Metal p-Complexes

DR. ARCHANA PANDEY

DEPARTMENT OF CHEMISTRY
BRAHMANAND COLLEGE, KANPUR
 CARBONYLS
• Structure and bonding, vibrational spectra
of metal carbonyls for bonding structural
elucidation, important reaction of metal
carbonyls, preparation, bonding structure.
• The p acceptor ligands possess vacant p
orbitals in addition to the lone pairs of
electrons. Such ligands are CO, NO, PR3,
CNR etc. These ligangs donate their lone
pairs to the metal atom to form a normal s
bond. In addition to it the vacant orbitals of
ligands accept electrons from the filled
metal orbitals to form a type of p bond.
• These ligands are thus called p-acid
ligands or p-acceptor ligands or p-bonding
ligands. The metal atoms in these
complexes are in low positive, zero or
negative oxidation states i.e. these ligands
stabilise lower oxidation states. Thus,
there is higher electron charge density on
the metal in it’s lower oxidation states than
in it's higher oxidation state. Therefore,
some of this excess electronic charge is
transferred from metal to the vacant
orbitals of the ligand by p bonding. S are
discussed below.
 Metal Carbonyls
• The electronic configuration of CO
molecule indicates that it contains a lone
pair of electrons on carbon and oxygen
atom each. Thus, carbon atom of CO
molecule can donate it's electron pair to a
transition metal atom (M) to form OC → M
coordinate bond. The compounds formed
by the combination of CO molecules with
transition metals are called metallic
carbonyls.
• Since the electrons forming OC → M bond are
supplied only by CO molecules, metal atom in
carbonyls is said to be in zero Oxidation state. In
metal carbonyls CO molecules act as neutral
ligands.
• Classification
• 1. Mononuclear carbonyls: These carbonyls
contain only one metallic atom per molecule and
have the general formula M (CO)y. Such
carbonyls are formed by the metals having even
atomic number e.g., Cr (CR)6, Fe (CO)5, Ni
(CO)4, Mo (CO)6, W (CO)6 etc. Some
characteristics of these carbonyls are:
 Properties
• These are generally colourless liquids or
solids with low m. pts. except V (CO)5
which is a black solid and Fe (CO)5 which
is a yellow solid.
• These are readily soluble in organic
solvents like benzene, ether, light
petroleum etc.
• These can be vaporised without
decomposition.
• These carbonyls are more volatile than the
others.
 Polymolecular carbonyls
• These carbonyls contain more than one
metal atom per molecule. They are
represented by the general formula Mx
(CO)y.
• Some authors call the carbonyls having
two metal atoms as bridged carbonyls
and represented as M2(CO)y while those
containing more than two metal atoms as
polynuclear carbonyls. The polynuclear
carbonyls may be of two types:
• (a) Homonuclear carbonyls: These
carbonyls contain only one type of the metal
atoms e.g., Fe3 (CO)12, Ru3 (CO)12, Ir4 (CO)12
etc.
• (b) Heteronuclear carbonyls: These
contains more than one type of metal atoms
e.g., Mn Co (CO)9, Mn Re (CO)10 etc.
• Some characteristics of these carbonyls are:
These carbonyls are generally insoluble in
organic solvents.
These decompose at ‘or below their melting
point.
General methods of preparation
1. By direct synthesis: By direct combination
of CO with finely divided transition metals
under suitable conditions of temperature and
pressure forms carbonyls. For example:
xM  yCO  M x (CO) y

Fe  5CO 200O
C Fe(CO)
100atm 5

Co 8CO 200O C Co (CO)

100atm 2 8

Ni  4CO 30O C Ni(CO)

