Treatment of H2OsBr6 with C5Me5H in tert-butyl alcohol affords the dinuclear osmium(III) species (C5Me5)2Os2Br4, 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 singlet−triplet splitting of >800 cm-1. The reactions of (C5Me5)2Os2Br4 with oxygen, bromine, lithium anilide, acetonitrile, and norbornadiene (NBD) are described, affording the compounds (C5Me5)2Os2(μ-O)Br4, (C5Me5)OsBr4, (C5Me5)2Os2(μ-NPh)2Br2, (C5Me5)Os(MeCN)3BPh4, and (C5Me5)Os(NBD)Br; the crystal structures of the bridging oxo and bridging imido complexes are given.