[6].	Safety	information	for	chromium	hexacarbonyl	is	given	
in	Table	1[7].		
	
Table	1.	Safety	information	of	chromium	hexacarbonyl.		
Safety	
parameter	

Safety	Data Explanation	

Symbol	

GHS06	

Acute	toxicity	(oral,	dermal,	
inhalation),	categories	1,2,3	

Signal	word Danger ‐
Hazard	
statement	

H301 a.	Letter	"H"	(for	"hazard	statement").	
b.	A	number	‘3’	for	health	hazard.		
c.	Two	numbers	‘01’	for	explosive	
properties	(00‐10).		

Precautionary	
statements	

P301	+	P310	 P301	IF	SWALLOWED:	P310	
Immediately	call	a	POISON	CENTER	or	
doctor/physician.		

Hazard	Code	 T	 For	‘Toxic’	
Risk	
Statements	

22 Harmful	if	swallowed	

 
1.4.	Molybdenum	hexacarbonyl	
	

Molybdenum	 hexacarbonyl	 (also	 called	 molybdenum	
carbonyl)	is	the	chemical	compound	with	the	formula	Mo(CO)6	
(Scheme	4).	This	colorless	solid,	like	its	chromium	and	tungsten	
analogues,	is	noteworthy	as	a	volatile,	air‐stable	derivative	of	a	
metal	in	its	zero	oxidation	state.		
	
1.4.1.	Structure		
	

Mo(CO)6	 adopts	 an	 octahedral	 geometry	 consisting	 of	 six	
rod‐like	 CO	 ligands	 radiating	 from	 the	 central	 Mo	 atom	
(Scheme	4).	A	recurring	minor	debate	in	some	chemical	circles	
concerns	 the	 definition	 of	 an	 "organometallic"	 compound.	
Usually,	 organometallic	 indicates	 the	 presence	 of	 a	 metal	
directly	bonded	via	a	M‐C	bond	to	an	organic	fragment,	which	
must	in	turn	have	a	C‐H	bond.	By	this	strict	definition,	Mo(CO)6	
is	not	organometallic.		
	

 
 

Scheme	4	
 

1.4.2.	Properties	
	

Mo(CO)6	 is	 a	 white	 crystalline	 solid;	 melts	 at	 150	 oC;	
insoluble	in	water,	soluble	in	most	organic	solvents.		

 
1.4.3.	Preparation	
	

Mo(CO)6	 is	 prepared	 by	 the	 reduction	 of	 molybdenum	
chlorides	 or	 oxides	 under	 a	 pressure	 of	 carbon	 monoxide,	
although	 it	 would	 be	 unusual	 to	 prepare	 this	 inexpensive	
compound	 in	 the	 laboratory.	 The	 compound	 is	 somewhat	 air‐
stable	 and	 sparingly	 soluble	 in	 nonpolar	 organic	 solvents.	
Molybdenum	 hexachloride	 is	 reacted	 with	 carbon	 monoxide	
under	pressure	in	the	presence	of	zinc	or	aluminum	powder	in	
anhydrous	ether	at	150	oC	[8].		

 

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1.4.4.	Applications	in	inorganic	and	organometallic	
synthesis	
	

Molybdenum	 hexacarbonyl	 is	 widely	 used	 in	 electron	
beam‐induced	deposition	 technique.	 It	 is	 easily	vaporized	and	
decomposed	 by	 the	 electron	 beam	 providing	 a	 convenient	
source	 of	 molybdenum	 atoms	 [9]	 Mo(CO)6	 is	 also	 a	 popular	
reagent	 in	organometallic	 synthesis	 [10]	because	one	or	more	
CO	 ligands	 can	 be	 displaced	 by	 other	 donor	 ligands	 [11].	 For	
example,	 Mo(CO)6	 reacts	 with	 2,2'‐bipyridine	 to	 afford	
Mo(CO)4(bipy).	 UV‐photolysis	 of	 a	 THF	 solution	 of	 Mo(CO)6	
gives	Mo(CO)5(THF).	Many	metal	carbonyls	are	similarly	photo‐
activatable.		

Mo(CO)6,	 [Mo(CO)3(MeCN)3],	 and	 related	 derivatives	 are	
employed	as	catalysts	in	organic	synthesis.	For	example,	these	
catalysts	 can	 be	 used	 for	 alkyne	 metathesis	 and	 the	 Pauson‐
Khand	reaction.		
 
1.4.5.	Safety	and	handling	
	

Like	 all	 metal	 carbonyls,	 Mo(CO)6	 is	 dangerous	 source	 of	
volatile	 metal	 as	 well	 as	 CO.	 It	 diffuses	 readily	 into	 plastic	
stoppers	 [12‐14].	 Safety	 Information	 for	 molybdenum	
hexacarbonyl	is	given	in	Table	2.		
	
1.5.	Tungsten	hexacarbonyl	
	

Tungsten	 hexacarbonyl	 (also	 called	 tungsten	 carbonyl)	 is	
the	 chemical	 compound	with	 the	 formula	W(CO)6	 (Scheme	5).	
This	 complex	 gave	 rise	 to	 the	 first	 example	 of	 a	 dihydrogen	
complex	[15].		
	
1.5.1.	Structure	
	

W(CO)6	 adopts	 an	 octahedral	 geometry	 consisting	 of	 six	
rod‐like	 CO	 ligands	 radiating	 from	 the	 central	 W	 atom	 with	
dipole	moment	0.0	D	(Scheme	5).		
	
	
Table	2.	Safety	information	of	molybdenum	hexacarbonyl.		
Safety	
parameter	

Safety	Data	 Explanation	

Symbol	

GHS06	

Acute	toxicity	(oral,	dermal,	
inhalation),	categories	1,2,3	

Signal	word	 Danger	 ‐	
Hazard	
statement	

H300‐H310‐H330	 a.	Letter	"H"	(for	"hazard	statement").	
b.	A	number	‘3’	for	health	hazard.		
c.	Two	numbers	‘01’	for	explosive	
properties	(codes	from	00‐10).		
d.	Two	numbers	‘30’	for	flammability	
(codes	from	20	to	30).		

Precautionary	
statements	

P260‐P264‐P280‐
P284‐P301	+	P310‐
P302	+	P350	

P260	Do	not	breathe	
dust/fume/gas/mist/vapours/spray.		
P264	Wash	hands	thoroughly	after	
handling.		
P280	Wear	protective	
gloves/protective	clothing/eye	
protection/face	protection.		
P284	Wear	respiratory	protection.		
P301	IF	SWALLOWED:	P310	
Immediately	call	a	POISON	CENTER	or	
doctor/physician.		
P302	IF	ON	SKIN:	P350	Gently	wash	
with	plenty	of	soap	and	water.		

Hazard	Code	 T+	 For	‘Very	Toxic’	
Risk	
Statements	

26/27/28	 26:	Very	Toxic	by	inhalation	
27:	Very	Toxic	in	contact	with	skin		
28:	Very	Toxic	if	swallowed	
	

	
	

Scheme	5	
	
1.5.2.	Properties	
	

This	 colorless	 compound,	 like	 its	 chromium	 and	
molybdenum	 analogs,	 is	 noteworthy	 as	 a	 volatile,	 air‐stable	
derivative	 of	 tungsten	 in	 its	 zero	 oxidation	 state.	W(CO)6	 is	 a	
white	crystalline	solid;	melts	at	170	oC;	sublimes;	boil	at	175	oC.	
It	is	insoluble	in	water	and	alcohol;	soluble	in	fuming	nitric	acid	
and	most	organic	solvents.		

 
1.5.3.	Preparation	
	

W(CO)6	 is	 prepared	 by	 the	 reduction	 of	 WCl6	 under	 a	
pressure	of	carbon	monoxide.	It	would	be	rare	to	prepare	this	
inexpensive	compound	in	the	laboratory	because	the	apparatus	
is	 expensive	 and	 the	 compound	 can	 be	 purchased	 cheaply.	
Dallas	T.	Hurd	prepared	tungsten	hexacarbonyl	when	tungsten	
hexachloride	 is	reacted	with	carbon	monoxide	under	pressure	
in	the	presence	of	zinc	or	aluminum	powder	in	anhydrous	ether	
at	100	oC	[8].		

 
1.5.4.	Applications	in	inorganic	and	organometallic	
synthesis	
	

All	 reactions	 of	 W(CO)6	 commence	 with	 displacement	 of	
some	 CO	 ligands	 in	W(CO)6.	W(CO)6	 behaves	 similarly	 to	 the	
Mo(CO)6	 but	 tends	 to	 form	 compounds	 that	 are	 kinetically	
more	robust.	One	derivative	is	the	dihydrogen	complex	W(CO)3	
[P(C6H11)3]2(H2)	reported	in	1982	by	Kubas	[15]	Three	of	these	
CO	 ligands	 can	 be	 displaced	 by	 acetonitrile	 [16].	 W(CO)6	 has	
been	 used	 to	 desulfurize	 organosulfur	 compounds	 and	 as	 a	
precursor	to	catalysts	for	alkene	metathesis.		

 
1.5.5.	Safety	and	handling	
	

Like	 all	 metal	 carbonyls,	 W(CO)6	 is	 dangerous	 source	 of	
volatile	 metal	 as	 well	 as	 CO	 [9,17].	 Safety	 Information	 for	
Tungsten	hexacarbonyl	is	given	in	Table	3.		
	
1.6.	Importance	of	IR	spectroscopy	in	characterizing	metal	
carbonyls	
	

The	 most	 important	 technique	 for	 characterizing	 metal	
carbonyls	is	infra‐red	spectroscopy.	The	C‐O	vibration,	typically	
called	νCO,	occurs	at	2143	cm‐1	for	CO	gas.	The	positions	of	the	
νCO	 band(s)	 for	 the	 metal	 carbonyls	 are	 inversely	 correlated	
with	the	strength	of	the	π‐bonding	between	the	metal	and	the	
carbon.		

In	addition	to	their	frequency,	the	number	of	the	νCO	bands	
is	diagnostic	of	structure	of	the	complex.	Octahedral	complexes,	
e.g.	Cr(CO)6,	exhibits	only	a	single	νCO	band	in	 its	 IR	spectrum.	
Spectra	 for	 complexes	 of	 lower	 symmetry	 are	more	 complex.	
For	example,	the	IR	spectrum	of	Fe2(CO)9displays	CO	bands	at	
2082,	2019	and	1829	cm‐1.		

In	 cluster	 carbonyls,	 νCO	 is	 a	 sensitive	 probe	 for	 the	 CO	
coordination	geometry.	For	bridging	(μ2)	 ligands	νCO	is	usually	
shifted	 by	 100‐200	 cm‐1	 to	 lower	 wavenumbers	 compared	 to	
the	signatures	of	μ1‐CO.	Bands	for	face	capping	(μ3)	CO	ligands	
appear	 at	 even	 lower	 energies.	Typical	 values	 for	 IR	 bands	 in	
cluster	carbonyls	are	given	in	Table	4	[18].		
	
	

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Table	3.	Safety	information	of	tungsten	hexacarbonyl.		
Safety	
parameter	

Safety	Data	 Explanation	

Symbol	

GHS06

Acute	toxicity	(oral,	dermal,	
inhalation),	categories	1,2,3	

Signal	word	 Danger	
Hazard	
statement	

H301‐H311‐H331	 a.	Letter	"H"	(for	"hazard	statement").		
b.	A	number	‘3’	for	health	hazard.		
c.	Two	numbers	‘01’	for	explosive	
properties	(00‐10).		
d.	Two	numbers	‘11’	for	Acute	
Toxicity	properties	(10‐13).		
e.	Two	numbers	‘31’	for	flammability	
(codes	from	20	to	30).		

Precautionary	
statements	

P261‐P280‐P301	+	
P310‐P311	

P261	Avoid	breathing	
dust/fume/gas/mist/vapours/spray	
P280	Wear	protective	
gloves/protective	clothing/eye	
protection/face	protection.		
P301	IF	SWALLOWED:	P310	
Immediately	call	a	POISON	CENTER	
or	doctor/physician.		
P311	Call	a	POISON	CENTER	or	
doctor/physician.		

Hazard	Code	 T	 For	‘Toxic’	
Risk	
Statements	

23/24/25	 23:	Toxic	by	inhalation	
24:	Toxic	in	contact	with	skin	
25:	Toxic	if	swallowed	

Safety	
Statements	

36/37‐45	 36:	Wear	suitable	protective	clothing	
37:	Wear	suitable	gloves	
45:	In	case	of	accident	or	if	you	feel	
unwell,	seek	medical	advice	
immediately	(show	label	where	
possible)	
 

	
Table	4.	Typical	IR‐bands	for	νCO	in	cluster	carbonyls.		
Compound	 νCO	(cm‐1)	
CO	 2143	
[Ti(CO)6]‐2	 1748	
[V(CO)6]‐	 1859	
[Cr(CO)6]	 2000	
[Mn(CO)6]+	 2100	
[Fe(CO)6]2+	 2204	
[Fe(CO)5]	 2022,	2000	

	
As	 one	 goes	 from	 a	 terminal	 CO‐bonding	 mode	 to	 2‐

bridging	 and	 finally	 3‐bridging,	 there	 is	 a	 relatively	 dramatic	
drop	 in	 the	 CO	 stretching	 frequency	 seen	 in	 the	 IR.	Note	 that	
these	 ranges	 are	 typical	 for	 “neutral”	 transition	 metal	
complexes	with	an	average	amount	of	electron	density	on	 the	
metal	center	(Table	5).	Bridging	carbonyls	tend	to	have	weaker	
and	broader	IR	bands.		
	
Table	5.	Detecting	effect	of	electron	density	on	metal.		
dx	 Complex	 νCO(cm‐1)	
	 Free	CO	 2143	
d10	 [Ag(CO)]+	 2204	
	 [Ni(CO)4]	 2060	
	 [Co(CO)4]‐	 1890	
	 [Fe(CO)4]2‐	 1790	
	 [Mn(CO)6]+	 2090	
d6	 [Cr(CO)6]	 2000	
	 [V(CO)6]‐	 1860	
	

As	the	electron	density	on	a	metal	center	increases,	more	π‐
back	 bonding	 to	 the	 CO	 ligand(s)	 takes	 place.	 This	 futher	
weakens	the	C‐O	bond	by	pumping	more	electron	density	 into	
the	formally	empty	carbonyl	π*	orbital.	This	increases	the	M‐CO	
bond	 strength	 making	 it	 more	 double‐bond‐like,	 i.e.,	 the	
resonance	structure	M=C=O	assumes	more	importance.		

This	can	clearly	be	seen	on	Table	6	that	illustrates	the	effect	
of	charge	and	electronegativity	on	the	amount	of	metal	to	CO	π‐
back	bonding	and	the	CO	IR	stretching	frequency.	
	
	

Table	6.	Detection	of	ligand	donation	effects.		
Complex	 νCO(cm‐1)	
Mo(CO)3(PF3)3 2090,	2055	
Mo(CO)3(PCl3)3 2040,	1991	
Mo(CO)3	[P(OMe)3]3	 1977,	1888	
Mo(CO)3(PPh3)3	 1934,	1835	
Mo(CO)3(NCCH3)3	 1915,	1783	
Mo(CO)3(triamine)3	 1898,	1758	
Mo(CO)3(pyridine)3 1888,	1746	
	

The	 ability	 of	 the	 ligands	 on	 a	 metal	 to	 donate	 electron	
density	to	the	metal	center	certainly	has	considerable	effect	on	
the	absolute	amount	of	electron	density	on	that	metal.	This,	 in	
turn,	 naturally	 affects	 the	 π	 CO	 IR	 stretching	 frequencies	 in	
metal	carbonyl	complexes.	Ligands	that	are	trans	to	a	carbonyl	
can	 have	 a	 particularly	 large	 effect	 on	 the	 ability	 of	 the	 CO	
ligand	to	effectively	π	–back	bond	to	the	metal.	For	example	2	
transπ‐back	 bonding	 ligands	 will	 partially	 compete	 for	 the	
same	d‐orbital	electron	density,	weakening	each	others	net	M‐L	
π‐back	bonding	(Figure	5).		
	

	
	

Figure	5.	π‐Back	bonding	between	metal	and	CO	ligand.		
	

When	 the	 trans	 ligand	 is	 a	 π‐donating	 ligand,	 this	 can	
increase	 the	M‐CO	bond	strength	 (more	M=C=O	character)	by	
allowing	unimpeded	metal	to	CO	π‐back	bonding.	Pyridine	and	
amines	are	not	that	strong	π‐donors,	but	they	are	even	worse	π	
‐backbonding	 ligands.	 So	 the	 CO	 has	 virtually	 no	 competition	
for	π‐back	donation.		

Based	 on	 CO	 IR	 stretching	 frequencies,	 the	 following	
ligands	can	be	ranked	from	best	π‐acceptor	to	worst:	
	
NO+	>	CO	>	PF3	>	RN≡C	>	PCl3	>	P(OR)3	>	PR3	>	RC≡N	>	NH3	

	
Metal	carbonyls	have	the	general	formula	Mex(CO)y,	and	are	

formed	 by	 combination	 of	 the	 metal	 (Me)	 with	 carbon	
monoxide	 (CO).	Most	are	 solids	at	ordinary	 temperatures,	but	
nickel	 carbonyl,	 iron	 pentacarbonyl	 and	 ruthenium	
pentacarbonyl	 are	 liquids,	 and	 cobalt	 hydrocarbonyl	 is	 a	 gas.	
Most	 metal	 carbonyls	 react	 vigorously	 with	 oxygen	 and	
oxidizing	substances,	and	some	ignite	spontaneously.		

 
1.7.	Importance	of	NMR	spectroscopy	in	investigation	of	
metal	carbonyl	complexes	
	

NMR	 spectroscopy	 is	 useful	 for	 investigating	 complexes	
containing	spin‐spin	active	nuclei	(1H,	13C,	31P	and19F	etc.).	NMR	
spectroscopy	is	also	useful	 in	confirmation	of	 ligands	having	P	
and	F	atoms.	NMR	spectroscopy	is	also	useful	in	differentiating	
the	ligands	based	on	alkyl/aryl	groups.		
	
2.	Substitution	reactions	of	metal	carbonyl	complexes	
	

Ligand	 substitution	 reactions	 are	 essential	 for	 the	 use	 of	
transition	 metal	 organometallic	 compounds.	 Therefore,	 it	 is	
important	 that	 we	 know	 what	 factors	 affect	 the	 rates	 of	
reaction	 and	 why.	 Much	 information,	 both	 qualitative	 and	
quantitative,	 is	 available	 on	 the	 reactivity	 of	 organometallic	
compounds	 [19].	 What	 is	 needed	 are	 more	 detailed	 kinetic	
studies	 that	 give	 information	 on	 the	 mechanisms	 of	 ligand	
substitution,	and	on	what	factors	contribute	to	rates	of	reaction	
in	 these	 systems.	 Studies	on	different	 types	of	metal	 carbonyl	
substituted	systems	are	described	here.		

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M(CO)6	 are	 also	 a	 popular	 reagent	 in	 organometallic	
synthesis	[20]	because	one	or	more	CO	ligands	can	be	displaced	
by	other	donor	 ligands	[21].	Subasi	et.	al.	done	substitution	of	
CO	 ligand	 by	 the	 reaction	 of	 M(CO)5Br	 with	 a	 ligand	 (L)	 the	
products	of	 type	M(CO)4(L)Br	 formed	which	confirms	that	the	
coming	 ligand	 substitute	 the	 CO	 [22].	 Photolysis	 of	 the	
allenylidene	 pentacarbonyl	 chromium	 complexes	
[(CO)5Cr=C=C=C(R1)R2]	(R1=NMe2,	NPh2;	R2=NMe2,	OMe,	Ph)	in	
THF	in	the	presence	of	equimolar	amounts	of	XR3	(XR3=various	
phosphanes,	 P(OMe)3,	 AsPh3,	 SbPh3)	 affords	 cis‐allenylidene	
tetracarbonyl	XR3	complexes,	cis‐	[(CO)4(XR3)Cr=C=C=C(R1)R2]	
[23].	 The	 interaction	 between	 the	 cavitands,	 P(III)‐
phosphocavitands,	 and	 the	 Cr,	 Mo,	 W,	 and	 Mn	 carbonyl	
complexes	 found	possibility	of	 the	directed	coordination	of	all	
or	 some	 phosphorus	 atoms	 of	 the	 cavitand	 molecules	 with	
metals	(Figure	6)	[24].		
	

	
	

Figure	6.	The	figure	illustrating	the	formation	of	the	intramolecular	CO⋯Ph	
contact	in	phosphocavit	and	complex	of	molybdenum	hexacarbonyl	[24].	

