Treatment of H2OsBr6 with excess 1,5-cyclooctadiene ( cod) in boiling tert-butyl alcohol affords the polymer [OsBr2(cod)](x) ( 1), which reacts with acetonitrile to form the mononuclear adduct OsBr2(cod)(CH3CN)(2) ( 2). Polymer 1 reacts with potassium trispyrazolylborate (KTp) in ethanol to afford the hydride TpOs(cod) H (3) and the bromide complex TpOs(cod) Br ( 4). Bromide complex 4 reacts with sodium methoxide in methanol to afford TpOs(cod) OMe ( 5), which has been structurally characterized. Treatment of hydride 3 with methyl trifluoromethanesulfonate (MeOTf) in diethyl ether results in loss of methane and formation of the triflate complex TpOs(cod) OTf (6), which reacts with MgMe2 to give the methyl complex TpOs(cod) Me ( 7). The addition of bis( dimethylphosphino) methane (dmpm) to the known compound TpOs(PPh3)(2)Cl yields a mixture of the substitution products TpOs(eta(1)-dmpm)(PPh3) Cl ( 8) and TpOs(eta(2)-dmpm) Cl ( 9); the latter reacts with methyllithium to generate the methyl compound TpOs( dmpm) Me ( 10). NMR and IR data for these new compounds are reported. Crystal data for 5 . MeOH at - 80 degrees C are as follows: monoclinic, P2(1)/n, a = 10.728( 1) angstrom, b = 14.004(2) angstrom, c = 13.906( 2) angstrom, beta = 102.42( 6)degrees, V = 2040.3(5) angstrom(3), Z = 4, R-F = 0.0247 for I >= 2 sigma( I), and R-wF2 = 0.0539 for all data.