There are a number of cluster metal carbonyl compounds with metal-metal bonds joining three or more metals, and terminal CO, µ-CO (a bridge between two metals), and µ3-CO (a bridge capping three metals) are coordinated to the metal frames (refer to Section 6.3 (f)). Many cluster carbonyls are formed by a pyrolysis reaction of mononuclear or binuclear carbonyl compounds. Typical metal carbonyl compounds and their properties are shown