	
Some	 ligand‐ligand	 substitution	 reaction	 also	 been	

observed.	 The	 reaction	 of	 [M(CO)4(η2‐C2H2)]	 (M=Fe,	 Os)	
compounds	 with	 [(η5‐C5H5)(CO)2W≡CC6H5]	 proceeds	 by	
unexpected	 substitution	 of	 the	 acetylene	 ligand	 by	 the	
W≡CC6H5	 pseudo‐alkyne	 unit	 and	 formation	 of	 [MW(μ‐
CC6H5)(CO)6(η5‐C5H5)]	(M=Fe,	1;	Os,	2)	[25].	The	labile	complex	
W(CO)5(η2‐btmse)	 undergoes	 replacement	 of	 bis(trimethyl	
silyl)ethyne,	 btmse,	 by	 triphenylbismuthine	 in	 cyclohexane	
solution	at	an	observable	rate	in	the	temperature	range	of	35‐
50	°C	yielding	almost	solely	W(CO)5(BiPh3)	as	the	final	product	
[26].		

 
2.1.	Substitution	reactions	of	metal	carbonyls	in	presence	of	
decarbonylating	agents	
	

In	 organometallic	 chemistry,	 Me3NO	 is	 employed	 as	 a	
decarbonylating	agent	according	to	the	following	stoichiometry	
(Scheme	6).		

	

	
	

Scheme	6	
	

This	 reaction	 is	 used	 to	 decomplex	 organic	 ligands	 from	
metals,	e.	g.	from	(Diene)Fe(CO)3	[27].	It	is	also	used	in	certain	
oxidation	reactions,	e.	g.	 the	conversion	of	alkyl	 iodides	to	the	
aldehyde	 [28].	 Me3NO	 induced	 substitution	 reactions	 of	 (ƞ5‐
C5H5)MoMn(CO)8	with	P(OMe)3	and	t‐BuNC	results	in	facile	Mo‐
‐‐Mn	 bond	 cleavage	 products	 as	 well	 as	 the	 synthesis	 of	 (ƞ5‐
C5H5)Mo(CO)3Mn(CO)4	[P(OMe)3]	in	which	P(OMe)3	occupies	a	
site	trans	to	the	Mo‐‐‐Mn	bond	[29]	.		

 
2.2.	Photochemically	induced	substitution	reactions	of	metal	
carbonyls	
	

During	 early	 1960’s	 Strohmeier	 [30]	 published	 several	
papers	 on	 photochemical	 substitution	 reaction	 of	 metal	

carbonyls	 with	 their	 derivatives.	 Later	 additional	 interest	 in	
photolysis	 of	 group	 VI	 metal	 carbonyls	 developed	 among	
various	scientists	[31‐37].	Most	of	these	studies	have	centered	
around	the	photolysis	of	parent	hexacarbonyls	and	the	nature	
of	 the	 species	M(CO)5	generated	 in	 these	 reactions.	The	metal	
carbonyl	complexes	show	photochemical	 substitution	reaction	
proved	earlier	[38].	It	has	long	been	known	that	metal	carbonyl	
compounds	eliminate	a	CO	group	on	photolysis	in	UV	[39].		

Many	metal	 carbonyls	 are	 photo‐activatable.	 The	 hitherto	
unknown	complexes,	[M(CO)4(ƞ2‐H2L)],	[M=Cr;	1,	Mo;	2,	W;	3];	
have	 been	 synthesized	 by	 the	 photochemical	 reactions	 of	 VIB	
metal	carbonyls	[M(CO)6]	[M=Cr,	Mo,	W],	with	chiral	Schiff	base	
N,N'‐bis‐(2‐hydroxynaphthalene‐1‐carbaldehydene)‐(1R,2R)‐(‐
)‐diaminocyclohexane	(H2L)	in	THF	(Scheme	7)	[40].		
	

HC N

H

N CH

H

OH HO

M(CO)6 [M=Cr, Mo, W]

h

in THF

- 2 CO

HC N

H

N CH

H

M

CO
OC CO

OH OHCO

	
	

Scheme	7	
	

Many	 complexes	 based	 on	 above	 hypothesis	 have	 been	
prepared.	 Photochemical	 synthesis	 of	 metal	 carbonyl	
complexes	 of	 tetraalkyldiphosphine	 disulfides,	 [M2(CO)10(μ‐
R2P(S)P(S)R2)]	 and	 [M(CO)4(μ‐R2P(S)P(S)R2)](M	=	Mo,	W;	R	=	
Me)	[41]	and	metal	Carbonyl	complexes	of	M(CO)6	(M	=	Cr,	Mo,	
and	 W)	 with	 acetonemethanesulfonylhydrazone	 (amsh)	 and	
methanesulfonylhydrazone	 (msh)	 [42]	 have	 been	 done	
successfully	(Scheme	8).		
	

	
Scheme	8	

	
Photolytic	reaction	of	the	free	carbene	(L	=	1,3,4‐triphenyl‐

4,5‐dihydro‐1H‐1,2,4‐triazolin‐5‐ylidene)	 with	 the	 hexa‐
carbonyls	 of	 Cr,	 Mo	 and	 W	 the	 corresponding	 M(L)(CO)5	
complexes	 are	 generated.	 Depending	 on	 an	 excess	 of	 carbene	
also	the	cis‐(L)2Mo(CO)4	complex	was	obtained	[43].	Photolysis	
of	M(CO)6	(M=Cr,	Mo,	W)	in	the	presence	of	vinyl	ferrocene	in	
an	 n‐hexane	 solution	 at	 ‐15	°C	 yields	 M(CO5)(η2‐
vinylferrocene)metal(0)	 complexes	 as	 the	 sole	 product	
(Scheme	 9),	 which	 could	 be	 isolated	 and	 characterized	 by	
spectroscopic	techniques.	The	complexes	were	found	to	be	not	
very	stable	and	their	stability	increases	in	the	order	Cr<Mo<W.	

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The	 relatively	 stable	 W(CO)5(η2‐vinylferrocene)	 could	 be	
isolated	as	a	pure	solid	[44].		
	

	
	

Scheme	9	
	

John	K.	Ruff	and	Max	Lustig	 found	 that	 the	addition	of	 the	
difluorodithiophosphate	 anion	 to	 a	 solution	 of	 Cr(CO)5THF	 in	
the	absence	of	light	produces	only	trace	amounts	of	the	product	
[45].		

 
2.3.	Substitution	reactions	of	metal	carbonyls	in	presence	of	
complexation	agents	
	

Complexation	 agent	 or	 ligand	 is	 an	 ion	 or	 molecule	 that	
binds	 to	a	 central	metal	atom	to	 form	a	coordination	complex	
[46,47].	 Although	 the	 ligands	 can	 directly	 react	 with	 metal	
carbonyls	 to	 form	metal	 carbonyl	 complexes,	 but	 the	 reaction	
becomes	 more	 easy	 and	 gives	 higher	 yields	 when	 a	 M(CO)x	
transfer	 reagent	 is	 available.	 Many	 M(CO)x	 transfer	 reagents	
are	known	few	of	them	are	discussed	below.		
	
2.3.1.	M(CO)5	transfer	reagents	
	

The	 monodentate	 ligands	 which	 easily	 get	 replaced	 with	
incoming	ligands	are	termed	as	M(CO)5	transfer	reagents.		

 
2.3.1.1.	M(CO)5THF	
	

UV‐photolysis	 of	 a	 THF	 solution	 of	 Mo(CO)6	 gives	
Mo(CO)5(THF).	 A	 THF	 solution	 (150	 mL)	 of	 M(CO)6	 (0.71	
mmol)	was	 irradiated	with	UV	 light	under	N2	 stream	at	 room	
temperature	 for	 2	 h	 to	 generate	 the	M(CO)5THF	 intermediate	
[48].		

The	allenylphosphine	(L),	reacts	with	(THF)Cr(CO)5	to	form	
the	 complexes	 (L)‐Cr(CO)5[31].	 The	 complexes	 [Et4N]	
[M(CO)5SCPh3]	(M	=	Mo,	W)	were	prepared	by	the	reaction	of	
[Et4N][SCPh3]	 with	 the	 photogenerated	 M(CO)5THF	
intermediate	 at	 ambient	 temperatures.	 THF	 is	 substituted	 by	
reaction	of	M(CO)5THF	with	a	monodentate	ligand	(Scheme	10)	
[49].		
	

	
	

Scheme	10	
	

Reaction	of	1H‐1,3‐benzazaphospholes	with	M(CO)5	 (THF)	
yields	 the	 respective´1(P)‐metal	 carbonyl	 complexes	 (Scheme	
11)	[50].		

	

	
	

Scheme	11	
	

The	reaction	of	β‐ethynyl	α,β‐unsaturated	esters	or	amides	
with	 Cr(CO)5(L)	 (L=	 THF),	 derived	 from	 chromium	
hexacarbonyl,	 gives	 2‐pyranylidene‐chromium	 complexes	 in	
good	yields	(Scheme	12)	[51].		
	

	
Scheme	12	

	
Subasi	 et	 al.	 prepared	 complexes,	 [(μ	 ‐CO)2Cr2(η4‐H2L)2];	

[(μ	 ‐CO)M2(CO)2(η4‐H2L)2],	 [M	 =	 Mo;	 W];	 [(μ	 ‐CO)2Cr2(η4‐
H2L′)2],	and	[(μ	‐CO)M2(CO)2(η4‐H2L′)2],	[M	=	Mo;	W]	have	been	
synthesized	by	the	photochemical	reactions	of	photogenerated	
intermediate,	M(CO)5THF	(M	=	Cr,	Mo,	W)	with	two	oxygen	and	
nitrogen	 donor	 Schiff	 base	 ligands,	 N,N′‐bis(2‐
hydroxynaphtalin‐1‐carbaldehydene)‐1,2‐bis(p‐aminophe‐
noxy)ethane	 (H2L)	 and	 N,N′‐bis(2‐hydroxynaphtalin‐1‐
carbalde‐hydene)‐1,4‐bis(p‐aminophenoxy)butane	(H2	L′)	[52].	
They	also	report	the	complexes,	cis‐	[M(CO)2(η4‐H2L')],	[M	=	Cr;	
Mo;	W]	and	cis‐	[M(CO)2(η4‐H2L”)],	[M	=	Cr;	Mo;	W]	synthesized	
by	 the	 photochemical	 reactions	 of	 photogenerated	
intermediate,	M(CO)5THF	(M	=	Cr,	Mo,	W)	with	salen	type	Schiff	
base	 ligands,	 N,N'‐bis(salicylidene)‐1,4‐diaminobutane	 (H2L')	
and	 N,N'‐bis(2‐hydroxy‐3‐methoxybenzylidene)ethylene	
diamine	(H2L”)	[53].		

The	reactions	of	M(CO)6	with	ligands	using	THF	as	solvent	
also	give	same	results	as	found	by	reactions	of	M(CO)5THF	and	
ligand.	 Reaction	 of	 M(CO)6	 with	 3,5‐dimethyl‐4‐
ferrocenylmethylpyrazole	 under	 UV	 irradiation	 using	 THF	 as	
solvent	 yields	 monosubstituted	 group	 6	 metalcarbonyl	
complexes	 M(CO)5L	 (M	 =	 Cr,	 Mo	 or	 W;	 L	 =	 3,5‐dimethyl‐4‐
ferrocenylmethylpyrazole)	(Scheme	13)	[54].		

The	anionic	complexes	 [Et4N]	[M(CO)5SCPh3]	 (M	=	Mo,	W)	
were	 also	 prepared	 by	 direct	 thermal	 reaction	 of	 equimolar	
amounts	 of	 [Et4N]	 [SCPh3]	 and	 the	 hexacarbonyl	 parent,	
M(CO)6,	in	THF	solutions	[38].		

	

FcCH2

N

NH M(CO)6

THF, h
FcCH2

N

NH

M(CO)5

	
	

Scheme	13	
 

2.3.1.2	M(CO)5C60	
	

Fullerene	(C60)	also	shows	some	pathways	in	preparations	
of	organometallic	compounds.	The	fullerene	shows	substitution	
of	some	ligands	like	acetonitril	and	CO	or	vice	versa	with	some	
ligands	like	PPh3	and	PCy3.		

The	studies	on	chemical	reactivity	of	fullerene	C60	towards	
fac‐Mo(CO)3(MeCN)(dppe)	 and	Me(CO)4(dppe)	 shows	 that	 the	
C60	easily	replace	acetonitril	ligands	in	chlorobenzene	at	about	
80oC	to	give	fac‐Mo(CO)3(dppe)	(ƞ2‐C60),	whereas	fullerene	C60	
reacted	 with	 Me(CO)4(dppe)	 in	 refluxing	 chlorobenzene	 to	
afford	an	isomeric	mixture	of	fac‐	and	mer‐Mo(CO)3(dppe)(ƞ2‐
C60)	[55].		

The	 Lewis	 bases	 (L)	 triphenylphosphine	 (PPh3)	 and	
tricyclohexyl	 phosphine	 (P(Cy)3)	 displace	 60‐fullerene	 (C60)	

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from	 the	 complex	 fac‐(η2‐C60)(η2‐Phen)M(CO)3	 (M	 =	 Mo,	 W)	
(Phen	=	1,10‐phenanthroline)	(Scheme	14)	[56,57].		
	

	
	

Scheme	14	
	
2.3.2.	M(CO)4	transfer	reagents	
	

These	 reagents	 have	 their	 remarkable	 importance	 in	
synthesis	 of	 metal	 carbonyl	 complexes	 with	 ligands.	 These	
M(CO)4	transfer	reagents	form	some	complexes	which	were	not	
easily	formed	by	direct	reaction	of	M(CO)6	with	ligands.	In	this	
project	 we	 use	 M(CO)4NBD	 as	 M(CO)4	 transfer	 reagent	 and	
prepared	M(CO)4L	 type	 complexes	with	 TDD.	 If	 such	 reaction	
carried	 out	 with	 M(CO)6	 and	 TDD	 in	 equimolar	 quantities	
mono‐substituted	 complexes	 were	 formed.	 If	 we	 use	
metalcarbonyls	in	bimolar	quantity,	binuclear	complexes	were	
formed.		

In	other	preparation	we	use	M(CO)4COD	as	M(CO)4	transfer	
reagent	 with	 DTTT.	 DTTT	 is	 a	 monodentate	 ligand	 and	 form	
M(CO)5DTTT	 but	 we	 were	 keen	 to	 form	 bisubstituted	
complexes	 with	 DTTT,	 when	 we	 use	 M(CO)4COD	 as	 M(CO)4	
transfer	reagent,	bisubstituted	complexes	M(CO)4(DTTT)2	were	
formed.		

The	 thermal	 substitution	 of	 Mo(CO)4(η2:2‐COD)	 and	
Mo(CO)4(η2:2‐NBD)	with,	BFEDA	gave	the	same	product	which	
was	 isolated	 by	 using	 the	 same	 procedure.	 Thus,	 both	 of	
Mo(CO)4(η2:2‐NBD)	 and	 of	 Mo(CO)4(η2:2‐COD)	 can	 be	 used	 as	
Mo(CO)4	transfer	reagent	for	the	synthesis	of	Mo(CO)4(BFEDA)	
[58].		
	
2.3.2.1.	M(CO)4	NBD	
	
2.3.2.1.1.	Preparation	of	[M(CO)4(nbd)]	
	

R.	B.	King	prepared	[M(CO)4(nbd)]	by	refluxing	M(CO)6	and	
2,5‐norbornadiene	 (η2:2‐NBD)	 in	 25	 mL	 iso‐octane.	 And	 yield	
yellow	crystals	of	the	complex	on	crystallization	from	n‐hexane	
solution	 [59‐61].	 Bennett	 et	 al.	 later	 also	 prepared	
[M(CO)4(nbd)]	with	some	other	parallel	method	[62].		
	
2.3.2.1.2.	Uses	of	[M(CO)4(nbd)]as	M(CO)4	transfer	agent		
	

Reaction	 of	 norbornadiene	 complex	 C7H8Cr(CO)4	 with	
tris(dimethylamino)arsine	 (Tdas)	 at	 room	 temperature	 gives	
trans‐(Tdas)2Cr(CO)4	 which	 was	 isolated	 as	 yellow	 complex.	
While	the	reaction	of	M(CO)6	with	Tdas	gives	monosubstituted	
complex	(Tdas)Cr(CO)5	[63].		

[M(CO)4(nbd)]	 (M=Cr	 or	 Mo)	 react	 with	 L1	
(Ph2P(CH2)2O(CH2)2O(CH2)2PPh2)	 (Scheme	 15)	 to	 give	 the	
trans‐	[Cr(CO)4(L1)]	and	cis‐	[Mo(CO)4(L1)]	respectively,	both	of	
which	show	coordination	of	L1	through	the	P	atoms	only,	giving	
11‐membered	 ring	 metallocyclic	 complexes.	 The	 crystal	
structure	 of	 [Mo(CO)4(L1)]	 confirms	 the	 cis‐geometry	 of	 the	
product	[64].		
	

	
 

Scheme	15	
 

2.3.2.2.	M(CO)4	COD	
	

Fisher	 and	 Frohlich	 prepared	 [M(CO)4(COD)]	 by	 refluxing	
M(CO)6	and	1,5‐cyclooctadiene	(COD)	in	n‐heptane.	Cooling	the	
solution	 to	 ‐35oC	 yielded	 yellow	 crystals	 of	 M(CO)4(η2:2‐COD)	
[65].	R.	B.	King	also	prepared	[M(CO)4(COD)]	later	[66].		

The	 thermal	 substitution	 of	 1,5‐cyclooctadiene	 (COD)	 by	
bis(diphenylphosphino)alkanes	,	(C6H5)2P(CH2)nP(C6H5)2	(n	=	1,	
2,	 3)	 in	 M(CO)4(η2:2‐COD)	 complexes	 (M	=	Cr,	 Mo,	 W),	 were	
studied	 by	 Kayran	 et	 al.	 [67].	 The	 reactions	 of	 Cr(CO)4(COD)	
and	Mo(CO)4(NBD)	with	h5‐Cp*Ir(2,5‐dimethylthiophene)	give	
modest	 amounts	 of	 the	 [h6‐Cp*Ir(C,S‐2,5‐Me2T)]M(CO)3	
products	[68].		

 
2.3.2.3.	M(CO)4	(piperidine)2	
	

Mariusz	 et	 al.	 prepared	 M(CO)4(piperidine)2	 where	 (M	 =	
Mo,	 W)	 by	 thermal	 reaction	 of	 Mo(CO)6	 with	 piperidine	 and	
found	cis‐	bonding	 in	M(CO)4(piperidine)2.	The	 two	piperidine	
ligands	 in	 this	 yellow‐colored	 compound	 are	 labile,	 which	
allows	 other	 ligands	 to	 be	 introduced	 under	 mild	 conditions	
and	its	dimmers	[69].		

The	 reaction	 of	 [Mo(CO)4(piperidine)2]	 with	 triphenyl	
phosphine	in	boiling	dichloromethane	(b.p.	=	~40	°C)	gives	cis‐	
[Mo(CO)4(PPh3)2].	 This	 cis‐complex	 isomerizes	 in	 toluene	 to	
trans‐[Mo(CO)4(PPh3)2]	[70].	The	study	on	Carbonyl	complexes	
of	 manganese,	 rhenium	 and	 molybdenum	 with	
ethynyliminopyridine	 ligands	 found	 that	 [MBr(CO)5]	 reacts	
with	m‐ethynylphenylamine	and	pyridine‐2‐carboxaldehyde	in	
refluxing	 tetrahydrofuran	 to	 give,	 fac‐	 [MBr(CO)3(py‐2‐CH=N‐
C6H4‐m‐(C≡CH))]	 (M	=	Mn,	 1a;	 Re,	 2a).	 The	 same	 method	
affords	 the	 tetracarbonyl	 [Mo(CO)4{py‐2‐CH=N‐C6H4‐m‐
(C≡CH)}]	(3a)	starting	from	[Mo(CO)4(piperidine)2]	[71].		

Robert	 H.	 Guinn	 and	 Regina	 Gilliland	 [72]	 prepared	 cis‐	
[Mo(CO)4(PPh3)2]	 by	 reaction	 of	 cis‐[Mo(CO)4(pip)2]	 with	
triphenylphosphine	 (PPh3)	 (Scheme	 16)	 in	 dry	 CH2Cl2	 on	
refluxing	at	50	oC	for	15	min	using	previous	reference	of	Allen	
et	al.	[73].		
	
2.3.2.4.	M(CO)4	(CH3CN)2	
	

It	has	been	reported	that	nitrile	complexes	of	the	group	VIB	
metal	 carbonyls	 are	 prepared	 by	 photolytic	 displacement	 of	
carbonyl	 group	 to	 give	 mono‐	 and	 disubstituted	 derivatives	
[74,75]	or	by	displacement	of	diglyme	in	(Diglyme)Mo(CO)3	to	
give	 disubstituted	 derivatives	 [76].	 The	 photoproduction	 of	
metal	hexacarbonyl	derivatives	(M	=	Cr,	Mo,	W)	in	acetonitrile	
results	 the	 formation	 of	 species	 M(CO)4(CH3CN)2,	
M(CO)5(CH3CN)	and	in	the	cases	of	Mo	and	W	M(CO)3(CH3CN)3	
also	formed.	The	metal	hexacarbonyls	M(CO)6	(M	=	Cr,	Mo,	W)	
are	dissolved	in	acetonitrile	resulting	colorless	solutions	when	
exposed	 to	 ultraviolet	 radiations,	 they	 turn	 bright	 yellow	
almost	immediately	to	form	the	above	complexes	[77].		

