Reaction between CpW(CO)(3)SPh and RuCl2(PPh3)(x) (x = 3 and 4) in dichloromethane at room temperature afforded a single thiolato-bridged heterodinuclear complex, CP(CO)(2)W(mu-SPh)(mu-Cl)Ru(CO)(Cl)(PPh3) (1) in high yield. Irreversible isomerization of 1 to Cp(CO)(Cl)W(mu-SPh)(mu-Cl)Ru(CO)(2)(PPh3) (2), which exists as a mixture of stereoisomers 2a and 2b, occurs via chloride and carbonyl ligand migration during thermolysis of 1 in toluene at 80 degrees C. Compounds 1, 2a, and 2b were characterized by single-crystal X-ray diffraction analysis. Migration of g-acceptor carbonyl ligands from W atom to Ru atom during the conversion of CPW(CO)(3)SPh + RuCl2(PPh3)(3) -> 1 -> 2 with concomitant migration of sigma-donor chloride ligand from Ru to W atom in 1 is reported.