denoted ?, and their bridging mode. In ?²-CO complexes, both the carbon and oxygen are bonded to the metal. More commonly only carbon is bonded, in which case the hapticity is not mentioned.

The carbonyl ligand engages in a wide range of bonding modes in metal carbonyl dimers and clusters. In the most common bridging mode, denoted u₂ or simply u, the CO ligand bridges a pair of metals. This bonding mode is observed in the commonly available metal carbonyls: Co₂(CO)₈, Fe₂(CO)₉,