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CHAPTER NINE
Coordination Compounds
9.22
Discuss the nature of bonding in metal carbonyls.
Metal carbonyls are coordination complexes of transition metals with carbon
monoxide. These complexes contain CO ligands, such as nickel carbonyl (Ni(CO)4).
Carbon monoxide bonds to transition metals using pi* back-bonding."
A sigma bond arises from overlap of nonbonding electron pair on carbon with d, s, and p-orbitals
on the metal. A pair of π bonds arises from overlap of filled d-orbitals on the metal with a pair of
π-antibonding orbitals projecting from the carbon of the CO. This electron donation makes the
metal more electron rich, and in order to compensate for this increased electron density, a filled
metal d-orbital may interact with the empty pi* orbital on the carbonyl ligand to relieve itself of
the added electron density. This second component is called pi-backbonding or pi.. We can
describe the bonding of CO to a metal as consisting of two components. The first component is a
two electron donation of the lone pair on carbon into a vacant metal d-orbital. This electron
donation makes the metal more electron rich, and in order to compensate for this increased
electron density, a filled metal d-orbital may interact with the empty pi* orbital on the carbonyl
ligand to relieve itself of the added electron density. This second component is called pibackbonding or pi-backdonation.
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