Osmium complexes of arenes, acetonitrile, and chloride have been studied. The yield has been significantly increased for the synthesis of (eta(6)-C6H6)Os(NCCH3)Cl-2, an excellent starting material for the synthesis of Osbenzene complexes. Reactivity studies with this complex show that one chloride is displaced by Ag+ in acetonitrile to give [(eta(6)-C6H6)Os(NCCH3)(2)Cl](+) but both chlorides are displaced by Ag+ in acetone to give [(eta(6)-C6H6)Os(acetone)(3)](2+). The actone ligands of the latter complex can be displaced by acetonitrile to give [(eta(6)-C6H6)Os(NCCH3)(3)](2+) and by other arenes (1,3,5-triethylbenzene, hexamethylbenzene, durene, naphthalene) to give [(eta(6)-C6H6)Os(arene)](2+) complexes. Studies of thermal ligand exchange with the [(eta(6)-C6H6)Os(NCCH3)(3-x)Cl-x]((2-x)+) complexes indicate that chloride enhances the rate of thermal acetonitrile exchange. Photolysis of [(eta(6)-C6H6)Os(NCCH3)(3)](2+) and [(eta(6)-C6H6)Os(NCCH3)(2)Cl](+) in acetonitrile results in loss of benzene to produce the new complexes [Os(NCCH3)(6)](2+) and [Os(NCCH3)(5)Cl](+) in high yield. Photolysis of (eta(6)-C6H6)Os(NCCH3)Cl-2 initially results in the formation of Os(NCCH3)(4)Cl-2, which subsequently undergoes photochemical loss of chloride to give [Os(NCCH3)(5)Cl]Cl. Examination of this reaction by H-1 NMR spectroscopy indicates that the cis isomer of Os(NCCH3)(4)Cl-2 is produced. The photolysis of [(eta(6)-C6H6)Os(P(Ph)(3))(NCCH3)Cl](+) in acetonitrile also leads to release of benzene to form the cis product. Electrochemical data acquired for the new acetonitrile complexes ([Os(NCCH3)(6)](2+), [Os(NCCH3)(5)Cl](+), and Os(NCCH3)(4)Cl-2) show highly reversible, one-electron oxidations with the potential a linear function of the stoichiometry. The osmium complexes are about 0.5 V easier to oxidize than the corresponding ruthenium analogs.