Reaction	of	[(PPh2C5H4)Cp3Fe4(CO)4]	with	(CO)4W(CH3CN)2	
at	 ambient	 temperature	 affords	 [(CO)4W(PPh2C5H4)	
Cp3Fe4(CO)4]	 as	 the	 major	 product,	 together	 with	 a	 small	
amount	of	[(CO)5W(PPh2C5H4)Cp3Fe4(CO)4]	[78].		

Isocyanides	also	forms	extensive	families	of	complexes	that	
are	related	to	the	metal	carbonyls.	Typical	isocynide	ligands	are	
MeNC	and	t‐butylisocyanide	(Me3C‐NC).	A	special	case	is	CF3NC,	
an	 unstable	 molecule	 that	 forms	 stable	 complexes	 whose	
behavior	closely	parallels	that	of	the	metal	carbonyls	[79].		
	

	
	

Scheme	16	

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2.3.2.5.	M(CO)4(bipy)	
	

2,2’‐Bipyridine	 have	 been	 prepared	 by	 Smith	 et	 al.	 [80].	
Darensbourg	 et	 al.	 found	 that	 these	 complexes	 were	
synthesized	 by	 reacting	 [Mo(CO)6]	with	 excess	2,2’‐bipyridine	
(N‐N)	 ligand	 in	 deoxygenated	 toluene	 at	 reflux	 (Scheme	 17).	
Reaction	 times	 for	 the	 thermal	 synthesis	 of	 [Mo(CO)4(bipy)]	
complexes	vary	between	1.5	to	2.0	hours	[81].		
	

	
	

Scheme	17	
	

Later	David	Manuta	et.	al.	synthesize	Mo(CO)4(2,2'‐bipy)	by	
photolysis	of	a	solution	containing	the	parent	hexacarbonyl	and	
2,2'‐bipyridine(N‐N)	(Scheme	18)	[82].		
	

	
	

Scheme	18	
	

Fred	 Basolo	 and	 John	 McDonald	 found	 that	 the	 oxidative	
elimination	 reactions	 of	 M(CO)4L	 (M	 =	 Mo,	 W;	 L=	 2,2’‐
bipyridine)	with	mercuric	halides	yields	M(CO)3L(HgX)	and	CO	
using	 acetone	 as	 solvent	 [83].	 6‐Ferrocenyl‐2,2′‐bipyridine,	
prepared	by	the	reaction	of	lithiated	ferrocene	and	bipyridine,	
reacts	 with	 pentacarbonyl	 [η2‐bis(trimethylsilyl)ethyne]	
tungsten(0)	 to	 form	 tetracarbonyl(6‐ferrocenyl‐2,2′‐
bipyridine)tungsten(0)	which	was	isolated	as	analytically	pure	
substance	(Scheme	19)	[84].		
	

Fe

N

N
W

OC

OC
CO

CO
	

	
Scheme	19	

	
2.3.2.6.	M(CO)4(PMe3)2	
	

All	 metal	 carbonyls	 undergo	 substitution	 by	
organophosphorus	 ligands.	 For	 example,	 the	 series	 Mo(CO)6‐
x(PR3)x	is	well	known	for	various	phosphine	ligands	for	x	=	1,2	
and	 3.	 PF3	 behaves	 similarly	 but	 is	 remarkable	 because	 it	
readily	 forms	 homoleptic	 analogous	 of	 the	 binary	 metal	
carbonyls	i.	e.	Mo(PF3)6.		

Air	 stable	 and	 sublimable	 bis(phosphine)metal	
tetracarbonyls	 of	 chromium,	 molybdenum	 and	 tungsten	 have	
been	 prepared	 by	 a	 ligand	 displacement	 reaction	 with	 the	
octahydrotriborotetracarbonyl	 metalate	 anions	 (Scheme	 20)	
[85].		
	

	
	

Scheme	20	
	

Klanberg	and	Mutterties	 later	prepared	colorless	to	faintly	
yellowish	 crystalline	 complexes	 of	 type	 M(CO)4(PH3)2	 have	
been	synthesized	[86].		

Uses:	 A	 study	 on	 photodissociation	 of	 the	 phosphine‐
substituted	 transition	metal	 carbonylcomplexes	 Cr(CO)5L	was	
performed	 with	 time‐dependent	 DFT	 theory	 to	 explore	 the	
propensity	of	the	excited	molecules	to	expel	their	 ligands.	The	

influence	 of	 the	 PH3	 ligand	 on	 the	 properties	 of	 these	
complexes	is	compared	with	the	photodissociation	behavior	of	
the	carbonyl	complex	Cr(CO)6.	All	calculations	indicate	that	the	
excited	Cr(CO)5L	molecules	 (L	=	PH3,	PC2H5,	 and	PCH3)	prefer	
dissociation	of	their	phosphorus	substituent	over	that	of	an	CO	
ligand	 (Figure	 7).	 This	 suggests	 that	 the	 photochemical	
approach	may	be	a	viable	complement	 to	 the	 ligand	exchange	
and	redox	methods	that	are	currently	employed	to	demetalate	
transition	 metal	 complexed	 organophosphorus	 compounds	
[87].		
	

	
 

Figure	7.	Photodissociation	of	phosphine‐substituted	transition	metal	
carbonyl	complexes	[87].		

 
2.3.3.	M(CO)3	transfer	reagents	
	

Many	 ligands	 are	 known	 which	 act	 as	 M(CO)3	 transfer	
reagent.	Some	of	them	are	discussed	here.		
	
2.3.3.1.	M(CO)3(acetonitril)	
	

Methyl	cyanide	(acetonitril)	ligands	are	capable	to	displace	
two	 or	 three	 CO	 molecules	 from	 M(CO)6	 hence	 represent	
properties	 as	M(CO)4	 as	well	 as	M(CO)3	 transfer	 reagent.	 But	
the	 studies	 shows	 that	 M(CO)3	 tranfer	 properties	 are	 more	
common	for	acetonitril.		

Tate	et	al.	find	that	tri‐nitrile	derivatives	are	prepared	very	
conveniently	 by	 direct	 reaction.	 For	 instance	 on	 refluxing	
Mo(CO)6	 in	 excess	 acetonitrile	 under	 nitrogen,	 a	 near	
quantitative	 yield	 of	 (CH3CN)3Mo(CO)3	 is	 obtained	 with	
evolution	of	3	moles	of	gas	in	less	than	4	hr.	It	is	identical	with	
the	 compound	 prepared	 by	 Werner	 and	 Coffield	 [76]	 via	 a	
different	 route.	 Similarly	 W(CO)6	 give	 (CH3CN)3W(CO)3	 after	
40‐hr	reflux	which	shows	greater	air	stability.	(CH3CN)3Cr(CO)3	
also	been	prepared	but	it	shows	extremely	sensitivity	and	less	
stability.	These	compounds	serves	as	excellent	intermediates	in	
the	formation	of	new	compounds	not	available	by	other	routes	
[88].		

Elschenbroich	 and	 Salzer	 shows	 that	 upon	 refluxing	 in	 a	
solution	 of	 acetonitrile,	 Mo(CO)6	 converts	 to	 its	
tris(acetonitrile)	 derivative[Mo(CO)3(MeCN)3].	 The	 resulting	
air‐sensitive	compound	serves	as	a	source	of	"Mo(CO)3"	[89].		

Acrylonitrile	 complexes	 (CH2=CHCN)W(CO)5of	 tungsten	
carbonyl	 have	 been	 also	 prepared	 in	 which	 acrilonotril	 is	
bonded	 to	 tungsten	 by	mean	 of	 lone	 pair	 of	 electrons	 on	 the	
nitrogen.	The	complex	 is	prepared	by	stirring	 (CH3CN)W(CO)5	
overnight	 in	 excess	 acrylonitrile	 followed	 by	 vacuum	
evaporation	 of	 the	 displaced	 acetonitrile	 and	 excess	
acrylonitrile	[90].		

The	 reaction	 of	Mo(CO)3(CH3CN)3	 and	E2Ph2	 (E=Se	 or	 Te)	
yields	 the	 edge‐sharing	 bioctahedral,	 metal‐metal	 bonded	
Mo(I)	 products	 [Mo(CO)3(CH3CN)(μ‐EPh)]2	 [91].	 Duterium	
labeled	(fulvene)M(CO)3(M	=	Cr,	Mo)	complexes	were	prepared	
in	 high	 yields	 by	 reacting	 the	 duterated	 fulvene	 ligand	 with	
(MeCN)3M(CO)3	[92].		

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On	refluxing	(CH3CN)3W(CO)3	in	3‐hexyne,	2	equivalent	CO	
was	 evolved	 and	 results	 yellow	 colored	
(CH3CH2C=CCH2CH3)3W(CO)	[93].	Thermolysis	of	[(Mo(CO)4(μ‐
DPFB)2]	 (DPFB	 =	 1,4‐bis	 (diphyenylphosphino)‐2,5‐
difluorobenzene)	 in	 acetonitrile	 followed	 by	 addition	 of	
triphenylphosphine	 yields	 trans‐	 [Mo(CO)4(PPH3)2]	 and	 free	
DPFB	which	co‐crystallise	[94].		

 
2.4.	Substitution	reactions	of	cyclopentadienyl	and	
cyclohepatrienyl	complexes	of	M(CO)6	
	

The	n‐cyclopentadienyl	and	n‐arenes	groups	are	among	the	
most	common	class	of	 ligand	encountered	 in	organotransition	
metal	chemistry.	Transition	metal	complexes	with	these	ligands	
have	 been	 studied	 extensively	 and	 many	 of	 which	 have	
contributed	 to	 the	development	of	 areas	 such	as	homogenous	
catalyst,	 organic	 synthesis,	 new	 materials	 and	 biological	 and	
medical	sciences	[95].	A	variety	of	polyolephenic	and	aromatic	
hydrocarbons	 have	 been	 shown	 to	 react	 with	 carbonyls	 of	
Group	VI	metals	to	afford	π‐complexes.	Compounds	of	the	type	
L‐M(CO)3	[M	=	Cr,	Mo	or	W]	have	been	obtained	from	aromatic	
hydrocarbons	 [96‐100]	 and	 from	 cycloheptatriene	 [101,102],	
1,3,5‐cyclooctatriene	 [103,104]	 and	 even	 cyclononatetracene	
[100]	 In	 addition	 to	 complexes	 derived	 from	 conjugated	
trienes,	 metal	 complexes	 L‐M(CO)4	 involving	 non	 conjugating	
dienes	 1,5	 cyclooctadiene	 [105‐107]	 and	 bicycle‐
[2.2.1]heptadiene	[108,109]	have	been	reported	[110].		

 
2.4.1.	M(CO)4Cp	
	

Preparation:	 M(CO)4Cp	 was	 prepared	 by	 thermal	
substitution	 reaction	 of	 M(CO)6	 with	 cyclopentadiene	 [111].	
The	 ligand	 substitution	 reaction	 of	 M(CO)4cp	 proceed	 via	 CO	
substitution	method	(Scheme	21)	[112].		
	

	
	

Scheme	21	
	

Dicarbonyl(ƞ5‐cyclopentadienyl)iodoiron(II)	is	widely	used	
and	 easily	 prepared	 precursor	 for	 a	 variety	 of	 organometallic	
iron	complexes	[113].	McArdle	et	al.	and	Akita	et	al.	have	been	
prepared	 similar	 complexes	 with	 substituted	 Cp	 rings	
[114,115].	 Linkage	 isomerism	 in	 Carbonyl‐π‐cyclopentadienyl	
(thiocynato)metal	 complexes	 of	 manganese,	 chromium,	
molybdenum,	 tungsten	and	rhodium	has	also	been	studied	by	
Thomas	and	Andrew	[116].		

N‐Heterocyclic	 carbenes	 (NHC)	 (Scheme	 22)	 have	 proved	
to	be	excellent	ligand	in	transition	metal	complexes.	Photolysis	
of	 [CpMn(CO)3]	 in	presence	of	1,3,4,5‐tetramethylimidazoline‐
2‐thione	 SC3N2Me4	 produce	 thione‐bridged	 dinuclear	 complex	
[(CpMn(CO)2)2(SC3N2Me4)].	 Mn‐S‐Mn	 framework	 found	
responsible	 for	 dinuclear	 complexes.	Photoreaction	of	M(CO)6	
(M	=	Mo,	W)	with	SC3N2Me4	afforded	mononuclear	 complexes	
[M(CO)5(SC3N2Me4)].	It	was	predict	that	formation	of	dinuclear	
complex	was	somewhat	predicted	due	to	presence	of	Cp	ligand	
[117].		
	

	
	

Scheme	22	

2.4.2.	M(CO)3(ƞ6‐C7H8)	
	

Although	 Cycloheptatriene	 (C7H8)	 and	 cycloheptatrienyl	
(C7H7)	 transition	metal	 carbonyl	 complexes	have	been	known	
over	 six	 decades.	 The	 first	 cycloheptatriene	 complex	
Mo(CO)3(C7H8)	was	prepared	 by	Wilkinson	 et	 al.	 in	 1958	 and	
cycloheptatrienyl	complexes	shortly	after	[94].		
	
2.4.2.1.	Synthesis	of	η6‐cycloheptatriene	molybdenum	
tricarbonyl	[(η6‐C7H8)Mo(CO)3]	
	

Moderately	 air‐sensitive	 organometallic	 arene	 complex	
have	 been	 prepared	 from	Mo(CO)6The	molecular	 structure	 of	
(η6‐C7H8)Mo(CO)3	 has	 been	 determined	 by	 X‐ray	 diffraction	
[118]	and	the	compound	has	the	structure	indicated	in	Scheme	
23.	 Those	 of	 the	 closely	 related	 compounds	 (C6H6)Cr(CO)3	
[(benzene)	 chromium	 tricarbonyl]	 [119]	 and	 (Me6C6)Cr(CO)3	
[(hexamethylbenzene)	 chromium	 tricarbonyl]	 [120‐122]	 have	
also	 been	 established	 in	 detail	 and	 reveal	 a	 "piano	 stool"	
structure	 with	 the	 arene	 moiety	 π‐bonded	 to	 the	 metal	
(Scheme	23).	

	

	
	

Scheme	23	
	

Cycloheptatriene	in	cyclopentadiene/nonane	were	taken	in	
Schlenk	flask	or	round‐bottom	flask	and	add	Mo(CO)6	(2.	0	g.	7.	
6	mmol)	to	this	flask	and	heat	the	contents	of	the	flask	carefully	
to	 reflux	 for	2	hours.	At	 the	end	of	 the	 reflux	period,	 cool	 the	
reaction	 mixture	 to	 room	 temperature	 under	 a	 nitrogen	 gas	
flow.	 Red/orange/black	 (η6‐C7H8)Mo(CO)3	 solution	 formed	
which	 was	 crystallized	 on	 the	 alumina	 cake.	 Upon	 washing	
several	times	with	CH2Cl2	solvent	(η6‐C7H8)Mo(CO)3	crystallize	
as	red	solid.	The	red	solid	will	turn	black	and	decompose	if	it	is	
exposed	to	air	for	only	one	day	[123,124].		

The	 reaction	 of	 tris(dimethylamino)phosphine(Tdp)	 and	
tris(dimethylamino)arsine(Tdas)	with	the	C7H8M(CO)3	(M	=	Cr,	
Mo,	W)	in	hydrocarbon	solvents	has	been	investigated.	The	first	
step	of	this	reaction	appear	to	be	the	complete	displacement	of	
the	 coordinated	 olifin	 with	 the	 legands	 products	 with	 two	
legands	 in	 cis	 position	 are	 found	 to	 be	 unstable	 &	 undergo	
further	change	by	following	two	processes.		

1.	Rearrangement	of	cis	derivative	to	trans	derivative	
2.	Removal	of	a	ligand	by	decomposition	generated	carbon	

monoxide	[125].		
The	 trisubstituted	 metalcarbonyl	 complexes	 containing	

ligands	 which	 coordinate	 through	 bicovalent	 sulfur	 are	
reported.	 Direct	 reaction	 of	 the	 ligands	 with	
cycloheptatrienemolybdenumtricarbonyl	 [M(CO)3(ƞ6‐C7H8)],	
complexes	 of	 type	 Mo(CO)3L3	 were	 prepared	 where	 L	
representing	 dimethyl	 sulfide	 [(CH3)2S],	 diethyl	 sulfide	
[(C2H5)2S],	 thiourea	 [(CH2)2CS],	 thioacetamide	 [CH2CSNH2]	
[126].		

 
2.4.3.	M(CO)3(ƞ6‐C8H8)	
	

Preparation:	 Winstein	 et	 al.	 prepared	 cyclooctatetraene	
complexes	(ƞ6‐C8H8)Mo(CO)3,	in	which	this	olefin	is	bound	to	a	
metal	by	six	π‐electrons	on	six	carbon	atoms	[127].		

Cyclooctatetraene	 is	 displaced	 from	 C8H8Ru(CO)3	 by	 I2	 or	
P(OCH2)3CEt	 giving	 [Ru(CO)3I2]2	 and	 trans‐	
[P(OCH2)3CEt]2Ru(CO)3,	 respectively.	 Mercury(II)	 chloride,	
bromide,	or	 thiocyanate	also	react	with	C8H8Ru(CO)3	 to	afford	
the	 binuclear	 complexes[Ru(CO)3(HgX)X]2	 (XCl,	 Br,	 or	 SCN),	
which	undergo	bridge‐splitting	reactions	with	pyridine	without	

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breaking	 the	 Ru‐Hg	 bond,	 to	 give	 the	 complexes	
Ru(CO)3py(HgX)X	[128].		

 
3.	Kinetics	and	mechanism	of	substitution	reactions	in	
metal	carbonyls	
	

Ligand	 substitution	 reactions	 are	 essential	 for	 the	 use	 of	
transition	 metal	 organometallic	 compounds	 as	 catalysts.	
Therefore,	it	is	important	that	we	know	what	factors	affect	the	
rates	of	reactions	and	why.	Much	information,	both	qualitative	
and	 quantitative,	 is	 available	 [129]	 on	 the	 reactivity	 of	
organometallic	 compounds.	 More	 detailed	 kinetic	 study	 that	
give	 information	on	 the	mechanism	of	 ligand	substitution	and	
the	 factors	 contributes	 to	 rates	 of	 reaction	 aredescribed	here.	
Fred	 Basolo	 found	 that	 CO	 substitution	 reactions	 of	 metal	
carbonyls	proceed	via	 three	different	systems:	(a)	compounds	
where	 ring	 slippage	 seems	 to	 be	 involved	 in	 reaction;	 (b)	
heterocyclic	 metal	 carbonyls,	 and	 (c)	 17‐electron	 metal	
complexes	[130].		

 
3.1.	Ring	slipping	mechanism	
	

About	six	decades	ago	study	of	CO	exchange	using	14CO,	and	
of	 CO	 substitution	 with	 P‐ligands	 on	 binary	 metal	 carbonyls.	
Since	very	 little	work	[131]	of	this	type	had	been	done	at	that	
time,	 the	 observations	were	 often	 a	 source	 of	 some	 surprise.	
For	 example,	 Ni(CO)4	 was	 found	 [132]	 to	 exchange	 CO	 by	 a	
dissociative	process,	although	one	might	have	expected	 the	4‐
coordinate	complex	to	readily	expend	its	coordination	number	
to	 permit	 a	 low	 energy	 associative	 pathway	 for	 reaction.	 We	
discuss	 and	 understand	 this	 behavior	 in	 terms	 of	 Tolman’s	
[133]	16‐	and	18‐electron	rule.	An	associative	substitution	 for	
Ni(CO)4	would	require	the	formation	of	an	unstable	20‐electron	
transition	state	or	active	intermediate.		

We	 find	 [134]	 that	 CoNO(CO)3	 (Scheme	 24)	 and	
Fe(NO)2(CO)2,	 isoelectronic	 and	 isostructural	 with	 Ni(CO)4,	
both	 undergo	 CO	 substitution	 by	 an	 associative	 process.	 The	
interpretation	 given	 the	 observation,	 before	 the	 16‐,	 18‐
electron	rule	and	before	the	bent	metal	nitrosyl,	was	that	a	pair	
of	 electrons	 is	 localized	 on	 the	 nitrosyl	 group	 freeing	 a	 low	
energy	metal	orbital	for	nucleophilic	attack.		
	

	
	

Scheme	24	
	

Such	 a	 process	 that	 maintains	 an	 18‐electron	 count	
throughout	is	much	less	available	for	a	compound	with	only	CO	
ligands	because	N	 is	more	electronegative	than	C	and	because	
the	nitrosyl	ligand	is	formally	present	as	NO+.		