1atm 4
 2. By indirect synthesis involving Grignard’s
reagent: Job prepared chromium hexacarbonyl
Cr(CO)6 by treating ethereal solution of Grignard's
reagent with CO in presence of anh. CrCl3 Similar
reactions in presence of Mo (V) and W (V) chlorides
gave the corresponding hexacarbonyls.
According to Heiber, the primary reaction is as
follows:
4C6H5Mg Br + CrCl3+2CO →
G.R.
Cr (CO)2 (C6H5)4 + 3Mg Br Cl + Mg Br2
unstable
• The unstable intermediate is decomposed
in acid solution to give the hexacarbonyl.
3Cr(CO)2 (C6H5)4 +6H+ →
Cr(CO)6 + 2Cr3 + 12C6H5– +3H2
• The yield of the above reaction is low and
can be improved by using high carbon
monoxide pressure.
• 3. Indirect synthesis involving metal
compounds: Metal carbonyls can be
prepared by the react of CO with certain
metal compounds. For example:
2NiX4 + 2nCO → 2Ni (CO)n X + X2
2Ni(CO)n X + (4-2n) CO → Ni(CO)4 + NiX2
4. Synthesis by carbonylating the metallic
salts with CO in presence of reducing
agent: When salts like VCl3, CrCl3, CoS,
CoI2, Co CO3, Ru I3 etc. are treated with CO
(carbonylation) in presence of suitable
reducing agent like Na, Mg, Ag, Au, H2,
LiAIH4 etc., metallic carbonyls are obtained.
For example,
 100o C, 150atm
     
VCl  6CO  3Na V(CO)  3NaCl
3 H 3 PO 4 6

115o C,70atm
CrCl  6CO  LiAlH     Cr(CO)  LiCl  AlCl
3 4 6 3

200o C,200atm
2CoS 8CO  4Cu      Co (CO)  2Cu S
2 8 2

Sometimes CO acts both as a carbonylating

and reducing agent. For example, in the
preparation of Os (CO)5 and Re2 (CO)10 form Os
O4 and Re2 O7 respectively.
 25o
OsO  5CO  C, 350atm
     Os(CO)  2O
4 5 2

2Re O  10CO  Re (CO)  7O

2 7 2 10 2

Synthesis from other carbonyls:

When cooled solutions of iron and
osmium pentacarbonyls in glacial acetic
acid is exposed to ultraviolet light, Fe3 (CO)9
and Os2 (CO)9 are obtained respectively.
2Fe (CO)5 2Fe (CO)9 + CO
 
U .V . Light

2Os (CO)5
 
U .V . Light 2O2 (CO)9 + CO

By treating oxide of metals with CO under

pressure : Carbonyls of osmium and
rhenium are prepared by this method, e.g.

Os O4 + 9CO 
100O C, 50atm
Os (CO)5 + 4CO2

Re2O7 + 17CO  Re2

750O C, 200 atm
(CO)10 + 7CO2
Preparation of Mo (CO)6 and W (CO)6
from Fe (CO)5 :
Since CO groups present in Fe (CO)5
are labile, they can be replaced by treating
Fe (CO)5 with Mo Cl6 and WCl6.

100
 
O
C, ether
MoCl6 + 3Fe(CO)5 Mo(CO)6 + 3FeCl2 + 9CO

100
 
O
C, ether
WCl6 + 3Fe(CO)5 W (CO)6 + 3FeCl2 + 9CO
 Colour and Melting points of some carbonyls
Carbonyls Colour and state Melting point/Boiling point
V (CO)6 Black crystals Decomposes at 70oC,
sublimes in vacuum
Cr (CO)6 Colourless crystals Sublimes in vacuum
Mo (CO)6 Colourless crystals Sublimes in vacuum
W (CO)6 Colourless crystals Sublime in vacuum
Mn2 (CO)10 Golden crystals 1540- 1500C
Re (CO)10 Collurless cryseals Sublimes at 7400 C and
decomposes at 1770 C
Fe (CO)5 Yellow liquid B. P. 1030 C
Fe2 (CO)9 Bronze platelets Decomposes at 1000

