A series of mononuclear thio complexes of pentamethylcyclopentadienyl-molybdenum(VI) and -tungsten-(VT) have been synthesized via C-S bond-cleaving reactions of thiolates. Use of Li2S2 for sulfurization of Cp*MoCl4 resulted in the known dinuclear complex, anti-Cp*Mo-2(2)(S)(2)(mu-S)(2) (1), while the analogous reaction of Cp*WCl4 gave rise to anti-Cp*W-2(2)(S)(2)(mu-S)(2) (2) and (PPh4)[Cp*W(S)(3)] (3), the latter of which was isolated after the subsequent cation exchange reaction with PPh4Br. In contrast, the reaction of Cp*WCl4 with Li(2)edt (edt = SCH2CH2S) followed by treatment with PPh4Br generated 3 as the sole isolable product in high yield. A similar reaction between Cp*WCl4 and (LiSBu)-Bu-t afforded Cp*W(S)(2)((SBu)-Bu-t) (6), which turned out to be thermally unstable in solution and gradually degraded to 2. In these reactions of Cp*WCl4 with lithium thiolates, a facile C-S bond cleavage took place and the tungsten atom was oxidized from W(V) to W(VI). On the other hand, the Mo(IV) thiolate complexes, Cp*Mo((SBu)-Bu-t)(3) (4) and (PPh4)[Cp*Mo(edt)(2)] (5), were formed from the Cp*MoCl4/(LiSBu)-Bu-t and Cp*MoCl4/Li(2)edt/PPh4Br reaction systems. The complex 4 was readily oxidized by dry O-2 producing Cp*Mo(O)(2)((SBu)-Bu-t) (7) exclusively, while the reactions of 4 with NH2NMe2 and NH2NHPh occurred slowly to yield Cp*Mo(S)(2)((SBu)-Bu-t) (8). The hydrazines acted as oxidants, presumably by cleaving the N-N bond, and promoted the C-S bond rupture of tert-butyl thiolate and concomitant oxidation of molybdenum from Mo(IV) to Mo(VI). Elemental sulfur Ss and grey selenium also acted as oxidants in the reactions with 4, leading to a complex mixture of products. From the 4 + Ss reaction, the complexes 1, 8, and Cp*Mo(O)(S)((SBu)-Bu-t) (9) were produced, and the 4 + Se reaction lead to 8 and anti-Cp*Mo-2(2)(E)(2)(mu-E)(2) (10; E = S, Se). Finally treatment of 8 with Li2S2 and PPh4Br afforded (PPh4)[Cp*Mo(S)(3)] (11). We found that 11 was synthesized more easily by a one-pot reaction of 4, 1/4 equiv of S-8, and Li2S2 in THF. The trithio complexes, 3 and 11, reacted very cleanly with PhC=CPh generating (PPh4)[Cp*M(S)(S2C2Ph2)] (M = W (12), Mo (13)), respectively. A kinetic study of these reactions showed that they were first order in PhC=CPh and first order in 3 or 11 with appreciable negative entropies of activation and that the activation barrier was higher for the molybdenum reaction. The crystal structures of 3-8 and 11-13 were determined by the X-ray analysis.