More	readily	explained	after	the	nitrosyl	ligand	experience,	
the	 observations	 [135]	 that	 the	 compounds	 (ƞ5‐C5H5)M(CO)2	
(where	 M	 =	 Co,	 Rh,	 or	 Ir)	 undergo	 CO	 substitution	 by	 an	
associative	 pathway	 (Scheme	 25).	 Surprisingly,	 because	 these	
compounds	 are	pseudo‐isoelectronic	with	Fe(CO)5	which	only	
very	 slowly	 reacts	 thermally	 by	 a	 dissociative	 process	 [131].	
More	readily	interpreted,	because	after	the	nitrosyl	experience	
we	 were	 prepared	 to	 localize	 a	 pair	 of	 electrons	 on	 the	
cyclopentadienyl	ring	in	order	to	permit	nucleophilic	attack	of	a	
low	 energy	 metal	 orbital.	 Some	 years	 later	 it	 was	 suggested	
[136]	 that	 the	 pair	 of	 electrons	 localized	 on	 the	
cyclopentadienyl	 ring	 could	 be	 formulated	 as	 an	 ƞ3	 allyl‐ene	
system.	 There	 isnow	 X‐ray	 evidence	 [137]	 for	 such	 a	 cyclic	
ligand	 in	 the	 compound	 (ƞ5‐C5H5)(ƞ3‐C5H5)W(CO)2.	 This	
suggests	that	an	ƞ5→	ƞ3	→	ƞ5associative	pathway	is	involved.		
	

	
	

Scheme	25	
	

As	 expected	 for	 a	 mechanism	 that	 requires	 the	 flow	 of	
electron	density	from	the	metal	 into	the	cyclic	 ligand,	electron	
withdrawing	 substituents	 (NO2)	 on	 the	 ring	 enhance	 rates	 of	
CO	substitution	and	electron	donating	groups	(CH3)	on	the	ring	
retard	rates	of	substitution	(Scheme	26).		
	

	
	

Scheme	26	
	

Furthermore	 a	 relative	 rate	 increase	 of	 108	was	 observed	
for	 (ƞ5‐indenyl)Rh(CO)2	 over	 (ƞ5‐cyclopentadienyl)Rh(CO)2,	
and	 this	has	been	 termed	 the	 indenyl	 ligand	effect	 [138].	This	
had	 been	 previously	 observed	 (Scheme	 27)	 [139]	 and	
explained	on	 the	basis	of	 the	 fused	six‐ring	aromaticity	 in	 the	
transition	state	tending	to	drive	the	associative	reaction.		
	

	
	

Scheme	27	
	

These	 studies	 show	 it	 is	 possible	 to	markedly	 vary	 (over	
1010	 times)	 the	 rates	 of	 ligand	 substitution	 of	 organometallic	
compounds,	 and	 if	 necessary	 to	 fine	 tune	 these	 rates	 for	
optimum	homogenous	catalysis	conditions.		
 
3.2.	Heterocyclic	metal	carbonyl	mechanism	
	

Although	thousands	of	papers	have	been	published	on	the	
synthesis	and	reactions	if	ƞ5‐carbocyclic	metal	carbonyls,	much	
less	 work	 has	 been	 done	 with	 the	 corresponding	 ƞ5‐
heterocyclic	compounds	[140].	No	work	had	been	reported	on	
the	 kinetics	 and	 mechanisms	 of	 CO	 substitutions	 of	 these	
compounds,	so	 it	was	decided	to	mount	such	an	investigation.	
Compounds,	 corresponding	 to	 known	 ƞ5‐cyclopentadienyl	
metal	 carbonyls,	 were	 prepared	 and	 studied	 that	 have	 one	
heteroatom	(N,	P,	or	As)	in	the	5‐membered	ring.		

The	 reactions	 investigated	 [141]	 are	presented	 in	 Scheme	
28,	where	E	=	C,	N,	P	or	As,	and	L	=	PR3	or	P(OR)3.		
	

	
	

Scheme	28	
	

The	 compounds	where	E	=	C13,	 P	 or	As	do	not	 react	 even	
after	 3	 days	 at	 130	 oC.	 The	 (ƞ5‐N‐heterocycle)Mn(CO)3	 does	
reaction	at	these	conditions	with	a	second	order	rate	law,	first	
order	in	concentrations	of	the	complex	and	of	the	nucleophile.	
Values	of	second	order	rate	constant	at	130oC	in	decalin	are	1.2	
x	10‐6	Mol‐1Lit	Sec‐1	for	the	nucleophile	P(n‐Bu)3	and	2.0	x	10‐6	
mol‐1.L.	 sec‐1	 for	 P(OEt)3.	 The	 tentative	 interpretation	 given	
these	 observations	 that	 only	 the	 N‐heterocyclic	 compound	
reacts	is	that	N	is	the	most	electronegative	atoms	used,	e.	g.	N,	
3.0;	 C,	 2.5;	 P,	 2.1;	 As,	 2.0.	 Again	 the	 greater	 electron	
withdrawing	 property	 of	 the	 cyclic	 ligand	 with	 the	 more	
electronegative	N	atoms	seems	 to	 enhance	 the	 rate	of	 an	ƞ5→	
ƞ3→	ƞ5	mechanism.		

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Another	interesting	observation	[142]	was	made	when	the	
rates	 of	 reaction	 of	 three	 (ƞ5‐N‐heterocyclic)Mn(CO)3	 were	
compared	(Scheme	29).		
	

	
	

Scheme	29	
	

Addition	 of	 two	 methyl	 substituent	 which	 are	 electron	
donating	 to	 the	 pyrrolyl	 ligand	 are	 expected	 to	 decrease	 the	
rate	 of	 CO	 substitution	 by	 an	 associative	 ƞ5→	 ƞ3→	
ƞ5mechanism.	What	was	not	anticipated	is	that	the	positions	of	
the	methyl	groups	on	 the	 ring	would	have	such	a	 large	effect,	
the	2,5‐dimethyl	pyrrolyl	decreases	the	rate	of	substitution	100	
times	 more	 than	 does	 the	 3,4‐dimethyl	 pyrrolyl.	 We	 suggest	
this	 results	 form	 a	 preferred	 ring	 slippage	 towards	 the	
formation	of	an	ƞ3‐2‐azaallyl	bonding	mode	of	the	metal	moiety	
to	 the	 pyrrolyl	 ligand	 (Scheme	 30)	 in	 the	 transition	 state	 or	
active	intermediate	for	reaction.		
	

	
	

Scheme	30	
	

This	 slippage	 towards	 nitrogen	 results	 in	 a	 much	 more	
crowded	transition	state	for	(I)	than	for	the	corresponding	3,4‐	
dimethylpyrrolyl	 system,	and	 thus	a	much	greater	retardation	
in	the	rate	of	CO	substitution,	Support	for	ring	slippage	towards	
N	 is	 provided	 [143]	 by	 the	 X‐ray	 structure	 of	 (ƞ5‐3,4‐
dimethylpyrrolyl)Mn(CO)3which	 shows	 such	 slippage	 even	 in	
the	ground	state.		

Since	 only	 the	 N‐heterocycle	 compounds	 (ƞ5‐N‐
heterocycle)Mn(CO)3react	 rapidly	 enough	 even	 at	 130oC	 to	
conduct	a	kinetic	study,	it	has	not	been	possible	to	determine	if	
indeed	there	is	a	quantitative	correlation	between	the	rates	of	
CO	 substitution	 and	 the	 electronegativity	 of	 the	 hetero	 atom.	
This	should	be	possible	providing	we	could	prepare	 the	more	
reactive	compounds	of	the	type	(ƞ5‐heterocycle)M(CO)2,	where	
M	=	Co,	Rh,	or	 Ir.	An	attempt	(Scheme	31)	[144]	was	made	 to	
prepare	 (ƞ5‐2,5‐dimethylpyrrolyl)Rh(CO)2by	 a	 reaction	
analogous	to	that	used	for	the	synthesis	of	(ƞ5‐C5H5)Rh(CO)2.		
	

	
Scheme	31	

	
The	 desired	 and	 expected	 monomeric	 (ƞ5‐2,5‐

(CH3)2C4H2N)Rh(CO)2was	not	obtained,	so	we	have	as	yet	been	
unable	 to	 test	 the	 concept	 of	 reactivity	 versus	 the	
electronegativity	 of	 the	 heteroatom.	 However,	 the	 dimer	
obtained	 is	 the	 first	 example	 of	 a	 heterocyclic	bridging	 ligand	
between	two	metals	utilizing	both	the	σ	and	the	ƞ2‐π	bonding	
modes	 of	 the	 ligand	 to	 form	 a	 highly	 distorted	 metal‐olefin	
moiety.		
	
	
	

3.3.	17‐electron	metal	complexesmechanism	
	

It	 is	 well	 known	 that	 for	 molecular	 transition	 metal	
organometallic	 compounds	 18‐electrons	 is	 the	magic	 number	
for	maximum	stability.	Three	decade	ago	Brown	[145]	reported	
that	17‐electron	metal	carbonyl	radicals,	such	as	Mn(CO)5	and	
Re(C0)5,	are	substitution	labile.	These	radicals	were	generated	
photochemically	 in	 solution	 and	 studied	 in	 situ,	 so	 Shi	 &	
Richmond	[146]	decided	to	investigate	V(CO)6	which	is	a	stable	
solid	17‐electron	binary	metal	carbonyl	(Scheme	32).		

	
	

	
	

Scheme	32	
	

The	 reaction	 proceeds	 by	 an	 associative	 mechanism.	 The	
point	of	interest	here	is	that	it	was	possible	for	the	first	time	to	
quantitatively	 compare	 the	 rates	 of	 CO	 substitution	 of	
analogous	17‐	and	18‐electron	metal	carbonyls	(Scheme	33).		
	

	
	

Scheme	33	
	

This	 rapid	 rate	 of	 associative	 ligand	 substitution	 of	 a	 17‐
electron	over	a	corresponding	18‐electron	metal	complex	is	 in	
accord	with	the	lower	energy	barrier	for	the	reaction	process	of	
17‐electron	 →19‐electron	 compared	 to	 that	 of	 18‐electron	
→20‐electron.		

This	 investigation	 was	 extended	 to	 the	 reaction	 of	
vanadocene,	(ƞ5‐C5H5)V,	with	CO	to	give	the	17‐electron	adduct	
(ƞ5‐C5H5)2VC0	and	to	its	CO	exchange	kinetics	[147].	The	rate	of	
CO	exchange	is	fast	and	it	goes	by	an	associative	mechanism	as	
proposed	by	the	reaction	profile.		

Experiments	showed	that	the	rate	of	CO	exchange	with	(ƞ5‐
C5H5)VCO	 is	 greater	 than	 the	 rate	 of	 addition	 of	 CO	 to	
vanadocene,	which	accounts	for	the	higher	energy	barrier.	It	is	
believed	this	 is	caused	by	the	conformational	change	required	
when	CO	 is	 added	 to	vanadocene,	whereas	 the	associative	CO	
exchange	 takes	 place	 without	 the	 expenditure	 of	 energy	
required	to	bend	back	the	cyclopentadienyl	rings.		

A	dramatic	change	in	rate	and	in	mechanism	was	observed	
[148]	 when	 cyclopentadienyl	 was	 replaced	 by	 pentadienyl	 in	
these	systems.	The	compound	(ƞ5‐C5H7)VCO	is	substituted	inert	
and	substitution	takes	place	by	a	dissociative	pathway.		

This	 is	 the	 first	 report	 of	 a	 kinetic	 study	 of	 a	 17‐electron	
organometallic	compound	that	is	slow	to	react	and	that	reacts	
by	 a	 dissociative	 process.	 The	 absence	 of	 a	 facile	 associative	
substitution	 pathway	 in	 the	 pentadienyl	 compounds	 may	 be	
attributed	 to	 the	more	 rigid	 (ƞ5‐C5H7)2V	moiety,	 decreased	 V‐
CO	π‐backbonding,	or	reduced	electron	spin	delocalization,	 all	
of	which	make	the	meral	center	less	susceptible	to	nucleophilic	
attack.		
	
4.	Previously	reported	substituted	complexes	of	G‐6	metal	
carbonyls	
	
4.1.	Metal	carbonyl	complexes	with	monodentate	ligands	
	

The	 photochemical	 reactions	 of	 W(CO)6	 with	 alkynes	
HC≡CH,	 HC≡CMe	 and	 HC≡CCMe3	 give	 the	 unstable	 terminal	
alkyne	complexes	[W(CO)5(η2‐HC≡CR)],	[W(CO)4(η2‐HC≡CR)2],	
[W(CO)(η2‐HC≡CR)3],	 and	 vinylidene	 derivatives	
[W(CO)5(C=CHR)].		

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The	 studies	 show	 that	 alkynes	 behave	 as	 monodentate	
ligand	 [149].	 The	 metal	 carbonyl	 complexes	 with	 various	
monodentate	ligands	are	shown	below.		

	
4.1.1.	Monosubstituted	complexes	[M(CO)5L]	
	

A	 series	 of	 carbonylchromium,	molybdenum	and	 tungsten	
complexes	 containing	 ferrocenylpyrazole	 ligands,	 (M(CO)5L)	
(M=Cr,	Mo	or	W;	L	represents	ferrocenyl	pyrazole),	have	been	
prepared	by	the	photochemical	reactions	of	ferrocenylpyrazole	
ligands	with	M(CO)6	 [150].	 Tetraazaadamantane	 (taad)	 reacts	
with	 Group‐VI	 metal	 hexacarbonyls	 to	 give	 homonuclear	
(taad)M(CO)5	(M=	Cr,	Mo	and	W)	derivatives	[151].	[M(CO)4L]	
(M	=	Cr,	 Mo	 or	 W;	 L	=	1,5‐Diselena‐5‐ferrocenophane)	 have	
been	prepared.	The	structures	of	[M(CO)4L]	(M	=	Cr,	Mo)	show	
that	L	adopts	the	unusual	meso‐2	conformation	[152].		

Thermal	reactions	of	the	metal	hexacarbonyls	M(CO)6	(M	=	
Cr,	Mo	but	not	W)	with	tris(dimethylamino)arsine	(Tdas)	give	
monosubstituted	 derivatives	 TdasM(CO)5	 (M	 =	 Cr,	Mo)	 [153].	
[M(CO)5(DTTT)]	[M=Cr;	Mo;	W],	have	been	synthesized	by	the	
photochemical	reaction	of	metal	carbonyls	[M(CO)6]	(M=Cr,	Mo	
and	 W),	 with	 3,5‐dimethyl‐tetrahydro‐2H‐1,3,5‐thiadiazine‐2‐
thione	 (DTTT).	 The	 spectroscopic	 studies	 show	 that	 DTTT	
behaves	 as	 a	 monodentate	 ligand	 coordinating	 via	 the	 sulfur	
(C=S)	donor	atom	[154].	M(CO)5(2‐aminoazole),	where	M	=	Cr	
or	 W,	 showed	 preferred	 endocyclic	 imine	 coordination	 when	
provided	 with	 endocyclic	 soft	 thioether	 or	 endocyclic	 hard	
amine,	 endocyclic	 borderline	 imine	 and	 exocyclic	 hard	 amine	
coordination	sites	[155].		

The	synthesis	and	spectroscopic	data	of	six	Group	6	metal	
complexes	 of	 type	 of	 M(P{R}{TMS}2)(CO)5,	 M	=	Cr,	 Mo,	 W;	
R	=	C6H5	 and	 M	=	Cr;	 R	=	H	 with	 the	 silylphosphine	 ligands,	
P(C6H5){Si(CH3)3}2	 and	 P(H){Si(CH3)3}2	 are	 reported	 [156].	
Photolysis	of	hexacarbonylchromium(0)	in	the	presence	of	2,6‐
diaminopyridine	 in	 toluene	 solution	 at	 10	 °C	 yields	
pentacarbonyl(2,6‐diaminopyridine)chromium(0),	which	could	
be	 isolated	 from	 solution	 as	 plate‐like	 crystals	 (Scheme	 34).	
The	 2,6‐diaminopyridine	 ligand	 is	 bonded	 to	 the	 chromium	
atom	through	one	of	the	NH2	groups	[157].		
	

	
	

Scheme	34	
	

Fully	 alkyl	 substituted	 pyranylidene	 carbene	 complexes	
from	M(CO)6	(M	=	Cr,	Mo,	W)	and	the	readily	available	1‐lithio‐
1,3‐butadiene,	 bearing	 a	 leaving	 group,	 via	 an	 intramolecular	
trapping	of	metallaacylate	 intermediates.	These	carbenes	have	
been	 demonstrated	 to	 have	 interesting	 and	 unique	 reactivity	
(Scheme	35)	[158].		

	

	
	

Scheme	35	
	

4.1.2.	Bisubstituted	complexes	[M(CO)4L2]	
	

Tungsten(0)	 carbonyl	 complexes	 with	 1‐(diphenyl‐
phosphanyl)‐1′‐vinylferrocene	 (Scheme	 36	 (A))	 and	 1‐
(diphenylphosphanyl)‐1′‐(dimethylvinylsilyl)ferrocene	
(Scheme	 36	 (B))	 as	 P‐monodentate	 donors,	 [W(CO)4(L‐κP)2],	
have	been	prepared	[159].		
	

	
	

Scheme	36	
	 	

A	 series	 of	 germylene	 and	 stannylene	 (Me2NCH2CH2O)2E	
(E	=	Ge,	 E	=	Sn)	 complexes	 of	 group	 6	 metals	 carbonyls	
L·M(CO)n	(M	=	Cr,	Mo,	W,	n	=	5	or	4	)were	prepared	[160].	The	
pentacarbonyl(η2‐bis(trimethylsilyl)ethyne)tungsten(0),	
W(CO)5(η2‐btmse),	reacts	with	tricyclohexylphosphine,	PCy3,	to	
yield	W(CO)5(PCy3)	and	W(CO)4(PCy3)2.	The	former	complex	is	
the	alkyne	substitution	product,	while	the	latter	one	is	formed	
from	the	conversion	of	its	labile	cis‐isomer,	which	is	generated	
by	 further	 reaction	 of	 the	 CO	 substitution	 product,	 cis‐
W(CO)4(η2‐btmse)(PCy3),	with	a	second	PCy3	molecule	(Scheme	
37)	[161].		
	

	
	

Scheme	37	
	

In	 trans‐	 [Mo(CO)4(PPh3)2],	 little	 deviation	 from	 ideal	
octahedral	 geometry	 is	 seen	and	 the	Mo‐‐‐P	bond	 length	of	2.	
500(1)	 Å	 is	 significantly	 shorter	 than	 those	 found	 in	 the	 cis	
isomer	[162].	Reaction	of	norbornadiene	complex	C7H8Cr(CO)4	
with	 tris(dimethylamino)arsine	 (Tdas)	 at	 room	 temperature	
gives	 trans‐(Tdas)2Cr(CO)4	 which	 was	 isolated	 as	 yellow	
complex.	 Reactions	 of	 disubstituted	 derivatives	 trans‐
(Tdas)2Cr(CO)4	with	carbonmonoxide	at	room	temperature	and	
pressure	 gives	 monosubstituted	 derivatives	 (Tdas)Cr(CO)5	
[63].		
	
4.1.3.	Trisubstituted	complexes	[M(CO)3L3]	
	

The	 trisubstituted	 metalcarbonyl	 complexes	 containing	
ligands	 which	 coordinate	 through	 bicovalent	 sulfur	 are	
reported.	 Direct	 reaction	 of	 the	 ligands	 with	
cycloheptatrienemolybdenumtricarbonyl,	 Mo(CO)3L3	 were	
prepared	 where	 L	 representing	 dimethyl	 sulfide	 [(CH3)2S],	
diethyl	 sulfide	 [(C2H5)2S],	 thiourea	 [(CH2)2CS],	 thioacetamide	
[CH2CSNH2].	 The	 thiourea	 and	 thioacetamide	 compounds	 are	
stable	for	periods	of	atleast	a	year	in	vacuum	and	for	a	week	or	
move	 in	 the	 air	 under	 normal	 laboratory	 condictions.	 The	
diethylcomplexes	are	stable	for	at	least	a	few	weeks	in	vacuum	
but	decompose	within	a	few	hours	in	air.	The	dimethyl	sulfide	
compounds	is	immediately	decomposed	by	air	and	is	stable	for	
few	hours	under	vacuum	[126].		
	
4.2.	Metal	carbonyl	complexes	with	bidentate	ligands	
	

The	metal	 carbonyl	 complexes	with	 bidentate	 ligands	 are	
variety	 of	 homo‐	 and	 bimetallic	 metal	 carbonyl	 compounds	
(Scheme38).	 Amandaet	 al.	 investigated	 metal	 carbonyl	

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complexes	 containing	 1,1‘‐bis(diphenylphosphino)ferrocene	
(dppf)	 with	 the	 general	 formulas	 M(CO)5(dppf)	 (1a−c),	
M(CO)4(dppf)	(2a−c),	and	(CO)5M(dppf)M‘(CO)5	(3a−f)	(M,	M‘	=	
Cr,	Mo,	W)	[163].		
	

	
	

Scheme	38	
	
4.2.1.	Mononuclear	complexes	[M(CO)4L]	
	

Interaction	 of	 1‐(2‐pyridylazo)‐2‐naphthol	 (PAN)	 with	
[Mo(CO)6]	 in	 air	 resulted	 in	 formation	 of	 the	 tricarbonyl	 oxo‐
complex	 [Mo(O)(CO)3(PAN)]	 (Scheme	 39(A)).	 In	 presence	 of	
triphenyl	 phosphine	 (PPh3),	 the	 reaction	 of	 PAN	 with	 either	
Mo(CO)6	 or	 Ru3(CO)12	 gave	 [Mo(CO)3(PAN)(PPh3)]	 (Scheme	
39(B)),	 and	 [Ru(CO)2(PAN)(PPh3)].	 Taher	 et	 al.	 reported	 that	
PAN	behaves	as	a	bidentate	 ligand	coordinating	via	 the	two	N	
donor	atom	[164].		