Fe3 (CO)12 Dark green crystals Decomposes at ~ 140oC

Co2(CO)8 Orange crystals M.P. 51oC

Ni(CO)4 Colourless liquid B.P. 43oC

 General Properties of Carbonyls
Physical properties :
 The metal carbonyls are crystalline solids
except NI(CO)4, Fe (CO)5, Ru (CO)5 and Os
(CO)5 which are liquids of ordinary temperature.
 Generally monomeric carbonyls are colourless
while polymeric are coloured and unstable as
shown in Table 6.1.
 Since they are covalent in nature, most of them
are soluble in organic solvents.
 They are diamagnetic in nature except V (CO)6
which is paramagnetic due to the presence of
one unpaired electron.
The metals in carbonyls are in zero
oxidation state.
These are poor conductors of electricity.
They can be sublimed or distilled at low
temperature with decomposition.
CHEMICAL PROPERTIES:
Substitution reactions : Some or all CO groups
present in the carbonyls can be replaced by
monodentate ligands like py, PCl3, CH3OH, alkyl
or aryl isocyanide (CNR) etc. For example.
Ni(CO)4 + 4PCl3 Ni(PCl3)4 + 4CO
Ni(CO)4 + 4CNR Ni(CNR)4 + 4CO
Fe(CO)5 + 2py Fe(CO3) (py)2+2CO
Bidentate ligands like NO2, o-phen,
diars etc. replace two or more CO groups at
a time. For example :
Ni(CO)4 + 2NO2 Ni(NO2)2 + 4NO
Ni(CO)4 + o-phen Ni(CO)2 (o-phen) + 2CO
Ni(CO)4 + diars Ni(CO)2 (diars) + 2CO
Fe(CO)5 + diars Fe(CO)2 (diars) + 2CO
 Action of NaOH or Na metal
 Aqueous or alcoholic solution of NaOH reacts
with Fe (CO)5 to form carbonylate anion, [H Fe
(CO)4]–
 Fe(CO)5 (Fe = 0) + 3NaOH
Na+ [H+Fe2- (CO)4]- + Na2CO3 + H2O
 Na-metal in liquid NH3 converts Cr (CO)6, Mn2
(CO)10, Fe2 (CO)9, Co2 (CO)8, Fe(CO)12 etc. into
carbonylate anions and in this conversion
carbonyls are reduced, e.g.,
Cr (CO)6 + 2Na Na2+ [Cr2-(CO)5]2- + CO
Cr = 0 Cr = -2
Mn2 (CO)10 + 2Na 2Na2+ [Mn-(CO)5]-
Mn = 0 Mn = -2
Action of halogens: Most of the carbonyls react
with halogens to form carbonyl halides. For
example :
Fe (CO)5 + X2 Fe(CO)4 X2 + CO
Mo (CO)6 +Cl2 Mo(CO)4 Cl2 + 2CO
Mn2 (CO)10 + X2 2Mn (CO)5 X
X = Br, I
• Both Ni(CO)4 and Co2(CO)8 react with
halogens and form metallic halides.
Ni(CO)4 + Br2 Ni Br2 + 4 CO
Co2(CO)8 +2Cl2 2CoCl2 + 8CO
Action of NO : Many carbonyls react with NO
to form metal carbonyl nitrosyls. For example :
200o C, 200atm
Fe(CO)5 + 2NO  Fe(CO)2(NO)2+ 3CO


