The reaction of mono- or dichloro-dimolybdenum(III) complexes [Mo2Cp2(mu-SMe)(2)(mu-Cl)(mu-Y)] (Cp = eta(5)-C5H5; 1, Y = SMe; 2, Y = PPh2; 3, Y = Cl) with NaBH4 at room temperature gave in high yields tetrahydroborato (8), hydrido (9) or metallaborane (12) complexes depending on the ancillary ligands. The correct formulation of derivatives 8 and 12 has been unambigously determined by X-ray diffraction methods. That of the hydrido compound 9 has been established in solution by NMR analysis and confirmed by an X-ray study of the mu-azavinylidene derivative [Mo2Cp2(mu-SMe)(2)(mu-PPh2)(mu-N = CHMe)] ( 10) obtained from the insertion of acetonitrile into the Mo – H bond of 9. Reaction of NaBH4 with nitrile derivatives, [Mo2Cp2(mu-SMe)(4-n)(CH3CN)(2n)](n+) (5, n = 1; 6 n = 2), afforded the tetrahydroborato compound 8, together with a mu-azavinylidene species [Mo2Cp2(mu-SMe)(3)(mu-N = CHMe)] (14), when n = 1, and the metallaborane complex 12, together with a mixed borohydrato- azavinylidene derivative [Mo2Cp2(mu-SMe)(2)(mu-BH4)(mu- N = CHMe)] ( 13), when n = 2. The molecular structures of these complexes have been confirmed by X-ray analysis. Preparations of some of the starting complexes (3 and 4) are also described, as are the molecular structures of the precursors [Mo2Cp2(mu-SMe)(2)(mu-X)(mu-Y)] (1, X/ Y = Cl/SMe; 2, X/ Y = Cl/PPh2; 4, X/Y = SMe/PPh2).