When acting as a ligand to a metal centre, carbon monoxide is capable of forming a metal-carbon σ bond via donation of a lone pair from carbon. It can also accept π-electron density from filled d-metal orbitals into the CO π antibonding orbital. Hence CO can be considered a Lewis σ base and a Lewis π acid, which is capable of stabilising low oxidation state metals. This type of π bonding is referred to as π backbonding.
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