Co2(CO)8 + 2NO165o C,200atm

  2Co(CO)2(NO)+2CO

Action of H2: When Mn2(CO)10 and Co2(CO)8

are treated with H2, they are reduced to
carbonyl hydrides.
 Different Metal Carbonyls
Carbonyls of VIB groups : These form
carbonyls of the type M (CO)6 where M = Cr, Mo
and W. Chromium also forms Cr (CO)5.
Chromium hexacarbonyl, Cr (CO)6
Preparation :
 It is prepared by Job’s method by passing CO2 at
50 atm. Pressure and at room temperature into a
suspension of chromic chloride in ether which
has been treated with phenyl magnesium
bromide at – 70oC.
 Chromium hexacarbonyl may be prepared by
treating a solution of chromic salt dissolved in
ether with Al (C2H5)3 and CO at high temperature
and pressure.
 It may also be obtained by treating chromic or
chromous salt of an organic acid in pyridine
solution with CO at 80-170oC and 100-300 atm.
Pressure in the presence of powdered zinc or
magnesium.
 It may also be prepared by treating CrCl3 with
CO in the presence of reducing agent like LiAl
H4.
CrCl3+CO+LiAI H4 175o C,70atm
  Cr(CO) + LiCl + AlCl
6 3
Properties:
 It is a colourless crystalline solid which sublimes
at 100oC.
 It is soluble in organic solvents such as ether,
benzene, chloroform, carbon tetrachloride etc.
 It is not attacked by air, aqueous alkalies, dilute
acids concentration HCl and concentration
H2SO4. It is decomposed by chlorine or
concentration HNO3.
 When treated with sodium metal or NaBH4 in
liquid NH3, it is reduced to form carboxylate
anion.
Cr(CO)6 + 2Na liq 
.NH 3 Na2[Cr 2-(CO) ]2-+CO
5
Cr = 0 Cr = -2
 Some CO groups of Cr (CO)6 can be replaced
by pyridine.
 Chromium pentacarbonyl, Cr (CO)5
 Ultraviolet irradiation of the hexacarbonyl
solution supports the existence of Cr (CO)5. It
has square pyramidal structure.
 Molybdenum hexacarbonyl and Tungsten
hexacarbonyl
 Preparation:
 Both may be prepared by Job’s method i.e., by
treating MoCl6 or W Cl6 with CO in presence of
phenyl magnesium bromide.
 Both may also be prepared by the action of
metallic Mo or W on CO at 225oC and 200 atm.
Pressure.
 Mo (CO)6 and W (CO)6 may be prepared by the
action of CO on the metal in presence of
sulphide of copper or iron.
 Mo (CO)6 and W (CO)6 may also be prepared by
the action of K3 Mo Cl9 and K3 W Cl9 with carbon
monoxide.
Properties :
 Both are colourless crystalline solids.
 Mo (CO)6 sublimes at 40oC and boils at 156.4oC
while W (CO)6 sublimes at 50oC and boils at
1750C.
 They are soluble in organic solvents such as
ether, benzene, chloroform, carbon tetrachloride
etc.
 They do not react with air, aqueous alkalies
acids except concentration HNO3.
 Both are decomposed by chlorine and bromine.
 Some CO groups present in Mo (CO)6 and W
(CO)6 are replaced by pyridine.
py
M (CO)6  M(CO)5py py 
py
M2(CO)7(py)3  M(CO)3(py)3
Where M = Mo and W
 Both react with sodium metal in liquid NH3 to
form ionic carbonyls.
M(CO)6 + 2Na  Na2+ [M(CO)6)]2- + CO
liq.NH3
 Carbonyls of VII B group
The form carbonyls of the general formula M2
(CO)10 where M = Mn, Tc and Re.
Manganese carbonyl, Mn2(CO)10
Preparation :
 By treating Mn I2 and Mg with CO in ether under
pressure.
25o C, 210 atm
2Mnl2 + 2Mg + 10CO Mn2(CO)10+2MgI2
 By carbonylating MnCl2 with CO in presence of
(C6H5)2CO Na.
o C, 140 atm
MnCl2 + 10CO + 4(C6H5)2 CONa 165 

Mn2(CO)10 + 4(C6H5)2CO + 4 NaCl

 Properties
 It is a golden yellow crystalline solid which melts
at 155oC.
 It sublimes in vacuum and is soluble in organic
solvents.
 Action of halogens: It reacts with halogens to
form carbonyl halides, Mn (CO)5X.
Mn2 (CO)10 + X2 (X =Br, I) 2Mn (CO)5X
 Action of hydrogen: Mn2 (CO)10 is reduced by
H2 to form carbonyl hydride.
o C, 200 atm - o
Mn2 (CO)10 (Mn=0) + H2 200 2[Mn (CO)5H]
 Technetium carbonyl, Tc2 (CO)10
Higgins and co-worker in 1961 reported the
formation of Tc2(CO)10. It is similar to Mn2(CO)10
and Re2(CO)10. A few iodine derivative to Tc2
(CO)10 have also been prepared.
Rhenium carbonyl, Re2(CO)10
Preparation: (i) By the action of rhenium
heptoxide with CO at 200oC and 200 atm.
Pressure.
Re2 O7 + 17CO Re2(CO)10 + 7CO2
(ii) By the reaction of potassium perrhenate and
rhenium hepta sulphide, Re2S7 with CO under
pressure.
 Properties
 Re2 (CO)10 is a colourless solid.
 It sublimes at 140oC and melts at 177oC.
 It is soluble in organic solvents.
 It is not attacked by alkalies or cold and
concentrated mineral acids.
 Action of halogens: It forms rhenium carbonyl
halide, Re (CO)5 X when treated with halogens.
Re2(CO)10 + X2 2Re (CO)5X
where X = any halogen.
 Substitution reaction : It reacts with pyridine to
form Re (CO)3 (py)2
Re2(CO)10 + 4py 2Re (CO)3 (py)2+ 4CO
 Carbonyl of VIII group
Carbonyls of Iron : Three carbonyls of iron are
known viz. Fe(CO)5, Fe2(CO)9 and Fe3(CO)12
 Iron pentacarbonyl, Fe(CO)5:
 It is prepared by the action of CO on iron powder
at 100-200oC and 200 atm. pressure.
Fe + 5CO Fe (CO)5
 By the action of CO on ferrous iodide or ferrous
sulphide at 100oC atm. Pressure in the presence
of copper.
FeI2 + 5CO + 2Cu Fe(CO)5 + Cu2I2
FeS + 5CO + Cu Fe(CO)5 + CuS
 Properties
 It is a yellow liquid which boils at 103oC.
 It is soluble in methyl alcohol, ether, acetone and
benzene but insoluble in water.
 It decomposes at 250oC to give iron.
2Fe(CO)5 250 oC Fe + 5CO

