Heating (NH4)2MS4 (M = W, Mo) in A',7V-dimethylformamide leads to the formation of the trinuclear ions M3S92~. Under similar conditions but in the presence of 1,2-ethanedithiol (edtH2) or o-aminobenzenethiol (abtH2) the dinuclear ions M2S4(edt)22" or M2S4(abt)22" are formed. The complex (PPh4)2Mo3S9 (Ph = C6H5) crystallizes in the space group PI (Z = 1) with unit cell dimensions a = 9.100 (2) Á, b = 10.531 (5) Á, c = 13.286 (6) Á, a = 87.02 (4)°, ß = 84.99 (2)°, and 7 = 83.56 (2)°. The structure shows two external tetrahedral MoS4 units chelating a central square-pyramidal MoS unit. The Mo-Mo-Mo angle is 155.0 (1)°. Key distances (Á) are as follows: Mo-S(terminal to central Mo) = 2.086 (4); Mo-S(terminal to external Mo) = 2.134 (4), 2.145 (4), 2.136 (3), 2.148 (3); Mo-S(bridge to external Mo) = 2.231 (3), 2.247 (3), 2.251 (3), 2.234; Mo-S(bridge to central Mo) = 2.429(3), 2.369 (3), 2.389 (3), 2.388 (3); Mo-Mo = 2.982 (1), 2.924 (1). The cyclic voltammetry of the trinuclear complexes is reported. The complexes M3S92~a re formulated as mixed-valence VI, IV, VI compounds, and a mechanism is proposed for their formation.The chemistry of Mo-S compounds has attracted much attention in the past,1 and recent reports have revealed the new binary anions Mo3S(S2)62~,2 Mo2(S2)62',2 MoS(S4)22',3 and Mo2S4(S2)(S4)2".4 In these complexes Mo is bound to S in the form of sulfido (S2~), persulfido (S22-) or tetrasulfido (S42") ligands, illustrating the structural variety that is possible when the chemically versatile elements Mo and S are present.5The simplest binary Mo-S moiety is the MoS42" ion. While this ion has been known for many years, its chemical reactivity has only recently been appreciated. MoS42" has found extensive use in the syntheses of heteronuclear Fe-Mo-S complexes such as Fe(MoS4)23"6•7 and Mo2Fe6S9(SEt)83".8 In part the proliferation of compounds of these types is a result of attempts to find analogues of the Mo-Fe center of the nitrogenase enzyme system.9 Indeed, some of the compounds prepared show spectroscopic (EXAFS and EPR) features6,10 similar to those exhibited by the Fe-Mo center in the native enzyme11 and in the isolated Fe-Mo cofactor.12,13 However, interest in MoS42" extends beyond its interaction with Fe as it also forms complexes with elements in the Ni, Cu, and Zn subgroups14,15 as well as with Mo16 itself. In many of these complexes, the MoS4 moiety remains intact and can be regarded as a bidentate ligand.In this paper we report the thermal reaction of MoS42' in A.TV-dimethylformamide, which yields the new binary Mo-S(1) Stiefel, E. I.