	

	
	

Scheme	39	
	

The	ability	of	the	dithiafulvenylphosphine	(P‐DTF)	to	react	
as	 a	 monodentate	 (P)	 or	 a	 bidentate	 (P,S)	 ligand	 with	 metal	
carbonylcomplexes	 such	 as	 Mo(CO)6	 and	 MnBr(CO)5	 was	
investigated	(Scheme	40)	[165].		

The	 reaction	 of	 bis(3,5‐dimethyl‐1,2,4‐triazol‐1‐
yl)methane,	CH2(3,5‐Me2Tz)2,	with	M(CO)6	(M=Cr,	Mo	or	W)	in	
refluxing	 DME	 produces	 CH2(3,5‐Me2Tz)2M(CO)4	 in	 moderate	
yield	[166].		

Metal	 carbonyl	 complexes	 MnBr(CO)3(3,3′‐R‐2,2′‐
biquinoline)	 containing	 heterocyclic	 nitrogen	 ligands	 (3,3′‐R‐
2,2′‐biquinoline)	(R=CH3	and	CH3;	H	and	H;	‐‐‐CH2‐‐‐CH2‐‐‐	or	‐‐‐
CH2‐‐‐CH2‐‐‐CH2‐‐‐)	 were	 synthesized.	 The	 ligand	 (3,3′‐R‐2,2′‐
biquinoline)	behave	as	bidentate	ligand	coordinating	via	two	N	
donor	atoms	(Scheme	41)	[167].		

	

	
	

Scheme	40	

	
	

Scheme	41	
	

The	synthesis	 of	metal	 carbonyl	 complexes	 containing	 the	
heterocyclic	 nitrogen	 ligand	 4′‐phenyl‐2,2′:6′,2″‐terpyridine	
are	reported.	Complexes	of	the	[M(CO)4(L)]	type	(M=	Mo(0),	or	
W(0);	 and	 L=4′‐phenyl‐2,2′:6′,2″‐terpyridine)	 were	 prepared	
by	 photosubstitution	 or	 by	 thermolytic	 reactions	 [168].	 Re(I)	
carbonyl	 complex	 fac‐	 [Re(CO)3(dpop′)Cl]	 with	 the	 nominally	
bi‐dentate	ligand	dipyrido(2,3‐a:3′,2′‐j)phenazine	(dpop′)	were	
prepared	(Scheme42)	[169].		
	

	
	

Scheme	42	
	

Tungsten(0)‐carbonyl	complexes	of	naphthylazoimidazoles	
[W(CO)5(α‐NaiR‐N)]	 are	 reported.	 The	 complexes	 have	 been	
transformed	 to	 [W(CO)4(α‐NaiR‐N,N′)]	at	higher	 temperatures	
where	 α‐NaiR‐N	 dentes	 a	 monodentate	 imidazole‐N	 donor	
ligand	and	α‐NaiR‐N,N′	denotes	a	bidentate	imidazole‐N,	azo‐N	
donor	 ligand	 [170].	 The	 chiral	 diimine	 ligand	 C5H4N‐2‐
C(C6H5)=N{(1S,2S)‐C5H8‐2‐OCH2C6H5)}	 was	 prepared	 via	 the	
reaction	 of	 2‐benzoyl	 pyridine	 and	 (1S,2S)‐(+)‐2‐
benzyloxycyclopentylamine.	 The	 ligand	 reacts	with	M(CO)6	 in	
toluene	 to	 give	 the	 respective	 chiral	 metal(0)	 complexes	
[M(CO)4(L‐κN,N′)]	(M	=	Mo	and	W)	[171].		

N,N′‐Bis(ferrocenylmethylene)ethylenediamine	 (bfeda)	
yields	M(CO)4(bfeda)	 complexes	 (M	=	Cr,	Mo,	W)	which	 could	
be	isolated.	In	the	case	of	tungsten,	W(CO)5(bfeda)	is	formed	as	
intermediate	 and	 then	 undergoes	 the	 ring	 closure	 reaction	
yielding	 the	 ultimate	 product	 W(CO)4(bfeda)	 [172].	 Upon	
treatment	 of	 functionalized	 ligands	 Ph3SnCH(SPh)(3,5‐Me2Pz)	
and	 Ph3SnCH(SPy)(3,5‐Me2Pz)	 (Pz	=	pyrazol‐1‐yl	 and	 Py	=	2‐
pyridyl)	 with	 group	 6	 metal	 carbonyl	 complexes,	 markedly	
different	reactions	are	observed.	Reaction	of	1‐arylthiomethyl‐
3,5‐dimethylpyrazole	 [ArSCH2(3,5‐Me2Pz),	 Ar	=	phenyl	 or	 2‐
pyridyl,	 Pz	=	pyrazol‐1‐yl]	 with	 Mo(CO)6	 produces	 complexes	
ArSCH2(3,5‐Me2Pz)Mo(CO)4,	 while	 similar	 reaction	 with	
W(CO)6	yields	analogous	complexes	ArSCH2(3,5‐Me2Pz)W(CO)4	
(Scheme	43)	[173].		
	

	
	

Scheme	43	
	

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The	 Mo(CO)4DAB	 where	 DAB=	 diacetyldihydrazone,	
glyoxalcyclohexylamine,	 diazabutadiene	were	prepared	where	
DAB	 behave	 as	 bidentate	 ligand	 [174].	 Reaction	 of	
bis(diisopropylamino)carbene	with	ether	substituted	Mo‐,	and	
W‐carbonyls	leads	to	η2‐bonded	carbene	complexes	which	can	
be	converted	in	contrast	to	the	analogous	chromium	complex	in	
a	CO‐atmosphere	to	imine	complexes	(Scheme	44)	[175].		
	

	
	

Scheme	44	
	

The	 complexes	 [M(CO)4(ƞ2‐H2L)]	 [M	 =	 Cr;	 Mo;	 W]	 have	
been	 synthesized	 by	 photochemical	 reactions	 of	 VIB	 metal	
carbonyls	[M(CO)6]	[M	=	Cr;	Mo;	W]	with	N,N’‐bis(salicylidene)‐
1,2‐bis‐(o‐aminophenoxy)ethane	 (H2L)	 in	THF.	The	H2L	 ligand	
was	found	coordinated	to	the	central	metal	as	bidentate	ligand	
via	 the	 central	 azomethine	 nitrogen	 atoms	 [176].	 The	
interaction	of	 the	group	VI	metal	hexacarbonyls	with	electron	
donors	 has	 been	 widely	 explored.	 These	 reaction	 generally	
require	 thermal	 activation,	 although	 in	 some	 cases	 photolytic	
activation	was	 observed.	 Photolytic	 preparation	 of	 some	 new	
anionic	 derivatives	 of	 chromium,	 molybdenum	 and	 tungsten	
hexacarbonyls	 involving	 both	 the	 difluorodithiophosphate	
anion	 and	 difluorothiosulphate	 anion	 as	 ligands.	 The	 reaction	
between	 metal	 hexacarbonyl	 and	 complexing	 anion	 in	
methylene	 chloride	 solution	 results	 the	 formation	 of	
M(CO)4PF2S2‐	complexes.	These	complexes	were	found	to	be	air	
stable	 in	 solid	 state,	 although	 solutions	 are	 oxidized	 by	
atmospheric	oxygen.	The	stability	increases	from	chromium	to	
tungsten	derivatives	[177].		
	
4.2.2.	Binuclear	complexes	[M2(CO)10L]	
	

Pentacarbonylpyrazinetungsten(0),	 (CO)5W(pyz),	 is	 not	
stable	 in	 solution	 in	 polar	 solvents	 such	 as	 acetone	 or	
dichloromethane	 and	 undergoes	 conversion	 to	 a	 bimetallic	
complex,	 (CO)5W(pyz)W(CO)5[178].	 Density	 functional	 theory	
has	 been	 used	 to	 explore	 possible	 homoleptic	 binuclear	 Cr	
carbonyls	 Cr2(CO)n	 (n	=	11,	 10,	 9,	 and	 8)	 using	 the	 pure	 DFT	
method	BP86	and	the	hybrid	Hartree‐Fock	DFT	method	B3LYP.	
The	binuclear	Cr2(CO)11	 is	computed	to	be	thermodynamically	
unstable	 with	 respect	 to	 dissociation	 into	 mononuclear	
fragments.	 The	 following	 binuclear	 Cr	 carbonyls	 were	 found	
with	 metal‐metal	 multiple	 bonding	 ‐	 Cr2(CO)10,	 Cr2(CO)9,	 and	
Cr2(CO)8	[179].		

IR	 spectroelectrochemical	 studies	 of	 the	 metal	 carbonyl	
dimers	 Mn2(CO)10	 and	 [CpM(CO)3]2	 (M	 =	 W,	 Mo).	 Cross‐
coupling	and	ligand	substitution	reactions	of	electrochemically	
generated	organometallic	radicals	were	reported	[180].		

The	 transition	 metal	 carbonyl	 complexes	 of	 1,3‐bis(di‐R‐
stibino)propanes	 (R	=	Me	 or	 Ph)	 (distibinopropanes)	with	 the	
carbonyls	 of	 Cr,	Mo,	W,	 Fe,	 Mn	 and	 Ni	were	 prepared.	 These	
include	 cis‐[M(CO)4{R2Sb(CH2)3SbR2}]	 (M	=	Cr,	 Mo	 or	 W),	
[{Fe(CO)4}2{μ‐R2Sb(CH2)3SbR2}],	 [{Ni(CO)3}2{μ‐R2Sb(CH2)3	
SbR2}]	 (Scheme	 45),	 [Co2(CO)6{Ph2Sb(CH2)3SbPh2}],	 [Co2(CO)4	
{Me2Sb(CH2)3SbMe2}3]	 [Co(CO)4]2	 and	 [Mn2(CO)8{Ph2Sb(CH2)3	
SbPh2}].	The	studies	show	that	distibinopropanes	are	bidentate	
ligands	[181].		

Sunlight	irradiation	of	the	reactions	of	[M(CO)6],	M=Cr,	Mo	
and	 W	 with	 dimethylglyoxime	 (H2dmg)	 in	 THF	 were	
investigated.	The	reaction	of	[Cr(CO)6]	with	H2dmg	resulted	in	
the	formation	of	the	square	pyramidal	complex	[CrO(H2dmg)2].	
The	H2dmg	coordinated	the	metal	from	the	two	nitrogen	atoms.	
The	 corresponding	 reactions	 of	 molybdenum	 and	 tungsten	
carbonyls	 yielded	 the	 binuclear	 oxo	 complex	 [(H2dmg)O2M(μ‐

O)2MO2(H2dmg)]	 [182].	 The	 structure,	 bonding,	 and	
photochemistry	 of	 the	 Group	 VI	 cyclopentadienyl	 metal	
carbonyl	 compounds	 of	 the	 general	 form	 (η5‐C5H5)2M2(CO)6,	
where	M=Cr,	Mo,	 and	W,	 and	 their	 ring‐substituted	 and	 ring‐
coupled	derivatives	is	critically	reviewed	[183].		
	

	
	

Scheme	45	
	

Binuclear	metal	carbonyl	complexes	have	been	reported	via	
cis‐bridging	bidentate	coordination	of	 the	 ligand	between	 two	
metals	 in	 {M2(CO)4	 [μ‐Ph2P(S)P(S)Ph2]}(M	 =	 Cr,	 Mo	 and	 W)	
[184,185].	Lithiation	of	ligand	CpPhCHCHPhCp	and	addition	of	
(MeCN)3M(CO)3	 (M	=	Mo,	 W)	 followed	 by	 oxidation	 with	 2	
equiv.	 of	 ferrocenium	 tetrafluoroborate,	 afford	 (PhCHCHPh)‐
coupled	 bis(cyclopentadienyl)	 dinuclear	 molybdenum	 and	
tungsten	complexes	[186].		
	
4.3.	Metal	carbonyl	complexes	with	tridentate	ligands	
	

The	bicyclic	triene	5,6‐dimethylenebicyclo	[2.2.1]heptene‐2	
has	 been	 found	 to	 react	 with	 chromium,	 molybdenum	 and	
tungsten	 hexacarbonyls	 to	 produce	 volatile	 red‐to‐orange	
crystalline	 solids	 of	 composition	 of	 C9H10M(CO)3	 [187].	
Reactions	 of	 M(CO)6,	 where	 M	=	Cr	 and	 Mo	 with	 Schiff	 bases	
prepared	 by	 the	 condensation	 of	 ethanolamine	 with	 either	
acetylacetone	 or	 benzoylacetone	 were	 investigated.	 The	
reactions	of	Cr(CO)6	in	benzene	resulted	in	the	formation	of	the	
tricarbonyl	 drivatives	 [Cr(CO)3(HL)],	 HL	=	acaceaH	 or	 baceaH.	
The	HL	proved	 to	 act	 as	 a	 tridentate	 ligand	 [188].	 Salts	of	 (2‐
(dimethylamino)ethyl)cyclopentadienyl	 (CpN)	 group	 VI	 metal	
carbonyl	 anions	 (	 [M(CO)3(η5‐CpN)]‐	 (M	 =	 Cr,	 Mo,	 W))	 were	
reported	(Scheme	46)	[189].		

The	modification	 of	bis(pyrazol‐1‐yl)methane	 by	 sulfur	 or	
selenium	on	the	methine	carbon	has	been	successfully	carried	
out	 by	 the	 reaction	 of	 the	 bis(pyrazol‐1‐yl)methide	 anion,	
prepared	 in	 situ	 by	 the	 reaction	 of	 bis(pyrazol‐1‐yl)methane	
with	 n‐BuLi,	 with	 elemental	 sulfur	 or	 selenium.	 These	
bis(pyrazol‐1‐yl)methylthiolate(Scheme	 47)	 or	 selenolate	
anions	reacted	with	M(CO)6	(M	=	Mo	or	W)	in	THF	and	formed	
their	tricarbonyl	metal	anionsCH(Pz)2EM(CO)3‐	[190].		
	

	
	

Scheme	46	
	
	
	

	
	

Scheme	47	

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The	potential	use	of	haptotropic	shifts	to	control	rotational	
barriers	 has	 been	 initiated	 by	 the	 synthesis	 of	 (9‐indenyl)	
triptycene	 and	 its	 chromium	 tricarbonyl	 derivative.	 In	 the	
metal	 complex,	 the	 chromium	carbonyl	moiety	 coordinates	 to	
one	of	the	blades	of	the	triptycene	unit	rather	than	to	the	more	
crowded	indenyl	group	(Scheme48)	[191].		
	

	
	

Scheme	48	
	

Organotransition‐Metal	 Complexes	 of	 tridentate	 ligands	
were	 synthesized	 by	 Shiu	 and	 Chou	 and	 their	 structure	 and	
reactivity	 of	 first	 intramolecular	 coordination	 compound	with	
weakely	 bound	 ƞ2‐arene	 have	 been	 done.	 They	 found	 that	
thermolysis	 of	 (PhHCPz’2)Mo(CO)4	 in	 DME	 gives	
(PhHCPz’2)Mo(CO)3	(Pz’	=	3,5‐dimethylpyrazol‐1‐yl)	[192].		
	
5.	Some	cluster	complexes	of	G‐6	metal	carbonyl	
complexes	
	

The	phrase	cluster	was	coined	by	F.	A.	Cotton	 in	 the	early	
1960s	to	refer	to	compounds	containing	metal‐metal	bonds.	In	
1990	Mingos	and	Wales	explain	cluster	as	a	compound	contains	
a	 group	 of	 two	 or	 more	 metal	 atoms	 where	 direct	 and	
substantial	metal	metal	bonding	is	present	[193].	A	cluster	is	an	
ensemble	 of	 bound	 atoms	 intermediate	 in	 size	 between	 a	
molecule	 and	 a	 bulk	 solid.	 Transition	metals	 and	main	 group	
elements	form	strong	clusters	[194].	The	main	cluster	types	are	
"naked"	 clusters	 (without	 stabilizing	 ligands)	 and	 those	 with	
ligands.	For	transition	metal	clusters,	typical	stabilizing	ligands	
include	 carbon	 monoxide,	 halides,	 isocyanides,	 alkenes,	 and	
hydrides.	 For	 main	 group	 elements,	 typical	 clusters	 are	
stabilized	 by	 hydride	 ligands.	 Transition	 metal	 clusters	 are	
frequently	 composed	 of	 refractory	 metal	 atoms.	 In	 general	
metal	 centers	 with	 extended	 d‐orbitals	 form	 stable	 clusters	
because	 of	 favorable	 overlap	 of	 valence	 orbitals.	 Thus,	metals	
with	 a	 low	 oxidation	 state	 for	 the	 later	 metals	 and	 mid‐
oxidation	 states	 for	 the	 early	 metals	 tend	 to	 form	 stable	
clusters.	 Polynuclear	 metal	 carbonyls	 are	 generally	 found	 in	
late	 transition	 metals	 with	 low	 formal	 oxidation	 states.	
Braunstein	et	al.	gives	a	review	of	Metal	clusters	 in	chemistry	
[195].	

Types	of	clusters	formed	by	metal	carbonyls:	Generally	the	
clusters	 referred	 to	 have	 more	 than	 one	 metal	 atom.	 These	
metal	clusters	may	be	homonuclear	or	heteronuclear.		
	
5.1.	Homonuclear	clusters	
	

Homonuclear	 refers	 to	 a	 cluster	 with	 only	 one	 metal	
element.	 Dinuclear	 metal	 (I)	 carbonyl	 thiolates	
[W2(SPh)2(CO)8],	 [Mo2(SPh)3(CO)6]−,	 [Mo2(SPh)2(CO)6	
(S2CNEt2)]−,	 [W2(SPh)2(CO)6DPPM]	 and	 a	 polymer	 [W2(SPh)2	
(CO)6DPPM]n	 were	 synthesized	 by	 the	 reaction	 of	
M2(SPh)2(CO)8]	(M=Mo,	W)	with	appropriate	NaSR	(SR=SC6H5,	
S2CNEt2)	 ligands	 in	 the	 presence	 of	 tetra‐alkylammonium	
halide,	 or	 with	 diphenylphosphinemethane	 (DPPM=(C6H5)2P	
CH2P	 (C6H5)2)	 in	 certain	 solvents	 [196].	 The	 32‐electron	
complex	[Mo2Cp2(μ‐COMe)(μ‐PCy2)(CO)2]	(Cp	=	η5‐C5H5)	reacts	
at	 333	K	 with	 terminal	 alkynes	 RC≡CR	 to	 give	 the	
corresponding	 derivatives	 [Mo2Cp2{μ‐κ2:η3‐CRCHC(OMe)}(μ‐

PCy2)(CO)2],	 (R	=	p‐tol,	 CO2Me),	with	 good	 yields	 (Scheme	49)	
[197].		
	

	
Scheme	49	

	
Di‐nuclear	 molybdenum	 and	 tungsten	 metal	 cluster	

complexes	 [Et4N]2	 [Mo2(SC6H11)2(CO)8],	 [Et4N]	
[Mo2(SC6H11)3(CO)6],	 [Ph4P]	 [Mo2(SC6H11)3(CO)6],	
[(CH3)3PhCH2N]	 [Mo2(SC6H11)3(CO)6],	 [Et4N]2	
[W2(SC6H11)2(CO)8],	 [W2(SC6H11)2(CO)8]	 and	 [Et4N]	
[W2(SC6H11)3(CO)6]	 had	 been	 synthesized	 with	
cyclohexanthiolate	 ligand	 [198].	 Treatment	 of	
M(allyl)(Cl)(CO)2(py)2	 (M	=	Mo,	W)	with	1	equiv.	 of	potassium	
pyrazolates	 in	 tetrahydrofuran	 at	 −78	°C	 afforded	
M(allyl)(R2pz)(CO)2(py)n	 (R2pz	=	3,5‐disubstituted	 pyrazolate;	
n	=	1,	 2;	 R=	 t‐Bu)	 in	 68‐81%	 yields.	 Analogous	 treatment	 of	
Mo(allyl)(Cl)(CO)2(NCCH3)2	 with	 1	equiv.	 of	 potassium	
pyrazolates	(R2pz	where	R=	t‐Bu)	in	tetrahydrofuran	at	−78	°C	
afforded	[Mo(allyl)(R2pz)(CO)2]2	in	79%	yield	(Scheme	50).	An	
X‐ray	 crystal	 structure	 analysis	 of	 [Mo(allyl)(R2pz)(CO)2]2	
showed	 a	 dimeric	 structure	 bridged	 by	 two	 μ‐η2:η1‐tBu2pz	
ligands	[199].		
	