 Action of ultra-violet light: It forms enneacarbonyl di
iron (o), Fe2(CO)9 when cooled solution of Fe(CO)5 in
glacial acetic acid is irrdiated with u.v. light.
2Fe(CO)5 u.v.light
  Fe2(CO)9 + CO
 Hydrolysis: It is hydrolysed by water, acids and weak
bases.
Fe(CO)5 + H2SO4 FeSO4 + 5CO + H2
 Reaction with halogens : It reacts with halogens (X2) to
form stable tetracarbonyl halides, Fe(CO)4X2.
 Reaction with cyclopentadiene: It reacts with
cyclopentadine at 300oC to give p-complex ferrocene.
Fe(CO)5 + 2C5H6 (C6H5)2 Fe + 5CO + H2
 Reaction with ethylene diamine: It reacts with ethylene
diamine to form an addition product, Fe (CO)5 en.
Fe (CO)5 + en Fe(CO)5 (en)
 Reaction with halides: With halides like CCl4, SO2Cl2,
SnCl4, SbCl5 and CuCl2, it forms halides of Fe(II). Thus,
the oxidation state of Fe increases from zero to + 2. For
example:
Fe(CO)5 + 2CCl4 Fe+2Cl2 + C2Cl6 + 5CO
Fe(CO)5 + SnCl4 Fe(CO)4Cl2 + SnCl2 + CO
Fe(CO)5 + 2CuCl2 FeCl2 + 2CuCl + 5CO
 Carbonyl of Ruthenium
• It forms three carbonyls viz. Ru (CO)5, Ru2 (CO)9 and Ru3
(CO)12.
• Ruthenium pentacarbonyl, Ru (CO)5:
 Preparation:
 It is prepared by the action of CO on reduced ruthenium at
200oC and 200 atm. Pressure.
Ru + 5CO 200o C, 200 atm
       Ru (CO)5
 It is also prepared by the action of CO on Rul3 mixed with
finely divided silver at 170oC and 450 atm. pressure.
 Properties :
 It is a colourless liquid having b.p. – 220C.
 It is insoluble in water but soluble in chloroform, alcohol and
benzene.
 It decomposes to form Ru2 (CO)9 and Ru3 (CO)12.
 It reacts with halogens to give Ru CO Br and CO.
 Ruthenium ennea carbonyl, Ru2(CO)9
Preparation:
• It is prepared by irradiating pentacarbonyl to
ultraviolet light.
• It is also prepared by heating solution of penta
carbonyl in benzene at 50oC.
Properties:
 It is yellow crystalline solid which is volatile.
 With iodine, it forms Ru (CO)2 I2.
 With NO, it gives Ru (NO)5.
 Ruthenium dodeca carbonyl, Ru3 (CO)12
Preparation :
 By heating Ru (CO)5 at 50oC.
 By exposing Ru (CO)5 to ultra violet light. It is a
green crystalline solid.
Carbonyl of Rhodium:
 If forms two carbonyls viz. Rh2 (CO)8 and Rh4
(CO)11.
Rhodium octacarbonyl, Rh2 (CO)8.
Preparation :
 It is prepared by the action of CO on metallic
rhodium under high pressure.
 It may also be prepared by the action of CO on
rhodium tri iodide at 100oC and high pressure in
presence of halogen acceptor such as metallic
copper.
 Carbonyl of osmium
 It forms two carbonyls viz. Os (CO)5 and Os2
(CO)9.
Osmium pentacarbonyl, Os (CO)5
Preparation :
 It is obtained by the action of CO on OsI3 at 120oC
and 200 atm. Pressure in presence of copper.
 It is also obtained by the action of CO on OsO4 at
100oC atm. pressure.
OsO4 9CO Os (CO)5 + 4CO2
 It is a colourless liquid having m.p. – 150oC
 Osmium enanea carbonyl, Os2 (CO)9
 Preparation : It is obtained by the action of CO on
osmium tri iodide in presence of copper.
Properties:
 It is yellow crystalline solid.
 It sublimes without decomposition and melts at
224oC.
 It is more stable towards heat than Ru2(CO)9.
Carbonyl of Iridium :
 It forms two carbonyls viz, Ir2 (CO)8 and Ir4 (CO)12.
Iridium octacarbonyl, Ir2 (CO)8 :
 Preparation: It is prepared by reacting K Ir2 Br6 or
K Ir2 I6 with CO at 200oC and 200 atm.
 Properties
 It is a yellow crystalline solid.
 It decomposes at 200oC.
 Chemically it is inert.
Carbonyls of Platinum
Preparation:
 When CO is passed over platinous chloride at
250oC, a mixture of Pt Cl2 2CO and 2Pt Cl2. 3CO
is obtained.
 3Pt Cl2 + 5 CO 25 oC

