Under exclusion of air the thioether and thiol complexes [Ru(SRR′)(PPh3)′S4′] (′S4′2- = 1,2-bis(2-mercaptophenylthio)ethane (2—)) easily react with CHCl3 yielding [Ru(Cl)(PPh3)(′S4′—CHCl2)] in which one thiolato atom of the ′S4′ ligand is diastereospecifically dichloromethylated. In the presence of air, however, [RuIII(Cl)(PPh3)′S4′] is formed.The molecular structures of [Ru(Cl)(PPh3)(′S4′-CHCl2)] · 2CHCl3 and [RuIII(Cl)(PPh3)′buS4′] have been determined by X-ray crystallography. ′buS4′2- (= 1,2-bis(3,5-di(t-butyl)-2-mercaptophenylthio)ethane(2-)) is the t-butyl derivative of the ′S4′ ligand. Reasons for observed diastereospecifity of alkylation are discussed.