Objectives

Bonding in Metal

The homoleptic carbonyls (compounds containing carbonyl ligands only) are formed by most of the transition metals. These carbonyls

Carbonyls

have simple, well defined structures. Tetracarbonylnickel(0) is

tetrahedral, pentacarbonyliron(0) is trigonalbipyramidal while
hexacarbonyl chromium(0) is octahedral.
Decacarbonyldimanganese(0) is made up of two square pyramidal Mn(CO)₅ units joined by a Mn – Mn bond. Octacarbonyldicobalt(0) has a Co – Co bond bridged by two CO groups (Fig.9.13).


261 Coordination Compounds

CO


Ni
OC _CO
CO
CO
OC
Fe _CO
OC
CO ^O

Ni(CO)₄
Tetrahedral
Fe(CO)₅
Trigonal bipyramidal O

CO
CO ^CO
CO CO
_CO CO O

Fig. 9.13
Structures of some representative
Cr
CO _CO
OC ^{Mn Mn} CO CO CO
[Co₂(CO)₈]

homoleptic metal ^CO
CO CO

carbonyls.
Cr(CO)₆ Octahedral
[Mn₂(CO)₁₀]

Fig. 9.14: Example of synergic bonding interactions in a carbonyl complex.
The metal-carbon bond in metal carbonyls possess both  and  character. The M–C  bond is formed by the donation of lone pair of electrons on the carbonyl carbon into a vacant orbital of the metal. The M–C  bond is formed by the donation of a pair of electrons from a filled d orbital of metal into the vacant antibonding * orbital of carbon monoxide. The metal to ligand bonding creates a synergic effect which strengthens the bond between CO and the metal (Fig.9.14).

7.