The diamagnetic cobalt(III) dimethyl complex, cis,mer-(PMe(3))(3)Co(CH(3))(2)I, was found to promote selective C-C bond formation, affording ethane and triplet (PMe(3))(3)CoI. The mechanism of reductive elimination has been investigated by a series of kinetic and isotopic-labeling experiments. Ethane formation proceeds with a rate constant of 3.1(5) x 10(-5) s(-1) (50 degrees C) and activation parameters of Delta H(double dagger) = 31.4(8) kcal/mol and Delta S(double dagger) = 17(3) eu. Addition of free trimethylphosphine or coordinating solvent strongly inhibits reductive elimination, indicating reversible phosphine dissociation prior to C-C bond-coupling. EXSY NMR analysis established a rate constant of 9(2) s(-1) for phosphine loss from cis,mer-(PMe(3))(3)Co(CH(3))(2)I. Radical trapping, crossover, and isotope effect experiments were consistent with a proposed mechanism for ethane extrusion where formation of an unobserved five-coordinate intermediate is followed by concerted C-C bond formation. An unusual intermolecular exchange of cobalt-methyl ligands was also observed by isotopic labeling.