Chalcogen-stabilized dimolybdaboranes 3-5 (3: [(Cp*MO) 2B4H5Se(Ph)], 4: [(Cp*MO) 2B4H3Se2(SeCH2Ph)] and 5: [(Cp*MO)2B3H6(BSR)(μ- η1-SR)] (R = 2,6-(tBu)2-C6H 2OH)) have been isolated from the mild pyrolysis of dichalcogenide ligands, RE-E'R (R = Ph: E = S, E' = Se; R = CH2Ph, [2,6-(tBu) 2-C6H2OH]: E = E' = Se, S) and [(Cp*MO)2B4H8], 2, an intermediate generated from the reaction of [Cp/MoCl4] (1) (Cp* = η5-C5Me5), with [LiBH4.thf]. The geometry of [(Cp*MO)2B4H5Se(Ph)] is similar to that of [(Cp*MO)2B5H9], in which one BH3 unit on the open face is replaced by a triple bridged selenium atom. All the compounds have been characterized in solution by 1H, 11B, 13C NMR and IR spectroscopy and elemental analysis. The structural types were unequivocally established by X-ray crystallographic analysis of compounds 3-5. © 2011 Elsevier Ltd.