The carbonyl ligand engages in a range of bonding modes in metal carbonyl dimers and clusters. In the most common bridging mode, denoted "μ"2 or simply "μ", 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, Fe3(CO)12, and Co4(CO)12. In certain higher nuclearity clusters, CO bridges between three or even four metals. These ligands are denoted "μ"3-CO and "μ"4-CO. Less common are bonding modes in which both C and O bond to the metal, such as "μ"3"η"2.