Oxidative cleavage of metal-metal bonds in heteronuclear clusters. Reaction of tin tetrachloride with (.mu.-H)2(.mu.3-S)Ru3(CO)9 and the crystal and molecular structures of (.mu.-H)2(.mu.3-S)Ru3(CO)9 and (.mu.-H)2(.mu.3-S)(.mu.-Cl)Ru3(CO)8(SnCl3)