PtCl2. 2CO + 2Pt Cl2. 3CO
 Pure PtCl2.Co is obtained by subliming the
higher carbonyl chloride through a tube heated
to 250oC.
 PtCl2. 2CO PtCl2.CO + CO
2PtCl2.3CO 2PtCl2.CO + CO
 PtCl2.2CO is also obtained by heating Pt foil
or sponge at 240-250oC with CO and Cl2.
Pt + 2CO + Cl2 PtCl2.2CO
 These carbonyls are easily decomposed by
water and hydrochloric acid.
PtCl2.CO + H2O Pt + 2HCl + CO2
PtCl2.CO + HCl H [PtCl3.CO]
PtCl2.CO + HCl H [PtCl3.CO] + CO
 Structures of Metallic Carbonyls
Effective atomic number rule or EAN rule:
The most satisfactory formula of carbon
monoxide is : C :: O :
 From the above structure of carbon monoxide, it
seems that the lone pair of electrons on the
carbon atom can be used to form a dative bond
with certain metals (M←C=O). In the formation
of M←C= O bonds, the electrons are denoted
by carbon monoxide molecules and the metal
atoms thus said to have zero oxidation state.
The number of carbon monoxide molecules
which can combine with one metal atom is
controlled by the tendency of the metal atom to
acquire the effective atomic number of the next
inert gas. This is given for stable monomeric
carbonyls.
 E.A.N. = m + 2y = G.
where
m = atomic number of the metal M.
y = number of CO molecules in one
molecule of carbonyl M (CO)y
G = atomic number of the next inert gas
(a) Mononuclear carbonyls having
metallic atom even atomic number : The
metals having even atomic number obey
EAN rule and hence form mononuclear
carbonyls as shown in Table.
Mononuclear carbonyls of metal atoms having
even atomic number
Metal At. No. of No. of electron E.A.N. (m+2y) At. no. of next
carbonyl the metal donated by CO inert gas (G)
(m) molecules (2y)

Cr (CO)6 24 2 x 6 =12 24 + 12 = 36 Kr (36)

Fe (CO)5 26 2 x 5 = 10 26 + 10 = 36 Kr (36)
Ni (CO)4 28 2x4=8 28 + 8 = 36 Kr (36)
Mo (CO)6 42 2 x 6 = 12 42 + 12 = 54 Xe (54)

Ru (CO)5 44 2 x 5 = 10 44 + 10 = 54 Xe (54)

W (CO)6 74 2 x 6 = 12 74 + 12 =86 Rn (86)

