A compound in which groups of metal atoms are joined together by metal-metal bonds. The formation of such compounds is a feature of the chemistry of certain elements, particularly molybdenum and tungsten, but also vanadium, tantalum, niobium, and uranium. Isopoly compounds are ones in which the cluster contains atoms of the same element; heteropoly compounds contain a mixture of different elements. It is sometimes possible to predict which cluster compounds are stable using electron counting rules.