Synthesis and reactivity of the osmium(III) pentamethylcyclopentadienyl complex (C5Me5)2Os2Br4.: X-ray crystal structures of (C5Me5)2Os2Br4, (C5Me5)2Os2(μ-O)Br4, and (C5Me5)2Os2(μ-NPh)2Br2

Treatment of H2OsBr6 with C5Me5H in tert-butyl alcohol affords the dinuclear osmium(III) species (C5Me5)(2)Os2Br4, 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) A 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)(2)Os2Br4 with oxygen, bromine, lithium anilide, acetonitrile, and norbornadiene (NBD) are described, affording the compounds (C5Me5)(2)Os-2(mu-O)Br-4, (C5Me5)OsBr4, (C5Me5)(2)Os-2(mu-NPh)(2)Br-2, [(C5Me5)Os(MeCN)(3)][BPh4], and (C5Me5)Os(NBD)Br; the crystal structures of the bridging oxo and bridging imido complexes are given.