Abstract The reaction of RuCl 3 with 4 molar equivalents of Na[Co(CO) 4 ] gives a mixed cluster Na[RuCo 3 (CO) 12 ] in high yield. The cluster readily undergoes a cation exchange by bulky cations such as [(Ph 3 P) 2 N] + , while protonolysis by phosphoric acid produces HRuCo 3 (CO) 12 . The X-ray structure of [(Ph 3 P) 2 N][RuCo 3 (CO) 12 ] shows that the four metals are located at the corners of a tetrahedron. These mixed clusters are very effective for the homologation of methanol compared with Co 2 (CO) 8 . On the other hand, the Co 2 (CO) 8 Ru 3 (CO) 12 bimetallic catalyst shows remarkably high catalytic activity for hydroformylation of olefins such as cyclohexene compared with Co 2 (CO) 8 alone. The synergistic effects for cobalt and ruthenium on these carbonylation reactions dre discussed. Metalloselective substitution reactions were observed when HRuCo 3 (CO) 12 was treated with amines or phosphines. The substitution of amines for the carbonyl ligand takes place preferentially at the ruthenium atom whereas the substitution of phosphines occurs exclusively at the cobalt atoms.