Carbonyl ligand

A carbonyl ligand is coordinated carbon monoxide, usually attached through carbon. Sigma donation and pi backbonding cooperate in the metal–carbon bond. Terminal and bridging forms have different electron-counting descriptions.

For a series of comparable metal carbonyls, greater metal-to-CO pi backbonding populates CO antibonding orbitals, weakens its internal bond and lowers its infrared stretching frequency. Coupled normal modes and changing geometry must be considered when comparing observed bands.

A carbonyl attached primarily to one metal but with a secondary interaction with another. Formula alone does not determine whether terminal, bridging or semibridging CO is present.