Treatment of H 2 OsBr 6 with C 5 Me 5 H in tert-butyl alcohol affords the dinuclear osmium(III) species (C 5 Me 5 ) 2 Os 2 Br 4 , a mono(pentamethylcyclopentadienyl) complex that serves as the key synthetic entry into a wide array of "half-sandwich" complexes of osmium. The X-ray crystal structure shows it to contain two bridging and two terminal bromide ligands, with the Os-Br bond distances being shorter for the bridging bromide ligands than for the terminal bromide ligands. The Os-Os distance of 2.970(1) Å is indicative of a single osmium-osmium bond. The compound is weakly paramagnetic in solution and in the solid state, and the magnetic susceptibility determined over the range 4-300 K gives a singlettriplet splitting of >800 cm -1 . The reactions of (C 5 Me 5 ) 2 Os 2 Br 4 with oxygen, bromine, lithium anilide, acetonitrile, and norbornadiene (NBD) are described, affording the compounds (C 5 Me 5 ) 2 Os 2 (µ-O)Br 4 , (C 5 Me 5 )OsBr 4 , (C 5 Me 5 ) 2 Os 2 (µ-NPh) 2 Br 2 , [(C 5 Me 5 )Os(MeCN) 3 ][BPh 4 ], and (C 5 Me 5 )Os(NBD)Br; the crystal structures of the bridging oxo and bridging imido complexes are given.