Osmium complexes of arenes, acetonitrile, and chloride have been studied. The yield has been significantly increased for the synthesis of (η6-C6H6)Os(NCCH3)Cl2, an excellent starting material for the synthesis of Os-benzene complexes. Reactivity studies with this complex show that one chloride is displaced by Ag+ in acetonitrile to give [(η6-C6H6)Os(NCCH3)2Cl]+ but both chlorides are displaced by Ag+ in acetone to give [(η6-C6H6)Os- (acetone)3]2+. The acetone ligands of the latter complex can be displaced by acetonitrile to give [(η6-C6H6)Os-(NCCH3)3]2+ and by other arenes (1,3,5-triethylbenzene, hexamethylbenzene, durene, naphthalene) to give [(η6-C6H6)Os(arene)]2+ complexes. Studies of thermal ligand exchange with the [(η6-C6H6)Os(NCCH3)3-xClx](2-x)+ complexes indicate that chloride enhances the rate of thermal acetonitrile exchange. Photolysis of [(η6-C6H6)Os(NCCH3)3]2+ and [(η6-C6H6)Os(NCCH3)2Cl]+ in acetonitrile results in loss of benzene to produce the new complexes [Os(NCCH3)6]2+ and [Os(NCCH3)5Cl]+ in high yield. Photolysis of (η6-C6H6)Os(NCCH3)Cl2 initially results in the formation of Os(NCCH3)4Cl2, which subsequently undergoes photochemical loss of chloride to give [Os(NCCH3)5Cl]Cl. