The reaction of [ν-C 5Me 5Mo(CO) 3] 2 with allyl chloride in refluxing THF gives the mononuclear complex [( ν-C 5Me 5)MoCl 2( ν 3-C 3H 5(CO)] ( 1). Thermolysis of complex 1 in THF or toluene in presence of dimethyldisulfide affords the unexpected complex [(ν-C 5Me 5)MoCl] 2(μ-Cl)(μ-O)(μ-SMe) ( 2). Similar reactions are observed with allyl bromide and give the analogous bromide complex 3. X-ray analysis establishes that complex 3 contains a Mo 2(μ-Br)(μ-O)(μ-SMe) core and that the C 5Me 5 ligands are cis with respect to the MoMo axis, as are the terminal Br groups. The formally double MoMo bond is relatively long (2.755(1) Å). An approximate plane of symmetry normal to the MoMo bond passes through the bridging O, Br and S donor atoms and relates the two C 5Me 5MoBr units. Complexes 2 and 3 react with HSMe to afford the μ-oxo-bis(μ-thiolato) products [(ν-C 5Me 5)MoX] 2(μ-O)(μ-SMe) 2 (X Cl ( 4), Br ( 5)) and the replacement of the chloride bridge in complex 2 by HSCH 2CH 2SH leads to [(ν-C 5Me 5)MoCl] 2(μ-O)(μ-SMe)(μ-SCH 2CH 2SH)] ( 6).