An early example of a supported metal–metal bond is cyclopentadienyliron dicarbonyl dimer, [(CH)Fe(CO)].  In the predominant isomers of this complex, the two Fe centers are joined not only by an Fe–Fe bond, but also by bridging CO ligands.  The related cyclopentadienylruthenium dicarbonyl dimer features an unsupported Ru–Ru bond. Many metal clusters contain several supported M–M bonds.  Some examples are Fe(CO) and Co(CO).