The reactivity Of [MoS4](2-) (1) toward PMe3 was explored in the presence and absence of proton donors. Whereas MeCN solutions of (Et4N)(2)[MoS4] and PMe3 are stable, in the presence of H2S such solutions catalyze formation of H-2 and SPMe3. Addition of NH4+ to such solutions afforded MoS2(PMe3)(4) (2), which can be prepared directly from (NH4)(2)[1]. Compound 2 is reactive toward thiols via a process proposed to involve the initial dissociation of one PMe3 ligand, a hypothesis supported by the relative inertness of trans-MoS2(dmpe)(2). Benzene solutions of 2 react with EtSH to give Mo-2(mu-S)(mu-SH)(PMe3)(4)(SEt)(3) (3Et). Analogous reactions with thiocresol (MeC6H4SH) and H2S gave Mo-2(mu-S)(mu-SH)(PMe3)(4)(SR)(3) (R = tol, H). Crystallographic analyses of 3Et, 3H, and 3tol indicate dinuclear species with seven terminal ligands and a Mo-2(mu-SR)(mu-S) core (r(Mo-Mo) = 2.748(1) Angstrom). From reaction mixtures leading to 3Et from 2, we obtained the intermediate Mo-2(IV)(mu-S)(2)(SEt)(4)(PMe3)(2) (4), an edge-shared bis(trigonal pyramidal) structure. Compounds 3H and 3Et react further with H2S to give Mo-4(mu(2)-S)4(mu(3)-S)(2)(PMe3)(6)(SH)(2) (5H) and o(0)4(mu(2)-S)(4)(mu(3)-S)(2)(PMe3)(6)(SEt)(2) (5Et), respectively. Analogously, W-4(mu(2)-S)(4)(mu(3)-S)(2)(PMe3)(6)(SH)(2) was synthesized from a methanol solution of (NH4)(2)WS4 With H2S and PMe3. A highly accurate crystallographic analysis of (NH4)(2)MoS4 (R-1 = 0.0193) indicates several weak NH...S interactions.