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Organometallic Chemistry: (Five classes)
16 and 18 electron rules; variety of ligands,

hapticity, Synthesis structure and reactivity of
metal carbonyls
Oxidative addition, reductive elimination,

Migratory insertion reactions,
Homogeneous catalysis:
(i) alkene hydrogenation,
(ii) alkene hydroformylation
(iii) Monsanto acetic acid process
(iv) Wacker process
CO as a ligand: s-donor p-acceptor
Back bonding
vacant filled vacant

filled
̶ ̶ + + ̶
+ +
+ ̶
+ ̶ +
̶
s-donor p-acceptor
Non-bonding Vacant p* orbital of CO
electron pair of CO
CO as a ligand: s-donor p-acceptor
Back bonding
̶ + + +
̶ + + ̶ ̶ ̶
+
+ ̶ ̶ + + ̶ +
Another diagram
C2H4 as a ligand: s-donor p-acceptor
Back bonding
R R R R
C C C C
R M R R R
M
filled vacant
+ ̶ ̶ + ̶
+
+
+ ̶ + ̶ +
̶
vacant filled
Counting of Valency Electrons
The first attempt to account for bonding in

transition metal complexes was made by Sidgwick
who extended the octet theory of Lewis.
Ligands were considered to be Lewis bases which

donated electrons (usually one pair per ligand) to
the metal which in turn acts as a Lewis acid.
Stability was assumed to be attendant of a noble

gas configuration for the metal.
The (EAN rule) 18 electron rule
The sum of the electrons on the metal plus the

electrons donated by the ligands was called the
Effective Atomic Number (EAN).
When this was equal to 36 (Kr), 54 (Xe) or 86 (Rn), the
EAN rule was said to be obeyed.
Alternatively:
When the metal achieves an outer-shell configuration
of ns2 (n-1)d10 np6 there will be eighteen electrons in
the valence orbitals and a closed, stable configuration
is obtained (18-electron rule)
This rule has considerable usefulness in predicting
the formulas of stable organometallic compounds and
is a fundamental concept towards the understanding
of the subject.
The 18-electron rule is a natural consequence of

the MO theory.There are also exceptions to this
rule where in organometallic compounds have 16
electrons and are yet stable (The 16 electron rule)!
Some exceptions to the rules arises when the metal

is sterically crowded by the ligands.
The 18-electron rule has remarkable utility for
predicting the structures and stabilities of
organometallic compounds. It is also possible to
predict whether there will be metal-metal bonds
and whether the ligand is terminal or bridging.
There are two methods for counting electrons:

1. Neutral atom method (neutral ligand method)
2. Oxidation state method (Donor pair method)
Either method may be used but their rules

SHOULD NOT be mixed.
To use either of the methods, one must know how

many electrons each ligand in a complex donates
to the metal.
Electron donated by ligands in neutral atom method
(neutral ligand method) and oxidation state method
(donor pair method) can be different
neutral atom method oxidation state method

One electron donor Two electron donor
It is not necessary to know the percentage of ionic or

covalent character of the bond
Electron donated by ligands in neutral atom method
(neutral ligand method) and oxidation state method
(donor pair method) can be same

two electron donor Two electron donor
CO as a ligand

Two electron donor Two electron donor
Chloride as terminal or bridging ligand
Dillip Kumar Chand 15
2 e donor
1 e donor
Neutral mononuclear metal carbonyls
V(CO)6 Cr(CO)6 Fe(CO)5 Ni(CO)4

Mo(CO)6 Ru(CO)5
W(CO)6 Os(CO)5
Cr =6 Fe = 8 Ni = 10
6 CO = 12 5 CO = 10 4 CO = 8
_______________________________________________
Total = 18 18 18
binuclear metal carbonyls

Mn2(CO)10 Fe2(CO)9 Co2(CO)8
Mn2(CO)10
Fe2(CO)9
Fe2(m2-CO)3(CO)6
Co2(CO)8 Co2(m2-CO)2(CO)6 Solid-state

2 Mn = 14 2 Fe = 16 2 Co = 18
10 CO = 20 9 CO = 18 8 CO = 16
1 Mn-Mn = 2 1 Fe-Fe = 2 1 Co-Co = 2
_______________________________________________
Total = 36 36 36
Per metal = 18 18 18
higher nuclear metal carbonyls

Mn2(CO)10 Fe2(CO)9 Co2(CO)8
Fe3(CO)12
Co4(CO)12
Fe3(CO)12
Ru3(CO)12
Os3(CO)12
Calculate the number of M-M bonds in Co4(CO)12.
Rh4(CO)12 and Ir4(CO)12. Draw their structures.
Electron count
4 Co = 36, 12 CO = 24; Total = 60
Requirement = 72, Shortage = 12
Number of M-M bonds = 12/2 = 6
Co4(m2-CO)3(CO)9
The structures of Co4(CO)12 and Rh4(CO)12 are
comparable.
Co4(m2-CO)3(CO)9
Rh4(m2-CO)3(CO)9
The structure of Ir4(CO)12 has no bridging CO.
Ir4(CO)12
Electron count
4 Ir = 36, 12 CO = 24; Total = 60
Requirement = 72, Shortage = 12
Number of M-M bonds = 12/2 = 6

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Organometallic Chemistry: (Five classes)

16 and 18 electron rules; variety of ligands,

Oxidative addition, reductive elimination,

vacant filled vacant

The first attempt to account for bonding in

Ligands were considered to be Lewis bases which

Stability was assumed to be attendant of a noble

The sum of the electrons on the metal plus the

The 18-electron rule is a natural consequence of

Some exceptions to the rules arises when the metal

There are two methods for counting electrons:

Either method may be used but their rules

To use either of the methods, one must know how

neutral atom method oxidation state method

It is not necessary to know the percentage of ionic or

neutral atom method oxidation state method

neutral atom method oxidation state method

V(CO)6 Cr(CO)6 Fe(CO)5 Ni(CO)4

V(CO)6 Cr(CO)6 Fe(CO)5 Ni(CO)4

Co2(CO)8 Co2(m2-CO)2(CO)6 Solid-state

V(CO)6 Cr(CO)6 Fe(CO)5 Ni(CO)4

The structure of Ir4(CO)12 has no bridging CO.

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