The reaction of mono- or dichloro-dimolybdenum(m) complexes [Mo 2Cp2(μ-SMe)2(μ-Cl)(μ-Y)] (Cp=η5-C5H5; 1, Y = SMe; 2, Y = PPh 2; 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 μ-azavinylidene derivative [Mo 2Cp2(μ-SMe)2(μ-PPh2)(μ-N= CHMe)] (10) obtained from the insertion of acetonitrile into the Mo-H bond of 9. Reaction of NaBH4, with nitrile derivatives, [Mo2Cp 2(μ-SMe)4-n(CH3CN)2n] n+ (5, n= 1; 6 =2), afforded the tetrahydroborato compound 8, together with a μ-azavinylidene species [Mo2Cp2(μ- SMe)3(μ-N=CHMe)] (14), when n= 1, and the metallaborane complex 12, together with a mixed borohydrato-azavinylidene derivative [Mo 2Cp2(μ-SMe)2(μ-BH4)(μ-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(μ-SMe)2(μ-X)-(μ-Y)] (1, X/Y = Cl/SMe; 2, X/Y = Cl/PPh2; 4, X/Y = SMe/PPh2).