	
	

Scheme	50	
	

Formation	of	 carbonylrhenium	cryptates	with	alkali	metal	
cations	 was	 studied.	 Coordination	 chemistry	 studies	 of	
[Ph2P(E)NP(E)Ph2]−,	 E	=	O,	 S,	 Se	 towards	 ReBr(CO)5	were	 key	
target	 of	 the	 study	 in	 last	 decade.	 The	 reaction	 of	 ReBr(CO)5	
with	 Li[Ph2P(O)NP(O)Ph2]	 afforded	 the	 cryptate	 Li	
[Re2(CO)6{μ‐Ph2P(O)NP(O)Ph2‐κ2O,O′}3];	 whereas	 K	
[Ph2P(O)NP(O)Ph2]	 reacted	 with	 ReBr(CO)5	 to	 give	 K	
[Re2(CO)6{μ‐Ph2P(O)NP(O)Ph2‐κ2O,O′}{Ph2P(O)NP(O)Ph2‐
κ2O,O′}2].	Other	 chalcogen	 ligands’	 salts	M	 [Ph2P(E)NP(E)Ph2],	
E	=	Se	 and	 S,	 M	=	K	 and	 Li	 gave	 dirhenium	 carbonyls	 with	
bromido	 and	 Ph2P(E)NP(E)Ph2,	 E	=	Se	 or	 S	 bridges	 upon	
reaction	with	ReBr(CO)5	[200].		
	
5.2.	Heteronuclear	clusters	
	

Heteronuclear	refers	to	a	cluster	with	at	least	two	different	
metal	 elements.	 These	 complexes	 are	 also	 known	 as	 mixed	
metal	 complexes.	 The	 oxygen	 transfer	 reaction	 of	 the	
heterometallic	 carbonyl	 cluster	 (μ3‐S)FeCo2(CO)9	 with	 bis(p‐
methoxyphenyl)telluroxide	 (BMPTO)	 gives	 bis(p‐
methoxyphenyl)telluride	 (BMPT)	 coordinated	 metal	 cluster	
products	 (μ3‐S)FeCo2(CO)8(BMPT)	 and	 (μ3‐S)	
FeCo2(CO)7(BMPT)2	 ,	 which	 react	 further	with	 PPh3	 to	 afford	
Ph3P‐substituted	 derivatives	 (μ3‐S)FeCo2(CO)8(PPh3)	 and	 (μ3‐
S)FeCo2(CO)7(PPh3)2,	 respectively	 [201].	 Mixed‐metal	
tetrahedral	 clusters	 [(μ3‐E)RuCoMo(CO)7(CpC(O)CH3)]((‐)‐
DIOP)	 (E=Se,	 S)	 were	 obtained	 by	 the	 reactions	 of	 [(μ3‐

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E)RuCo2(CO)9]	(E=Se,	S)	and	[(μ3‐E)RuCoMo(CO)8(CpC(O)CH3)]	
(E=Se,	 S)	 with	 the	 diphosphine	 compound	 (‐)‐DIOP	 [(2,2‐
dimethyl‐1,3‐dioxolane‐4,5‐diyl	 bis‐methylene)bisdiphenyl‐
phosphine]	 in	 n‐hexane	 or	 THF	 at	 room	 temperature,	
respectively	(Scheme	51)	[202].		
	

	
	

Scheme	51	
	

The	 tellurium‐containing	 trigonal	 bipyramidal	 clusters	
[Te2CrM2(CO)10]2‐	 (M	=	Cr;	Mo;	W)	has	been	synthesized	 from	
the	 ring	 closure	 reactions	 of	 the	 ring	 complexes	
[Te2Cr2M2(CO)18]2‐	(M	=	Cr;	Mo;	W)	(Scheme	52)	[203].		

	

	
	

Scheme	52	
	

Selenium‐capped	 trimolybdenum	 and	 tritungsten	 ring	
carbonyl	 clusters	 [Se2M3(CO)10]2−	 (M	=	Mo,	 W)have	 been	
synthesized	(Figure	8).	These	selenium‐capped	trimolybdenum	
cluster	 compound	 [Et4N]2	 [Se2Mo3(CO)10]	 (	 [Et4N]2)	 can	 be	
obtained	 from	 the	 reaction	 of	 the	 trichromium	 cluster	
compound	 [Et4N]2	 [Se2Cr3(CO)10]	 with	 4	 equiv.	 of	Mo(CO)6	 in	
refluxing	 acetone.	 On	 the	 other	 hand,	 when	 [Et4N]2	
[Se2Cr3(CO)10]	 reacted	 with	 4	 equiv.	 of	 W(CO)6	 in	 refluxing	
acetone,	 the	 planar	 cluster	 compound	 [Et4N]2	 [Se2W4(CO)18]	 (	
[Et4N]2)	 was	 isolated,	 which	 could	 further	 transform	 to	 the	
tritungsten	 cluster	 compound	 [Et4N]2	 [Se2W3(CO)10]	 (	 [Et4N]2)	
in	good	yield.	Alternatively,	these	clusterscould	be	formed	from	
the	 reactions	 of	 the	 monosubstituted	 products	 [Et4N]2	
[Se2Cr2M(CO)10]	(M	=	Mo;	W,	[Et4N]2)	with	3	equiv.	of	M(CO)6	in	
acetone,	respectively	[204].		

2,2′‐Bipyrimidine	 metal	 multinuclear	 complexes	
[((CO)4Mo)(μ‐1,2,3,4‐bipym)Pt(C≡CFc)2]	 with	 Mo(CO)6	 have	
been	 synthesized	 originating	 from	 cis‐	 [(bipym)Pt(C≡CR)2]	
(R	=	SiMe3)	in	presence	of	tetrahydrofuran	after	irradiation	for	
16	h	 at	 25	°C	 [186].	 Thermolysis	 of	 a	 toluene	 solution	
containing	 [Fe2W(CO)10(μ3‐S)2]	 (1)	 and	 [(η5‐
C5H5)W(CO)3(CCPh)]	(2),	at	80°C	yields	the	mixed	metal	cluster	
[Fe2(CO)6(μ3‐S)2W{(η5‐C5H5)W(CO)3(CCPh)}2]	 (Scheme	 53)	
[205].		
	

	
	

Figure	8.	Selenium‐capped	trimolybdenum	ring	carbonyl	clusters	[204].		
	
	

	
	

Scheme	53	
	

Reaction	 between	 HO2CfcPPh2M(CO)5	 (M	=	Cr,	 Mo,	 W;	
fc	=	ferrocene‐1,1′‐diyl)	 and	 {	 [Ti](μ‐σ,π‐C≡CSiMe3)2}CuMe	 at	
−30	°C	 gave	 the	 alkyne/ferrocene‐bridged	 heterotetrametallic	
compounds	 {[Ti](μ‐σ,π‐C≡CSiMe3)2}CuO2CfcPPh2M(CO)5	
(M	=	Cr,	Mo,	W)	(Scheme	54)	[206].		
	

	
	

Scheme	54	
	

The	 60‐electron	 tetrahedral	 clusters	 W2Ir2(μ‐L)(CO)8(η5‐
C5H4Me)2	 [L=dppe,	 dppf	 ]	 (Scheme	 55)	 have	 been	 prepared	
from	 reaction	 between	 W2Ir2(CO)10(η5‐C5H4Me)2	 and	 the	
corresponding	diphosphine	in	52	and	66%	yields,	respectively	
[207].		

Mixed	 ligand	 derivatives	 of	 MoIr3(μ‐CO)3(CO)8(η‐C5R5)	
(R=H,	 Me)	 and	 Mo2Ir2(μ‐CO)3(CO)7(η‐C5H5)2	 with	
stoichiometric	 amounts	 of	 the	 isocyanide	 ButNC	 afford	 the	
ligand	substituted	cluster	in	fair	to	good	yields	(13‐58%)	[208].	
Tetrahedral	 mixed‐metal	 cluster	 WIr3(CO)11(η‐C5H4Me)	 is	
prepared	 in	 8%	 yield	 from	 reactions	 between	 [W(CO)3(η‐
C5H4Me)]−	and	IrCl(CO)2(p‐toluidine)	under	an	Ar	atmosphere,	
and	 in	 46%	yield	 from	 reaction	 between	WH(CO)3(η‐C5H4Me)	
and	 IrCl(CO)2(p‐toluidine)	 under	 a	 CO	 atmosphere.	
WIr3(CO)11(η‐C5H4Me)	 reacts	with	diphenylacetylene	 to	 afford	
three	 products,	 namely	 tetrahedral	 WIr3(μ3‐η2‐
PhC2Ph)2(CO)7(η‐C5H4Me),	 butterfly	 WIr3{μ‐η4‐
C(Ph)C(Ph)C(Ph)C(Ph)}(CO)5(η‐C5H4Me)	 and	 the	 binuclear	
product	 Ir2{μ‐η4‐C(Ph)C(Ph)C(Ph)C(Ph)}(CO)5	 [209].	 The	
nickel‐molybdenum	 complex	 [(η5‐C5Me5)Ni=Mo(CO)3(η5‐
C5H4Me)]	can	be	considered	to	contain	a	partially	dative	nickel‐
molybdenum	double	bond.	This	complex	reacts	with	the	bulky	
terminal	alkyne	HC≡CCPh2(OMe)	(DPMP)	to	afford	the	alkyne‐
carbonyl	 coupled	 metallacyclic	 product	

(Sche
me	56)	(R	=	CPh2(OMe),	Ni‐Mo)	regioselectively	and	exclusively	
[210].		
	

	
	

Scheme	55	

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Scheme	56	
	

Branched	 dendritic	 metallo	 carbosiloxanes	 of	 type	
Si(OCH2CH2CH2SiMe2	[OCH2PPh2M(CO)n])4	(n	=	3:	M	=	Ni;	n	=	4:	
M	=	Fe;	 n	=	5:	 M	=	Mo,	 W)	 have	 also	 been	 prepared	 (Scheme	
57).	 These	 molecules	 possess	 phosphorus‐metal	 carbonyl	
groups	as	end‐grafted	units	[211].		
	

	
	

Scheme	57	
	
5.3.	Predicting	structures	of	cluster	compounds	
	

The	 polyhedral	 skeletal	 electron	 pair	 theory	 or	 Wade's	
electron	 counting	 rules	 predict	 trends	 in	 the	 stability	 and	
structures	of	many	metal	clusters.	The	electron	counting	rules	
were	originally	formulated	by	Kenneth	Wade	and	were	further	
developed	by	D.	M.	P.	Mingos	and	others;	 they	are	 sometimes	
known	 as	Wade's	 rules	 or	 the	Wade/Mingos	 rules.	 The	 rules	
are	 based	 on	 a	 molecular	 orbital	 treatment	 of	 the	 bonding	
[212‐215].	 Recently	 these	 rules	 are	 extended	 and	 unified	 for	
macropolyhedral	 boranes,	 metallocenes	 and	 borides.	 The	
unified	 electron	 counting	 rules	 are	 called	 Jemmis	mno	 rules	
[216‐217].		

Different	rules	(4n,	5n	or	6n)	are	invoked	depending	on	the	
number	 of	 electrons	 per	 vertex.	 The	 4n	 rules	 are	 reasonably	
accurate	in	predicting	the	structures	of	clusters	having	about	4	
electrons	 per	 vertex,	 as	 is	 the	 case	 for	 many	 boranes	 and	
carboranes.	 For	 such	 clusters,	 the	 structures	 are	 based	
ondeltahedra,	 which	 are	 polyhedrain	 which	 every	 face	 is	
triangular.	 The	 4n	 clusters	 are	 classified	 as	 closo‐,	 nido‐,	
arachno‐	 or	 hypho‐,	 based	 on	 whether	 they	 represent	 a	
complete	(closo‐)deltahedron,	or	a	deltahedron	that	 is	missing	
one	(nido‐),	two	(arachno‐)	or	three	(hypho)	vertices.		

However,	 hypho	 clusters	 are	 relatively	 uncommon	 due	 to	
the	 fact	 that	 the	 electron	 count	 is	 high	 enough	 to	 start	 to	 fill	
antibonding	 orbitals	 and	 destabilize	 the	 4n	 structure.	 If	 the	
electron	count	 is	close	 to	5	electrons	per	vertex,	 the	structure	
often	changes	to	one	governed	by	the	5n	rules,	which	are	based	
on	 3‐connected	 polyhedra.	 As	 the	 electron	 count	 increases	
further,	 the	 structures	 of	 clusters	 with	 5n	 electron	 counts	
become	unstable,	so	 the	6n	rules	can	be	 implemented.	The	6n	
clusters	 have	 structures	 that	 are	 based	 on	 rings.	 A	molecular	
orbital	 treatment	 can	 be	 used	 to	 rationalize	 the	 bonding	 of	
cluster	compounds	of	the	4n,	5n,	and	6n	types.		
	

5.3.1.	4n	Rules	
	

The	following	polyhedraare	the	basis	for	the	4n	rules;	each	
of	these	have	triangular	faces	[218].	The	number	of	vertices	in	
the	cluster	determines	what	polyhedron	the	structure	is	based	
on.	 Prediction	 of	 polyhedron	 structure	 based	 on	 number	 of	
vertices	in	cluster	id	given	in	Table	7.		
	
Table	7.	Prediction	of	polyhedron	structure	based	on	number	of	vertices	in	
cluster.		
Number	of	vertices	 Polyhedron	
4 Tetrahedron
5 Trigonal	bipyramid	
6 Octahedron
7 Pentagonal	bipyramid	
8	 D2d	(trigonal)	dodecahedron	
9	 Tricapped	trigonal	prism	
10	 Bicapped	square	antiprism	
11	 Octadecahedron	
12 Icosahedron	(bicapped	pentagonal	antiprism)
	

Using	 the	 electron	 count,	 the	 predicted	 structure	 can	 be	
found.	n	 is	 the	number	of	vertices	 in	 the	cluster.	The	4n	rules	
are	enumerated	in	Table	8.		
	
Table	8.	Structure	prediction	using	electron	count.		
Electron	
count	

Name Predicted	structure	

4n‐2 Bicapped	
closo	

n‐2	vertex	closo	polyhedron	with	2	capped	faces

4n Capped	
closo	

n‐1	vertex	closo	polyhedron	with	1	face	capped

4n+2 Closo Closo	polyhedron	with	n	vertices	

4n+4	 Nido	 n+1	vertex	closo	polyhedron	with	1	missing	
vertex	

4n+6	 Arachno	 n+2	vertex	closo	polyhedron	with	2	missing	
vertices	

4n+8	 Hypho	 n+3	vertex	closo	polyhderon	with	3	missing	
vertices	

	
When	 counting	 electrons	 for	 each	 cluster,	 the	 number	 of	

valence	 electrons	 is	 enumerated.	 For	 each	 transition	 metal	
present,	 10	 electrons	 are	 subtracted	 from	 the	 total	 electron	
count.	For	example,	in	Rh6(CO)16	the	total	number	of	electrons	
would	be	6(9)	+	16(2)	‐	6(10)	=	86	‐	6(10)	=	26.	Therefore,	the	
cluster	is	a	closo	polyhedron	because	n	=	6,	with	4n	+	2	=	26.		
Other	rules	may	be	considered	when	predicting	the	structure	of	
clusters:	

1. For	 clusters	 consisting	 mostly	 of	 transition	 metals,	
any	 main	 group	 elements	 present	 are	 often	 best	
counted	as	ligands	or	interstitial	atoms,	rather	than	
vertices.		

2. Larger	 and	 more	 electropositive	 atoms	 tend	 to	
occupy	 vertices	 of	 high	 connectivity	 and	 smaller	
more	electronegative	atoms	tend	to	occupy	vertices	
of	low	connectivity.		

3. In	 the	 special	 case	 of	 boron	 hydride	 clusters,	 each	
boron	 connected	 to	 3	 or	 more	 vertices	 has	 one	
terminal	 hydride,	 while	 a	 boron	 connected	 to	 2	
other	 vertices	 has	 2	 terminal	 hydrogens.	 If	 more	
hydrogens	are	present,	they	are	placed	in	open	face	
positions	 to	 even	 out	 the	 coordination	 number	 of	
the	vertices.		

4. For	 the	 special	 case	 of	 transition	 metal	 clusters,	
ligands	 are	 added	 to	 the	metal	 centers	 to	 give	 the	
metals	reasonable	coordination	numbers,	and	if	any	
hydrogen	 atoms	 are	 present	 they	 are	 placed	 in	
bridging	 positions	 to	 even	 out	 the	 coordination	
numbers	of	the	vertices.		

In	 general,	 closo	 structures	 with	 n	 vertices	 are	 n‐vertex	
polyhedra.	To	predict	the	structure	of	a	nido	cluster,	 the	closo	
cluster	 with	 n	 +	 1	 vertices	 is	 used	 as	 a	 starting	 point;	 if	 the	
cluster	is	composed	of	small	atoms	a	high	connectivity	vertex	is	
removed,	while	if	the	cluster	is	composed	of	large	atoms	a	low	

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connectivity	vertex	 is	 removed.	To	predict	 the	structure	of	an	
arachno	 cluster,	 the	 closo	 polyhedron	 with	 n	 +	 2	 vertices	 is	
used	as	the	starting	point,	and	the	n+1	vertex	nido	complex	 is	
generated	by	following	the	rule	above;	a	second	vertex	adjacent	
to	the	first	is	removed	if	the	cluster	is	composed	of	mostly	small	
atoms,	 a	 second	 vertex	 not	 adjacent	 to	 the	 first	 is	 removed	 if	
the	cluster	is	composed	mostly	of	large	atoms.		

The	 bookkeeping	 for	 deltahedral	 clusters	 is	 sometimes	
carried	 out	 by	 counting	 skeletal	 electrons	 instead	 of	 the	 total	
number	 of	 electrons.	 The	 skeletal	 orbital	 (electron	 pair)	 and	
skeletal	 electron	 counts	 for	 the	 four	 types	 of	 deltahedral	
clusters	are:	

 n‐vertex	closo:	(n+1)	skeletal	orbitals,	(2n+2)	
skeletal	electrons	

 n‐vertex	nido:	(n+2)	skeletal	orbitals,	(2n+4)	
skeletal	electrons	

 n‐vertex	 arachno:	 (n+3)	 skeletal	 orbitals,	
(2n+6)	skeletal	electrons	

 n‐vertex	 hypho:	 (n+4)	 skeletal	 orbitals,	
(2n+8)	skeletal	electrons	

The	 skeletal	 electron	 counts	 are	 determined	 by	 summing	
the	total	of	the	following	number	of	electrons:	

 2	from	each	BH	unit	
 3	from	each	CH	unit	
 1	 from	each	 additional	 hydrogen	 atom	 (over	

and	above	the	ones	on	the	BH	and	CH	units)	
 the	anionic	charge	electrons	

 
5.3.2.	5n	Rules	
	

As	 discussed	 previously,	 the	 4n	 rule	 mainly	 deals	 with	
clusters	with	electron	counts	of	4n+k,	in	which	approximately	4	
electrons	are	on	each	vertex.	As	more	electrons	are	added	per	
vertex,	 the	 number	 of	 the	 electrons	 per	 vertex	 approaches	 5.	
Rather	 than	 adopting	 structures	 based	 on	deltahedra,	 the	5n‐
type	 clusters	 have	 structures	 based	 on	 a	 different	 series	 of	
polyhedra	known	as	 the	3‐connectedpolyhedra,	 in	which	each	
vertex	is	connected	to	3	other	vertices.	The	common	types	of	3‐
connected	polyhedra	are	listed	in	Table	9.		
	
Table	9.	Prediction	of	polyhedron	structure	based	on	number	of	vertices	in	
cluster.		
Number	of	vertices	 Type	of	3‐connected	polyhedron
4	 Tetrahedron	
6	 Trigonal	prism	
8	 Cube	
10	 Pentagonal	prism	
12	 D2d	Pseudo‐octahedron	

	
Using	 the	 electron	 count,	 the	 predicted	 structure	 can	 be	

found.	n	 is	 the	number	of	vertices	 in	 the	cluster.	The	5n	rules	
are	given	in	Table	10.		
	
Table	10.	Structure	prediction	using	electron	count.		
Total	electron	count	 Predicted	structure	
5n	 n‐vertex	3‐connected	polyhedron	

5n+1	 n‐1	 vertex	 3‐connected	 polyhedron	with	 one	 vertex	
inserted	into	an	edge	

5n+2	 n‐2	 vertex	 3‐connected	 polyhedron	 with	 two	
vertexes	inserted	into	edges	

5n+k	 n‐k	 vertex	 3‐connected	 polyhedron	 with	 k	 vertexes	
inserted	into	edges	

 
5.3.3.	6n	Rules	
	

As	more	electrons	are	added	to	a	5n	cluster,	the	number	of	
electrons	 per	 vertex	 approaches	 6.	 Instead	 of	 adopting	
structures	 based	 on	 4n	 or	 5n	 rules,	 the	 clusters	 tend	 to	 have	
structures	goverened	by	the	6n	rules,	which	are	based	on	rings.	
The	rules	for	the	6n	structures	are	given	in	Table	11.		
	