Os (CO)4 76 2 x 5 = 10 76 + 10 = 86 Rn (86)
 On the basis of EAN it can be explained
why Ni atom does not form hexacarbonyl, Ni
(CO)6. It is because of the fact that EAN of Ni
atom in Ni (CO)6 would be equal to 28 + 2 x 6 =
40 which is not the atomic number of any of the
inert gases.
(b) Mononuclear carbonyls have the metallic
atom with odd atomic number: The metal
atoms having odd atomic number do not obey
EAN rule and hence can not form mononuclear
carbonyls. For example, some hypothetical
carbonyls are given in the table.
 Hypothetical mononuclear carbonyls
having atom with odd atomic number
Metal At. No. of No. of electron E.A.N. (m+2y) At. no. of
carbonyl the metal donated by next
(m) CO molecules inert gas
(2y) (G)

V (CO)6 23 2 x 6 = 12 23 + 12 = 35 Kr (36)

Mn (CO)6 25 2 x 5 = 10 25 + 10 = 25 Kr (36)

Co (CO)4 27 2x4=8 27 + 8 = 35 Kr (36)

(C)Polynuclear carbonyls: Sidgqick and
Bailey have shown that for polynuclear
carbonyls of the type Mx (CO)y, the general
formula is :
xm + 2y
G= = x-1
x
where
G = The atomic number of the next inert gas
x = The number of metal atoms
m = The atomic number of the metal
y = The number of CO molecules in one
molecule of the carbonyl
Co2(CO)8 :
Electrons from 2Co atoms = 2 x 27 = 54
Electrons from 8CO molecules = 2 x 8 = 16
Electrons from one Co-Co bond = 1x2 = 02
Fe2(CO)9 :
Electrons from 2Fe atoms = 2 x 26 = 52
Electrons from 9CO molecules = 2 x 9 = 18
Electrons from one Fe-Fe bond = 1x2 = 02
Fe3(CO)12 :
Electrons from 3Fe atoms = 3 x 26 = 78
Electrons from 12CO molecules = 2 x 12 = 24
Electrons from one 3Fe-Fe bonds = 2 x 3 = 06
 18-Electrons rule in metal carbonyls
• The formation of mononuclear
carbonyls by transition metals can also
be explained on the basis of 18 electron
rule. According to this rule a metal atom
with even atomic number combines with
a such number of CO molecules so that
the valence-shell (n-1) d, ns, np of the
metal atom acquires 18 electrons as
shown in table.
 Formation of mononuclear carbonyls
with metals having even at no. on the
basis of 18-electron rule
Carbonyls No.of the No. of electrons Total no. of
valence donated by CO electrons on the
electrons molecules (y) metal atoms
on the (x + y)
metal atom
Cr(CO)6 (Cr = 3d5, 4s1) 6 2 x 6 = 12 6 + 12 = 18

Fe(CO)5 (Fe = 3d6, 4s2) 8 2 x 5 = 10 8 + 10 = 18

Ni(CO)4 (Ni = 3d8, 4s2) 10 2x4=8 10 + 8 = 18

Mn2(CO)10 : (Mn = 3d54s2)

2Mn = 2 x 7 = 14 es
10CO = 2 x 10 = 20 es
Mn – Mn bond = 02 es
 Molecular orbital approach
• The nature of bond present in metal carbonyls
can be explained on the basis of molecular
orbital theory as shown below:
 Mononuclear carbonyls : These carbonyls have
linear M – CO bond in which CO molecule is
attached with metal atom through C atom. The C
atom undergoes sp hybridization to form two
hybrid orbitals which are denoted as Spa and
SPb. Now configurations of C and O atoms may
be written as below:
C = ls2, 2SP2a, 2SP1b, 2p1y, 2p0z
O = ls2, 2s2x, 2p1x, 2p1y, 2p2z
• Thus, the total number of bonding and
antibonding electrons are 6 and zero
respectively, the bond order of the molecule will
be 6-0/2 = 3. Hence the number of bonds
between C and O atoms in CO molecule is 3
i.e., one s and two p.
• The bond formation between metal atom and
CO molecule occurs as follows:
(a) An empty metal s orbital overlaps with filled
carbon s orbital to form MC = O bond as
shown in Fig.
(b) A filled d orbital of the metal overlaps
with empty antibonding p orbital of the
carbon atom to form MCO dative p
bond as shown in Fig.