The phrasecluster was coined by F.A. Cotton in the early 1960s to refer to compounds containing metal–metal bonds. In another definition a cluster compound contains a group of two or moremetal atoms where direct and substantial metal metal bonding is present. The prefixed terms “nuclear” and “metallic” are used and imply different meanings. For example, polynuclearrefers to a cluster with more than one metal atom, regardless of the elemental identities. Heteronuclear refers to a cluster with at least two different metal elements.
The maincluster types are "naked" clusters (without stabilizing ligands) and those with ligands. For transition metal clusters, typical stabilizing ligands include carbon monoxide,halides, isocyanides, alkenes, and hydrides. For main group elements, typical clusters are stabilized by hydride ligands.
Transition metal clusters are frequently composed ofrefractory metal atoms. In general metal centers with extended d-orbitals form stable clusters because of favorable overlap of valence orbitals. Thus, metals with a low oxidation statefor the later metals and mid-oxidation states for the early metals tend to form stable clusters. Polynuclear metal carbonyls are generally found in late transition metals with lowformal oxidation states. The polyhedral skeletal electron pair theory or Wade's electron counting rules predict trends in the stability and structures of many metal clusters.