Table	11.	Structure	prediction	using	electron	count.		
Total	electron	
count

Predicted	structure	

6n‐k n‐membered	ring	with	k/2	trans‐annular	bonds
6n‐4	 n‐membered	ring	with	2	trans‐annular	bonds	
6n‐2	 n‐membered	ring	with	1	trans‐annular	bond	
6n	 n‐membered	ring	
6n+2	 n‐membered	chain	(n‐membered	ring	with	1	broken	

bond)	
 

6.	Uses	of	G‐6	metal	carbonyl	substituted	complexes	
	

G‐6	Metal	hexacarbonyls	are	widely	used	in	electron	beam‐
induced	deposition	technique	‐	 these	are	easily	vaporized	and	
decomposed	 by	 the	 electron	 beam	 providing	 a	 convenient	
source	of	metal	atoms	[219].		

 
6.1.	As	catalyst	
	

Mo(CO)6,	 [Mo(CO)3(MeCN)3],	 and	 related	 derivatives	 are	
employed	as	catalysts	 in	organic	synthesis.	For	example,	these	
catalysts	 can	 be	 used	 for	 alkyne	 metathesis	 and	 the	 Pauson‐
Khand	reaction.		
	
6.1.1.	Metal	carbonyl	complexes	as	catalyst	
	

Most	of	 the	existing	metal	carbonyl	catalysts	 for	acetylene	
polymerizations	need	to	be	preactivated	by	chlorine‐containing	
additives	or	by	UV	irradiation	in	halogenated	solvents.	A	series	
of	 “simple”	 metal	 carbonyl	 catalysts	 of	 general	 structure	
M(CO)xLy	 (M	 =	 Mo,	 W;	 L=	 bis(trimethylsilyl)ethyne)	 were	
developed,	 none	 of	 which	 require	 additives	 or	 pre‐
photoirradiation,	 most	 of	 which	 are	 air‐	 and	moisture‐stable,	
and	 some	 of	 which	 work	 well	 in	 nonhalogenated	 solvents	
[220].	 Different	 catalyst	 derived	 from	 the	 reaction	 of	Mo	 and	
dimethyl	 malonate	 such	 as	 (CO)6‐nMo(CHE2)n	 [221]	 or	
(CO)5Mo(CHE2)	 [222‐225].	 Such	 catalysts	 appear	 to	 be	 of	
insufficient	 reactivity	 to	 initiate	 ionization.	However,	with	 the	
more	 reactive	 substrates,	 reaction	 proceeds	 normally	 [226].	
Allylic	 alkylation	 catalyzed	 by	 molybdenum	 complexes	 like	
Mo(bpy)(CO)4	 [227,228],	 forms	 a	 useful	 and	 frequently	
complementary	 alternative	 to	 the	 palladium‐catalyzed	
reactions.	Higher	selectivity	for	attack	at	primary	vs.	secondary	
carbon	of	a	π‐allyl	fragment	occurs	with	molybdenum	catalyst.	
The	synthesis	of	the	dioxo‐molybdenum(VI)	and	‐tungsten(VI)	
derivatives	MO2(S‐BINOL)(THF)n	and	MO2	[OCMe(CF3)2]2(THF)	
(M	=	Mo,	n	=	2;	W,	n	=	1)	is	reported	(Scheme	58).	André	Pontes	
et	 al.	 have	 examined	 the	 potential	 of	 these	 complexes	 as	
catalysts	 for	 the	 sulfoxidation	 of	methyl	 phenyl	 sulfide,	 olefin	
epoxidation	and	hydrosilylation	of	carbonyl	groups	[229].		

	

	
	

Scheme	58	
	

Alkene	 carbonyl	 complexes	 of	 the	 type:	 [M(CO)5(η2‐
alkene)],	 [M(CO)4(η2‐alkene)2],	 and	 [M(CO)3(η2‐alkene)3]	 have	
been	 prepared	 by	 photochemical	 reactions	 of	 Group	 6	
transition	metal	carbonyls	with	alkenes.	These	complexes	were	
found	 to	 exhibit	 some	 catalytic	 activity.	 These	 complexes	
catalyze	to	reactions	that	lead	to	the	formation	of	new	carbon‐
carbon	 bonds,	 such	 as	 metathesis,	 ring‐opening	 metathesis	
polymerization	 of	 cyclic	 olefins	 (ROMP),	 and	 the	 addition	 of	
haloalkanes	to	the	double	bond	of	olefin	[230].	The	thiacrowns	

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were	 synthesized	 by	 the	 catalytic	 ring	 opening	
cyclooligomerization	 of	 thiirane	 using	 some	 Group	 VI	 metal	
carbonyl	 complexes	 as	 catalyst.	 The	 Group	 VI	 catalyst	
precursors	 were	 transition	 metal	 complexes	 of	 the	 form	
M(NCCH3)(PR3)x(CO)5−x	 (x=0,	 M=Cr,	 Mo	 and	 W;	 x=1,	 M=Cr,	
R=Ph;	 M=W,	 R=Ph,	 or	 OC6H4‐p‐Me).	 The	 yields	 of	 the	
thiacrowns	 were	 significantly	 improved	 by	 the	 addition	 of	
dialkyl	 acetylenedicarboxylates	 to	 the	 reactions	 catalyzed	 by	
the	Group	VI	complexes	(Scheme	59)	[231].		
	

	
	

Scheme	59	
	

The	 catalytic	 activities	 of	 the	 complexes,	 Cp′M(CO)3X	
(Cp′	=	Cp	(η5‐C5H5),	Cp*	(η5‐C5Me5);	M	=	Mo,	W;	X	=	Cl,	CH3)	and	
Mo(CO)6	have	been	examined	 for	 the	oxidation	of	phenol	 (37.	
6	g/L)	using	H2O2	as	oxidant	in	aqueous	solution	(Scheme	60).	
Among	 them,	 CpMo(CO)3Cl	 and	 CpMo(CO)3(CH3)	 showed	 the	
highest	catalytic	activity	[232].		
	

	
	

Scheme	60	
	
6.1.2.	Metal	carbonyl	cations	as	catalyst	
	

Cationic	metal	carbonyls	[M(CO)m]n+,	which	have	a	positive	
metal	charge,	were	prepared	from	Cr,	Ag,	Au,	Pd,	Pt,	Rh	and	Ir.	
The	M‐C	bonds	of	these	compounds	were	mainly	formed	by	the	
σ‐dative	bond	of	CO,	and	then	the	ν	co	values	were	observed	at	
a	higher	wavenumber	than	the	free	CO.	These	unstable	cationic	
metal	carbonyls	were	used	as	catalysts	for	the	carbonylation	of	
olefins,	alcohols,	dienes,	aldehydes	and	saturated	hydrocarbons	
to	give	tert‐carboxylic	acids	in	high	yields	at	room	temperature	
and	atmospheric	CO	pressure	[233].		

There	has	been	a	rapid	development	 in	the	study	of	metal	
carbonyl	cations,	which	exhibit	distinguishing	characteristics	in	
comparison	with	 typical	 metal	 carbonyl	 complexes.	 Qiang	 Xu	
gives	an	overview	of	the	catalytic	application	of	metal	carbonyl	
cations,	including	the	metal	carbonyl	cluster	cations	[234].		
	
6.1.3.	Metal	carbonyl	clusters	as	catalyst	
	

Catalytic	 activity	 of	 transition	 metalcarbonyl	 clusters	
substituted	 with	 benzyne	 (and,	 for	 comparison,	 benzene	 and	
diene)	ligands	was	given	by	Sabrina	Brait	et	al.	Cluster‐surface	
analogies	 are	 helpful	 in	 gaining	 an	 understanding	 of	 the	
homogeneous	 and	 heterogeneous	 catalytic	 behaviour	 of	 the	
clusters	[235].		

Molybdenum	 hexacarbonyl	 was	 immobilized	 on	
polystyrene	 (2%	 cross‐linked	 with	 divinylbenzene)	 via	
imidazole	linkage	to	obtain	a	polymer‐supported	molybdenum	
carbonyl	catalyst	(2).	High	conversion	of	a	range	of	alkenes	to	
epoxides	was	achieved	with	tert‐butyl	hydroperoxide	and	 this	
catalyst.	 The	 catalyst	 is	 highly	 reusable	 and	 it	was	 reused	 10	
times	without	a	considerable	change	in	its	activity	[236].		

 
6.2.	In	nano	science	and	nanotechnologies	
	

The	possible	role	of	metal	carbonyl	clusters	in	nanoscience	
and	 nanotechnologies	 was	 investigated.	 Although	 a	 few	
nanosized	 metal	 carbonyl	 clusters	 (MCC)	 were	 already	
obtained	 in	 the	 seventies,	 these	 structurally	 and	

compositionally	 well‐defined	 molecules	 attracted	 the	 interest	
of	nanoscientists	only	as	possible	precursors	of	finely	dispersed	
metal	particles	for	heterogeneous	catalysis.	Cristina	et	al	give	a	
brief	 account	 of	 most	 recent	 results	 in	 the	 field	 of	 high‐
nuclearity	 metal	 carbonyl	 clusters	 and	 their	 emerging	
properties.	In	doing	that,	a	look	to	related	behaviour	of	ligand‐
stabilised	and	 ligand‐free	clusters	 (Figure	9),	 including	almost	
mono‐dispersed	 ligand‐stabilised	 metal	 nanoparticles,	 should	
be	done.	They	found	contribution	of	metal	carbonyl	clusters	to	
some	aspects	of	nanoscience	and	nanotechnologies	also	[237].		
	

	
	

Figure	 9.	 An	 ordered	 monolayer	 of	 differently	 charged	 individual	 MCC	
capacitors	 in	 consequence	 of	 their	 electron‐sink	 properties	 and	 the	
insulating	behavior	of	their	CO	shell	[237].		

	
An	 approach	 to	 noble	 metal‐metal	 oxide	 dumbbell	

nanoparticles	 based	 on	 seed‐mediated	 growth	 (Figure	 10).	
Metal	 oxides	 are	 grown	 over	 the	 presynthesized	 noble	metal	
seeds	by	thermal	decomposition	of	metal	carbonyl	followed	by	
oxidation	 in	 air.	 The	 as‐synthesized	 dumbbell	 nanoparticles	
have	 intrinsic	 epitaxial	 linkage	 between	 the	 metal	 and	 the	
oxide,	providing	enhanced	heterojunction	interactions	[238].		
	

	
	

Figure	 10.	Metal	 carbonyl	 catalyzed	 reaction:	 Preparation	 of	 metal‐metal	
oxide	dumbbell	nanoparticles	[238].		
	
6.3.	In	pharmaceuticals	
	

Metal	 carbonyls	 and	 their	 use	 as	 a	 new	 class	 of	
pharmaceuticals	 were	 studied	 [239].	 The	 hydrogenase	
enzymes	contain	CO	bound	to	iron,	apparently	the	CO	stabilizes	
low	oxidation	states	which	 facilitates	 the	binding	of	hydrogen	
[240,241].	Certain	metal	carbonyls	have	been	observed	in	trace	
amounts	 in	 landfills,	 where	 the	 reducing	 environment	 is	
compatible	 with	 their	 formation	 [242].	 Metal	 carbonyls	 have	
extensively	shown	various	antimicrobial	activity	against	E.	coli,	
S.	 aureus,	 B.	 subtilis,	 P.	 vulgaris	 etc.	 bacteria,	 C.	 albicans,	 T.	
rubrum,	T.	mentagrophytes,	A.	nigeretc.	fungi,	Cucumber	mosaic	
virus,	 insecticidal	 activity	 and	plant	 growth	 inhibitory	activity	
on	rice,	wheat	and	barley	plants.	Metal	carbonyl	complexes	also	
shown	 antitherapeutic	 activities	 like	 antitumor	 and	 cytotoxic	
activities	[242].		
	

6.4.	In	synthesis	of	other	complexes	
	
Metal	 carbonyls	 are	 found	 their	 important	 role	 in	 finding	

some	 specific	 products.	 Photochemical	 reactions	 of	
ferrocenylacetylene	with	 sulfur	 in	 presence	 of	 Mo(CO)6	 gives	
2,5‐diferrocenylthiophene	 and,	 with	 Cr(CO)6,	 2,6‐
diferrocenyldithiine	 formed.	 Formation	 of	 ferrocenyl‐
substituted	thioketone	complexes	was	observed	in	the	reaction	
of	 ferrocenylacetylene	 with	 water	 and	 sulfur,	 in	 presence	 of	
W(CO)6	 [243].	 Darensbourg	 reported	 the	 group	 6	
pentacarbonyl	 hydrides	 and	 various	 methods	 for	 their	
synthesis	[244‐246].		

	

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Scheme	64	
	

	
A	 major	 challenge	 that	 must	 be	 met	 for	 an	 asymmetric	

intermolecular	Pauson‐Khand	reaction	is	to	be	able	to	limit	the	
possible	 positions	 on	 the	 metal	 complex	 for	 the	 organic	
partners.	 Toward	 this	 end,	 the	 synthesis	 of	 monometallic	
systems	 derived	 from	 M(CO)6	 and	 two	 bidentate	 ligands,	 in	
which	 the	number	of	possible	coordination	sites	 is	 reduced	 to	
two	 this	 lead	 to	 enantioselective	 Pauson‐Khand	 reactions	
resulting	the	formation	of	Chiral	Cyclopentanones	(Scheme	61)	
[247].		
	

	
Scheme	61	

	
New	 polynuclear	 lanthanide(III)	 ionic	 clusters	 of	 general	

formula	 [Ln9(acac)16(OH)10]+	 [Mo2(CO)10(μ‐H)]−	 (Ln	=	Sm,	 Eu,	
Gd,	 Dy,	 Yb)	 and	 [Sm9(acac)16(OH)10]+	 [CrW(CO)10(μ‐H)]−	 is	
reported.	 The	 polynuclear	 complexes,	 prepared	 under	 pure	
nitrogen	 atmosphere	 by	 interaction	 of	 the	 hydridic	 metal	
carbonyls	with	the	β‐dichetonate	Ln(acac)3·3H2O	(Ln	=	Sm,	Eu,	
Gd,	Dy,	Yb)	[248].		

Novel	C3V ‐symmetric	scaffold,	trindane	has	been	efficiently	
synthesized	 from	 1,3,5‐tris(bromomethyl)‐2,4,6‐
tris(chloromethyl)benzene	in	six	steps	with	47%	overall	yield.	
The	control	of	all‐synstereochemistry	in	the	tribenzylation	step	
has	been	achieved	by	blocking	one	side	of	the	trindane	ring	by	
metal	carbonyl	complexes	(Scheme	62)	[249].		
	

	
	

Scheme	62	
	

Chromium	Fischer	carbene	complexes,	[Cr{=OMe(R)}(CO)5]	
have	 been	 utilized	 as	 a	 source	 of	 chromium	 carbonyls	 in	 the	
synthesis	 of	 chromium	 NHC	 (N‐heterocyclic	 carbene)	
complexes	 (Scheme	63).	The	method	can	be	employed	 for	 the	
synthesis	of	molybdenum	and	tungsten	NHC	compounds	[250].	

The	 reaction	 of	 Fischer	 carbene	 complex	 with	 o‐
alkynylpyridine	carbonyl	derivatives	involves	the	generation	of	
furo	 [3,4‐c]pyridine	 as	 transient	 intermediates	 through	 the	
coupling	 of	 o‐alkynylpyridine	 carbonyl	 derivatives	 with	
carbene	 complex	 and	 subsequent	 Diels‐Alder	 trapping	 with	
suitable	 dienophiles	 resulted	 in	 the	 formation	 of	 isoquinoline	

derivatives	 and	 the	 entire	 sequence	 can	 be	 run	 in	 one	 pot	
(Scheme	 64).	 When	 an	 olefinic	 tether	 was	 present,	
intramolecular	Diels‐Alder	cycloaddition	occurred	 followed	by	
ring	opening	to	yield	tricyclic	alcohols	[251].		

	
		

	
	

Scheme	63	
	

Phosphazene	 copolymers	 (Scheme	 65)	 carrying	 nitrile,	
pyridine	 and	 phosphine	 ligands	 have	 been	 synthesized	 by	
sequential	 substitution	 from	 [NPCl2]n.	 Their	 reactions	 with	
[W(MeOH)(CO)5]	 gives	 the	 corresponding	 tungstencarbonyl	
complexes,	 that	 undergo	 a	 complete	 decarbonylation	 below	
300	°C	forming	metal	containing	species	that	have	a	stabilizing	
effect	on	the	polymeric	matrices	[252].		
	

	
	

Scheme	65	
	

Polymer‐bound	 ruthenium	 and	 molybdenum	 carbonyl	
fragments	 were	 prepared	 and	 their	 use	 in	 a	 ligand	 exchange	
reaction	was	demonstrated	(Scheme	66)	[253].		
	

	
	

Scheme	66	
	

In	 preparation	 of	 metal	 carbonyl	 anions:	 Gibson	 et	 al.	
generated	 group	6	metal	 carbonyl	 anionsHM(CO)5‐by	 Ion‐Pair	
Extraction.	 Synthesis	 ofHM(CO)5‐(M	 =	 Cr	 or	 W)	 from	 M(CO)6	
utilizes	 tetraethylammonium	 hydrogen	 sulfate	 as	 the	 phase	
transfer	 aganet	 together	 with	 powdered	 KOH	 [254].	 The	
reaction	 times	 were	 short	 and	 the	 product	 yields	 were	 good	
[255].	Cherng	et	al.	have	synthesized	chromium	pentacarbonyl	
derivatives	of	arsenic	(V)	and	antimony	(V)	(Scheme	67)	[256].		

The	 tungsten	and	molybdenum	hexacarbonyls,	M(CO)6	 (M	
=	W,	Mo),	dissolve	in	fluorosulfonic	acid,	HSO3F,	to	generate	the	
tungsten	 and	molybdenum	 carbonyl	 cations,	 [M(CO)4]2+(solv),	
which	are	transformed,	by	exposure	to	an	NO	atmosphere,	into	
the	 tungsten	 and	 molybdenum	 carbonyldinitrosyl	 cations,	
[M(CO)(NO)2]2+(solv),	respectively	(Scheme	68)	[257].		

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388	 Kaushik	et	al.	/	European	Journal	of	Chemistry	3	(3)	(2012)	367‐394	
	
	

	
	

Scheme	67	
	

	

	
	

Scheme	68	
Mo(CO)4(LL)	 complexes,	 where	 LL	=	polypyridyl	 ligands	

such	 as	 2,2′‐bipyridine	 and	 1,10‐phenanthroline,	 undergo	
quasi‐reversible,	one‐electron	oxidations	in	methylene	chloride	
yielding	 the	 corresponding	 radical	 cations,	 [Mo(CO)4(LL)]+	
(Figure	 11).	 These	 electrogenerated	 species	 undergo	 rapid	
ligand	 substitution	 in	 the	 presence	 of	 acetonitrile,	 yielding	
[Mo(CO)3(LL)(CH3CN)]+	[258].		
	

	
	

Figure	11.	Generation	of	Metal	carbonyl	cations	from	Mo(CO)4(LL)	
complexes,	where	LL	=2,2′‐bipyridine	or1,10‐phenanthroline	[258].		

	
The	 cationic	 alkoxy	 and	 hydroxycarbyne	 complexes	

[M2Cp2(μ‐COR)(μ‐PR‘2)2]BF4	(Cp	=	η5‐C5H5;	M	=	W,	R	=	Me,	R‘	=	
Ph;	 M	 =	 Mo,	 R	 =	 Me	 and	 H,	 R‘	 =	 Et)	 and	 [Mo2Cp2(μ‐COR)(μ‐
COR‘)(μ‐PCy2)]BF4	(R	=	Me;	R‘	=	H,	Me,	Et)	are	obtained	in	high	
yield	 by	 the	 reaction	 of	 the	 corresponding	 neutral	
monocarbonyl	precursors	with	either	[Me3O]BF4	or	HBF4·OEt2	
in	dichloromethane	[253‐264].		
	
6.5.	Use	of	metal	carbonyl	complexes	to	achieve	high	
enantioselectivity	in	the	asymmetric	allylboration	of	
unsaturated	aldehydes	
	

The	 metal	 carbonyl	 units	 have	 been	 used	 as	 bulky	
substituents	 on	 chiral	 substrates	 enabling	 the	
diastereoselectivity	 of	 C‐C	 bond	 forming	 reaction	 at	 adjacent	
positions	 to	 be	 enhanced	 [265].	 The	 electronic	 and/or	 steric	
influence	of	 the	metal	 on	 the	unsaturated	organic	 ligand	 is	 of	
course	 central	 to	 the	 success	 of	 these	 and	 many	 other	
applications.	 William	 R	 Roush	 et	 al.	 report	 effect	 of	 metal	
carbonyl	 complexes,	 specifically	 the	 ability	 of	 such	 complexes	
to	 enhance	 the	 enantioselectivity	 of	 the	 asymmetric	
allylborations	of	unsaturated	(aryl	and	propargylic)	aldehydes.	
These	 reactions	 were	 in	 contrast	 to	 the	 previously	 reported	
examples	 in	 which	 the	 diastereoface	 selectivity	 of	 the	 metal	
carbonyl	 containing	 chiral	 substrates	 that	 is	 enhanced	 [265‐
275].	 The	 reactions	 in	which	metal	 carbonyl	 ligand	 of	 achiral	
substrates	lead	to	enhancement	of	the	enantiofacial	selectivity	

of	a	chiral	reagent	[276‐278].	Previous	studies	have	established	
that	 the	 asymmetric	 allylboration	 of	 aromatic	 aldehydes	 like	
benzaldehyde	proceed	with	only	moderate	stereoselectivity	of	
55‐72%	in	THF.	The	use	of	benzaldehyde	chromium	tricarbonyl	
complex	as	a	 surrogate	 for	C6H5CHO	 [279],	 in	 the	asymmetric	
allylboration	 with	 (R,R)‐1	 in	 toluene	 at	 ‐78	 oC	 followed	 by	
oxidative	decomposition	provide	(S)	in	greater	than	90%	yield.	
The	 (E)‐crotylboration	of	 chromium	 tricarbonyl	 complex	with	
(R,R)	was	also	provide	92%	yield	[280].		
	
