The d(3)-d(3) M(2)L(10) dimer Mo-2(mu-S)(mu-Cl)Cl-3(PMe(3))(5) (1) readily and reversibly loses a phosphine ligand to produce the M(2)L(9) species Mo-2(mu-S)(mu-Cl)Cl-3(PMe(3))(4) (2). An X-ray crystal structure of 2 reveals an unprecedented edge-shared structure, as opposed to the face-shared structure typical of M(2)L(9) dimers. The phosphine ligand lost from 1 is the one trans to the CL-sulfido ligand, which has an exceptionally long Mo-P bond distance in 1. Otherwise, the structure of 2 is similar to that of 1, suggesting an analogous Mo(III)-Mo(III) assignment, with an Mo-Mo bond of 2.6293(8) Angstrom and short Mo-S distances of 2.288(2) and 2.222(2) Angstrom. The analogous M(2)L(9) complexes Mo-2((mu-O)(mu-Cl)Cl-3(PMe(3))(4) and W-2(mu-S)(mu-Cl)Cl-3(PMe(3))(4) have also been observed. Mo-2(mu-S)(mu-Cl)Cl-3(PMe(3))(4) (2) reacts with chloride ion, acetonitrile, and carbon monoxide to give new M(2)L(10) dimer adducts, [Mo-2(mu-S)(mu-Cl)Cl-4(PMe(3))(4)]NMe(4) (5[NMe(4)]), Mo-2(mu-S)(mu-Cl)Cl-3(PMe(3))(4)(CH3CN) (6), and Mo-2(mu-S)(mu-Cl)Cl-3(PMe(3))(4)(CO) (7, 8a), the first two of which have been characterized by X-ray crystallography. The Mo-2(mu-S)(mu-Cl) core is maintained in each of these dimers, with very similar metrical data, but adduct formation is accompanied by rearrangement of the other ligands. Alkynes react with 2 to form M(2)L(10) dimers with bridging sulfide and alkyne ligands, such as Mo-2(mu-S)(mu-MeC=CMe)Cl-4(PMe(3))(4) (10a). The CO and 2-butyne adducts have labile phosphine ligands trans to the bridging sulfur atom, and M(2)L(9) species analogous to 2 can be isolated. The structures, diamagnetism, and lability of phosphine ligands trans to the mu-sulfido (or -oxo) in these dimers are suggested to arise from strong S-->M pi-bonding in the Mo-2(mu-S)(mu-X) core. Crystal data : for 2.C7H8, C2/c, Z = 8, a = 37.822(5) Angstrom, b = 9.6820(9) Angstrom, c = 22.249(3) Angstrom, V = 6635(3) Angstrom(3), refined to R = 4.3%, R(w) = 4.4%, GOF = 1.324; for 5[NMe(4)], P2(1)/n, Z = 4, a = 11.807(1) Angstrom, b = 16.105(2) Angstrom, c = 17.567(3) Angstrom, V = 3302.8(7) Angstrom(3), refined to R = 3.8%, R(w) = 5.4%, GOF = 1.10; for 6, P $($) over bar$$ 1 Z = 2, a = 8.578(1) Angstrom, b = 10.176(1) Angstrom, c = 17.562(2) Angstrom, V = 1420.7(7) Angstrom(3), refined to R = 3.6%, R(w) = 4.3%, GOF = 1.114; for 10a,, Pnma, Z = 4, a = 22.166(2) Angstrom,b = 13.979(1) Angstrom, c = 9.455(1) Angstrom, V = 2929.8(4) Angstrom(3), refined to R = 2.7%, R(w) = 3.8%, GOF = 0.71.