The nomenclature of the metal carbonyls depends on the charge of the complex, the number and type of central atoms, and the number and type of ligands and their binding modes. They occur as neutral complexes, as positively-charged metal carbonyl cations or as negatively charged metal carbonylates. The carbon monoxide ligand may be bound terminally to a single metal atom or bridging to two or more metal atoms. These complexes may be homoleptic, containing only CO ligands, such as nickel tetracarbonyl (Ni(CO)), but more commonly metal carbonyls are heteroleptic and contain a mixture of ligands.
Mononuclear metal carbonyls contain only one metal atom as the central atom. Except vanadium hexacarbonyl, only metals with even atomic number, such as chromium, iron, nickel, and their homologs, build neutral mononuclear complexes. Polynuclear metal carbonyls are formed from metals with odd atomic numbers and contain a metal–metal bond. Complexes with different metals but only one type of ligand are called isoleptic.
Carbon monoxide has distinct binding modes in metal carbonyls. They differ in terms of their hapticity, denoted η, and their bridging mode. In η-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 μ or simply μ, the CO ligand bridges a pair of metals. This bonding mode is observed in the commonly available metal carbonyls: Co(CO), Fe(CO), Fe(CO), and Co(CO). In certain higher nuclearity clusters, CO bridges between three or even four metals. These ligands are denoted μ-CO and μ-CO. Less common are bonding modes in which both C and O bond to the metal, such as μη.