Binary carbonyl clusters consist only of metal and CO. They are the most widely studied and used metal carbonyl clusters. They arise in general by the condensation of unsaturated metal carbonyls. Dissociation of CO from Ru(CO) would give Ru(CO), which could trimerize to Ru(CO). The reaction mechanisms are more complicated than this simple scenario. Condensation of low-molecular-weight metal carbonyls requires decarbonylation, which can be induced thermally, photochemically, or using various reagents. The nuclearity (number of metal centers) of binary metal carbonyl clusters is usually no greater than six.