6.6.	In	preparation	of	catalysts	
	

Cp*Mo(PMe3)3H	 undergoes	 carbonyl	 abstraction	 reactions	
with	a	variety	of	reagents,	 including	CO2,	 (CH2O)n,	HCO2H,	and	
MeOH	 to	 yield	 Cp*Mo(PMe3)2(CO)H.	 The	 Cp*Mo(PMe3)2(CO)H	
serves	as	a	catalyst	for	the	decomposition	of	HCO2H	to	CO2	and	
H2.	 [260]	 Preparation	 of	 alumina	 supported	 Pd‐W	 and	 Pd‐Mo	
catalysts	 was	 done	 photolytically	 from	 group	 VI	 metal	
carbonyls.	 Tungsten	 and	 molybdenum	 were	 introduced	 in	 γ‐
Al2O3	 and	 Pd/γ‐Al2O3	 using	 metalcarbonyls	 [W(CO)6	 and	
Mo(CO)6]	 photoactivation.	 The	 results	 showed	 that	 a	 reaction	
between	 the	 hexacarbonyls	 and	 the	 supported	 palladium	
particles	 takes	place	 leading	 to	bimetallic	 catalysts,	with	 clear	
palladium‐tungsten	 and	 palladium‐molybdenum	 interactions	
(Scheme	69).		
	

	
	

Scheme	69	
	

Both	 group	 VI	 metals	 strongly	 modify	 the	 chemisorption	
properties	 of	 palladium,	 while	 molybdenum	 seems	 to	 be	
present	 in	a	 reduced	state	 in	close	association	with	palladium	
[281].		

Molybdenum	 carbonyl	 complexes	 immobilized	 onto	
polymer‐bound	phosphines	were	easily	prepared	 in	 two	steps	
from	 commercially	 available	 chloromethylated	 polystyrene	
resin.	 These	 catalysts	 were	 found	 to	 efficiently	 catalyze	
epoxidation	 of	 a	 wide	 variety	 of	 alkenes,	 including	 aromatic	
and	 aliphatic	 terminal	 alkenes	 using	 tert‐butylhydroperoxide	
(Scheme	 70).	 The	 new	 heterogenized	 molybdenum	 carbonyl	
epoxidation	 catalysts	 can	 be	 easily	 recovered	 by	 simple	
filtration	and	show	no	appreciable	loss	of	activity	even	after	10	
times	 of	 recycling.	 Hence	 they	 show	 a	 new	 class	 of	 novel	
reusable	and	highly	active	epoxidation	catalysts	[282].		
	

	
	

Scheme	70	
	

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Polymer‐bound	 Schiff	 base	 ligandwas	 prepared	 by	
oxidation	 of	 chloromethylated	 polystyrene	 to	 aldehydic	
polystyreneand	 then	 reaction	 with	 ethylene	 diamine.	 The	
functionalized	 polystyrene	 was	 used	 to	 immobilize	 Mo(CO)6	
and	 polymer‐bound	 molybdenum	 carbonyl	 Schiff	 base	
catalystwas	 produced.	 This	 supported	 catalyst	 shows	 high	
activity	 in	 epoxidation	 of	 various	 alkenes	 in	 the	 presence	 of	
tert‐butylhydroperoxide	 (Scheme	 71).	 The	 supported	
molybdenum	 catalyst	 can	 be	 recovered	 and	 reused	 for	 eight	
times	without	loss	in	its	activity	[283].		
	

	
	

Scheme	71	
	
7.	Studies	done	on	metal	carbonyl	complexes	
	

Kinetics	 and	 mechanism	 of	 CO	 substitution	 reaction	 of	
M(CO)6	 (M=	 Cr,	 Mo,	 W)	 with	 (CH3)3NO	 in	 the	 absence	 and	
presence	 of	 triphenyl	 phosphine	 PPh3	 were	 incestigated.	 The	
rates	 of	 reaction	 are	 first‐order	 in	 concentrations	 of	 M(CO)6	
and	of	(CH3)3NO	but	zero‐order	in	PPh3	concentration.	The	rate	
of	 reaction	 decrease	 in	 the	 order	 W>Mo≈Cr.	 The	 mechanism	
was	 suggested	 with	 the	 formation	 of	 coordinatively	
unsaturated	 intermediates	 of	 the	 type	 M(CO)5,	 which	 then	
rapidly	react	with	an	entering	ligand	[284].		

Thermally	 activated	 ligand‐dissociation	 processes	 in	
substituted	group	6B	metal	carbonyl	derivatives	 involving	 the	
tightly	bound,	ubiquitous	phosphine	or	phosphate	ligands	have	
rarely	 been	 studied	 quantitatively.	 Kinetic	 and	 mechanistic	
studies	 of	 cis‐Mo(CO)2L2	 derivatieves	 (L	 =	 phosphine	 or	
phosphate)	are	presented	for	the	substitution	reaction	of	one	of	
the	 ligands	 (L)	 by	 carbon	 monoxide.	 These	 processes	 were	
observed	to	proceed	by	a	dissociative	mechanism	with	the	rate	
of	 substitution	 being	 greatly	 enhanced	 as	 the	 size	 of	 the	
phosphorus	 ligand	 increases	 within	 a	 series	 of	 phosphine	 or	
phosphate	derivates.	The	cis‐Mo(CO)4L2	species	were	found	to	
react	 stereo	 specifically	 with	 CO	 to	 afford	 cis‐Mo(CO)4L2	
derivative	was	observed	to	be	nonfluxional	during	 its	solution	
lifetime.	 Rate	 studies	 of	 phosphine	 dissociation	 in	 trans‐
Mo(CO)4(PPh3)2	are	reported	which	dekmonstrate	this	process	
to	be	less	facile	when	compared	with	the	analogous	process	in	
cls‐Mo(CO)4(PPh3)2	[285].	The	enthalpies	of	(i)	the	dissociation	
reactions	of	the	carbonyl	ligand	in	Cr(CO)n	[n	=	6,	5,	4],	Fe(CO)5	
,	and	Ni(CO)n	 [n	=	4,	3,	2],	(ii)	the	dissociation	reactions	of	the	
heteroligand	L	in	Cr(CO)5L	[L	=	CS,	Xe,	H2,	C2H4,	C2F4],	Cr(CO)3L	
[L	 =	 C6H6,	 C6Me6],	 and	 Fe(CO)4L	 [L	 =	 H2,	 C2H4],	 (iii)	 the	
deprotonation	 reactions	 of	 Cr(CO)3C6H6	 and	 Fe(CO)3C4H6,	 (iv)	
the	 protonation	 reaction	 of	 ferrocene,	 and	 (v)	 the	
hydrogenation	 reactions	 of	 Mn2(CO)10	 and	 Co2(CO)8	 were	
calculated	at	the	DFT/BP86,	MP2,	MP3,	SCS‐MP2,	and	SCS‐MP3	
levels	 and	 compared	 with	 the	 corresponding	 experimental	
data.	It	was	found	that	the	erratic	behavior	of	the	low‐order	MP	
approaches	 can	 be	 corrected	 by	 the	 newly	 developed	 spin	
component	 scaled	 (SCS)	 perturbation	 theory.	 The	 SCS‐MP3	
enthalpy	 of	 the	 hydrogenation	 reaction	 of	 Mn2(CO)10	 (5.8	
kcal/mol)	 agrees	 better	 with	 the	 experiment	 (8.7	 ±	 0.	 3	
kcal/mol)	 than	 the	 BP86	 value	 (1.7	 kcal/mol).	 The	 SCS‐MP3	
proton	affinity	of	metal‐protonated	ferrocene	(214.8	kcal/mol)	
and	of	the	agostic	form	(203.9	kcal/mol)	compare	well	with	the	

experimental	values	(206−213	kcal/mol)	and	contrary	to	MP2	
do	not	exclude	the	dynamic	behavior	of	protonated	ferrocene.	
It	 is	 suggested	 that	 for	 complex	 chemical	 systems	 including	
transition	metals	simultanous	application	of	DFT	and	SCS‐MP3	
methods	 may	 be	 helpful	 to	 increase	 the	 reliability	 of	 the	
predictions	[286].		

Vaporization	 thermodynamics	 of	 low	 and	 high	
(>400	g/mol)	 molecular	 weight	 (MW)	 carbonyls	 were	
compared.	A	gravimetric‐torsion	effusion	method	was	used	 to	
measure	 vapor	 pressures	 of	 carbonyls	 such	 Os3(CO)12,	
Rh6(CO)16,	Ru3(CO)12,	Co2(CO)8,	Cr(CO)6,	and	W(CO)6carbonyls.	
The	 studies	 found	 that	 Os3(CO)12,	 Cr(CO)6,	 and	 W(CO)6	 have	
shown	virtually	no	disproportionation	in	structure	[287].		

Dombek	and	Angelici	done	study	of	the	reaction	of	group	6	
metal	 thiocarbonyl	 complexes	 with	 nucleophiles.	 They	 found	
that	primary	amines	readily	react	with	M(CO)5(CS)	complexes	
where	 (M	 =	 Cr,	 Mo,	 W)	 to	 give	 isocyanide	 complexes,	
M(CO)5(CNR).	Secondary	amines	are	found	to	give	lower	yields	
of	 thioformamide	 complexes	 W(CO)5(S=C(H)NR2).	 Kinetic	
studies	 show	 a	 second	 order	 dependence	 in	 amine	
concentration.	It	was	suggested	that	the	rate‐determining	step	
is	the	addition	of	a	hydrogen‐bonded	amine	to	the	thiocarbonyl	
atom.	 The	 phosphine	 substituted	 analogues,	 cis‐	 and	 trans‐	
W(CO)4(CS)(PPh3)	 reacts	 much	 slower,	 presumably	 due	 to	
increased	electron	density	at	the	thiocarbonyl	ligand.	Azide	ion	
reacts	 rapidly	 with	 M(CO)5(CS),	 forming	 cleanely	 the	 known	
W(CO)5(CS).	A	number	of	weaker	nucleophiles	including	water,	
alcohols,	hydrazine,	aniline,	ammonia,	CH3SH	and	Ph3PO	do	not	
react	 with	 W(CO)5(CS).	 Stronger	 nucleophiles	 such	 as	 CH3Li,	
R2NLi	 and	 RO‐	 appear	 to	 add	 to	 the	 CS	 ligand,	 although	 the	
reactions	are	not	straightforward	[288].		

A	 range	 of	 microwave‐enhanced	 reactions	 of	 group	 VIb	
metal	 carbonyls	were	presented	by	Ardon	et	al.	 (Scheme	72).	
Molybdenum	 hexacarbonyl	 is	 particularly	 well	 suited	 to	 this	
procedure,	 with	 reactions	 generally	 proceeding	 cleanly	 and	
without	need	 for	an	 inert	atmosphere.	Reactions	of	chromium	
and	tungsten	hexacarbonyls	are	less	impressive	[289].		
	

	
	

Scheme	72	
	

Many	 carbonyl	 diimine	 complexes	 of	 first‐row	 transition	
metals	have	been	 found	 to	be	photoreactive	under	 irradiation	
into	 their	 lowest	 metal‐to‐ligand	 charge‐transfer	 (MLCT)	
absorption	 band	 (metal	 →	 α‐diimine),	 the	 observed	 primary	
reaction	 corresponding	 to	 a	 carbonyl	 loss.	 Metal‐to‐ligand	
charge‐transfer	(MLCT)	excited‐state	dynamics	of	Cr(CO)4(bpy)	
(bpy	 =	 2,2‘‐bipyridine)	 have	 been	 investigated	 through	 wave	
packet	 propagations	 on	 CASSCF/MR‐CCI	 potentials	 and	
compared	with	the	results	of	recent	femtosecond	time‐resolved	
spectroscopic	study	[290].		

Photoinduced	 CO‐loss	 and	 arene‐loss	 from	 (η6‐
C6H5Y)Cr(CO)3	 (Y	=	NH2,	OCH3,	H,	CHO	or	CO2CH3)	 result	 from	
discrete	photophysical	processes	whose	efficiency	depends	on	
the	solvent	(Scheme	73).	The	quantum	yield	(Φ)	for	arene	loss	
is	greater	in	halocarbon	than	in	hydrocarbon	solvents	[291].		
	

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390	 Kaushik	et	al.	/	European	Journal	of	Chemistry	3	(3)	(2012)	367‐394	
	

	
	

Scheme	73	
	

Zhang	 et	 al.	 studied	 cyclopentadienylmolybdenum	
carbonyls	Cp2Mo2(CO)n	 (Cp	=	η5‐C5H5;	n	=	6−1)	(Figure	12)	by	
density	 functional	 theory	 and	 predicted	 stability	 of	 metal	
carbonyl	 complexes	 [292].	 The	 two	 lowest	 energy	 structures	
predicted	for	Cp2Mo2(CO)6	lie	within	4	kcal/mol	of	each	other.	
Both	have	Mo−Mo	single	bonds	of	 lengths	3.	2−3.	3	Å	with	all	
terminal	carbonyl	groups	and	correspond	to	stable	compounds.	
Similarly,	 the	 lowest	 energy	 structure	 predicted	 for	
Cp2Mo2(CO)4	has	a	formal	Mo≡Mo	triple	bond	of	length	~2.	5	Å	
with	 four	 weakly	 semibridging	 carbonyl	 groups	 also	
corresponding	to	a	stable	compound	structurally	characterized	
by	X‐ray	diffraction.	The	pentacarbonyl	Cp2Mo2(CO)5,	which	 is	
not	known	experimentally	as	a	stable	compound	but	only	as	a	
transient	 intermediate,	 is	 shown	 to	have	a	 structure	with	one	
symmetrical	 bridging	 two‐electron	 donor	 and	 four	 terminal	
carbonyl	 groups	 as	 well	 as	 a	 formal	 Mo=Mo	 double	 bond.	
Furthermore,	 Cp2Mo2(CO)5	 is	 predicted	 to	 be	
thermodynamically	 unstable	 with	 respect	 to	 dispropor‐
tionation	 into	Cp2Mo2(CO)6	+	Cp2Mo2(CO)4.	The	 lowest	energy	
structure	 for	 Cp2Mo2(CO)3	 is	 a	 triplet	 with	 a	 formal	 Mo≡Mo	
triple	 bond.	 A	 higher	 energy	 singlet	 structure	 with	 one	 four‐
electron‐donor	 bridging	 carbonyl	 group	 is	 also	 found	 for	
Cp2Mo2(CO)3.		
	

	
	

Figure	 12.	 X‐ray	 crystallogh	 of	 cyclopentadienylmolybdenum	 carbonyls	
Cp2Mo2(CO)n	(Cp	=	η5‐C5H5;	n	=	6‐1)	[292].		
	

Intramolecular	carbonylcarbonyl	interactions	in	W,	Mo	and	
Fe	 complexes	 containing	 the	 η1‐N‐maleimidato	 ligand	 was	
found	 both	 for	 experimental	 and	 calculated	 structures.	 It	 is	
probably	 the	 first	 approach	 to	 explain	 this	 type	 of	
intramolecular	 interactions	 acting	 in	 organometallic	
compounds.	The	 investigations	 indicate	also	 the	differences	 in	
the	 character	of	bonding	between	 the	η‐N‐maleimidato	 ligand	
and	the	central	metal	atom	[293].		

The	 adsorption	 and	 thermal	 behavior	 of	 (η6‐
benzene)tricarbonyl	 chromium	 (0),	 (η6‐C6H6)Cr(CO)3,	 inside	
zeolite	Y	have	been	investigated.	The	thermal	decomposition	of	
(η6‐C6H6)Cr(CO)3	in	zeolite	Y	seems	strongly	dependent	on	the	

experimental	 conditions	 employed: 	 heating	 (η6‐C6H6)Cr(CO)3	
adsorbed	inside	zeolite	Y	in	a	Schlenk	tube	under	nitrogen	flow	
results	 in	 the	 formation	 of	 benzene	 and	 Cr	 metal,	 whereas	
heating	 the	 same	complex	 in	a	 closed	vessel	yields	a	different	
metal	carbonyl	species,	Cr(CO)6	[294].		

The	electronic	 description	of	octahedral	 (fac‐	 [M(CO)3L3]n,	
with	M	=	Re,	Ru,	and	Mn,	and	[Cr(CO)5L]n),	square‐planar	(cis‐	
[Pt(CO)2L2]n),	 and	 tetrahedral	 ([Ni(CO)3L]n)	 carbonyl	
complexes	(where	L	=	monodentate	ligand)	was	obtained.	The	
analysis	indicates	that	while	ligand	electronic	parameters	may	
be	 considered	 as	 a	 measure	 of	 the	 net	 donor	 power	 of	 the	
ligand,	the	net	transfer	of	the	electron	density	(or	charge)	does	
not	 occur	 from	 the	 ligand	 to	 the	 metal	 ion.	 In	 [M(CO)xLy]n	
carbonyl	species,	 the	charge	 transfer	occurs	 from	 the	 ligand	L	
to	 the	 oxygen	 atom	 of	 the	 bound	 carbon	 monoxides.	 This	
charge	 transfer	 translates	 into	changes	of	 the	polarization	 (or	
permanent	dipole)	and	the	covalency	of	the	C≡O	bonds,	and	it	
is	 this	effect	 that	 is	probed	in	IR	spectroscopy.	As	the	analysis	
shifts	 from	 IR	 radiations	 to	 electrochemical	 potentials,	 the	
parameters	best	describe	the	relative	thermodynamic	stability	
of	 the	 oxidized	 and	 reduced	 [M(CO)xLy]n/n+1	 species.	 No	
relationship	 is	 found	between	 the	metal	natural	 charge	of	 the	
[M(CO)xLy]n	fragments	analyzed	and	the	parameters	[295].		

The	reactivity	of	a	nucleophilic	nickel	acylate	complex	with	
a	 tungsten	 carbene	 complex,	 Fe(CO)5,	 Cr(CO)6,	 PPh3,	 and	 CO	
was	investigated.	With	the	tungsten	carbene	complex,	a	methyl	
transfer	 occurred.	 With	 the	 metal	 carbonyl	 complexes,	 the	
acylate	 group	 on	 the	 nickel	 and	 a	 carbonyl	 on	 the	 iron	 or	
chromium	 traded	 places.	 With	 the	 PPh3	 and	 CO,	 the	 acylate	
anion	was	replaced	by	the	phosphine	or	CO	ligand	(Scheme	74)	
[296].		
	

	
	

Scheme	74	
	
8.	Conclusion	
	

The	 G‐VIb	metal	 carbonyl	 complexes	 are	 of	 great	 interest	
toscientists	since	last	5	decades,	and	various	studies	have	been	
done	on	these	molecules.	The	successful	development	of	metal	
carbonyl	complexes	and	their	products	has	been	demonstrated	
by	 various	 examples	 of	 recent	 literature.	 The	 review	 article	
highlights	 various	 types	 of	 metal	 carbonyl	 complexes,	
complexation	agents	and	substitution	reactions	of	G‐VIb	metal	
carbonyls	with	various	ligands.	This	review	has	given	just	a	few	
examples	of	what	is	being	done	in	the	area	as	we	change	to	our	
work	of	the	future.		
	
Acknowledgements	
	

Sincere	 thanks	 are	 due	 to	 Dr.	 Chandrawati,	 Head,	
Department	 of	 Chemistry,	 Durga	 Prasad	 Baljeet	 Singh	 Post	
Graduate	 College,	 Anoopshahar,	 Bulandshahar,	 Uttarpradesh,	
India	 and	 Mr.	 Pradeep	 Kumar	 Mathur,	 Head,	 Department	 of	
Chemistry,	 Mahanand	 Mission	 Harijan	 College,	 Ghaziabad,	
Uttarpradesh,	 India,	 for	 providing	 research	 facilities.	 Authors	
are	 also	 thankful	 to	 Dr.	 Kushal	 Pal	 Singh,	 Principal,	 Durga	
Prasad	 Baljeet	 Singh	 Post	 Graduate	 College,	 Anoopshahar,	
Bulandshahar,	 Uttarpradesh,	 India,	 for	 his	 constant	
encouragement	to	carry	out	such	research	work.	Special	thanks	

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are	also	due	to	Central	Drug	and	Research	Institute,	Lucknow,	
India;	 Intertek,	 Mumbai,	 India	 and	 Tata	 Institute	 of	
Fundamental	Research,	Mumbai,	India	for	allocation	of	time	for	
various	analyses.		
	
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