Reactions of MoCl2(PMe3)4 with the sulfur atom donors SPMe3, SPPh3, (t)BuSH, and ethylene sulfide give the mu-sulfidodimer Mo2(mu-S)(mu-Cl)Cl3(PMe3)5. These reactions proceed by initial formation of the tetravalent terminal sulfido complex Mo(S)Cl2(PMe3)3 and its conproportionation with the divalent starting material. The X-ray crystal structure of Mo2(mu-S)(mu-Cl)Cl3(PMe3)5-1/6C7H8 shows an edge-shared bioctahedral dimer in which three phosphines are bound to one molybdenum and two to the other. The central Mo2(mu-S)(mu-Cl) core is symmetrical, indicating a Mo(III)-Mo(III) oxidation state assignment, and contains an Mo-Mo bond of 2.6956(6) angstrom and short Mo-S distances of 2.274(1) and 2.285(1) angstrom. Crystal data : R3BAR,Z= 18, a = 18.840(6) angstrom, b = 48.989(5) angstrom, V = 15058.8(11) angstrom3, refined to R = 3.5%, R(W) = 4.7%, and GOF = 1.047. The conproportionation of MCl2(PMe3)4 and terminal oxo and sulfido complexes M(E)Cl2(PMe3)3 is a general route to the molybdenum and tungsten mu-sulfido and mu-oxo dimers M2(mu-E)(mu-Cl)Cl3(PMe3)5, which are isostructural on the basis of their very similar H-1 and P-31{H-1} NMR spectra. Two isomeric mixed-metal dimers, MoW(mu-S)(mu-Cl)Cl3(PMe3)5, differing only in the occupation of the two different metal sites, are formed on reactions Of MO(S)Cl2(PMe3)3 with WCl2(PMe3)4 or W(S)Cl2(PMC3)3 with MoCl2(PMe3)4. The two combinations of sulfur atom donor and acceptor yield different isomers as their kinetic product, although equilibrium is quickly established (K(eq) = 0.62 at 24-degrees-C). These conproportionation reactions are examples of incomplete sulfur or oxygen atom transfer. In contrast, the reaction of Mo(O)Cl2(PMe3)3 with WCl2-(PMe3)4 results in complete oxygen atom transfer to give W(O)Cl2(PMe3)3 and MoCl2(PMC3)4. Conproportionation of the W(V) oxo complex W(O)Cl3(PMe3)2 with WCl3(PMe3)3 occurs by chlorine atom transfer to give the W(IV) monomers W(O)Cl2(PMe3)3 and WCl4(PMe3)2. The mechanisms of these reactions and the preferences for complete vs incomplete atom transfer and monomeric vs dimeric products are discussed.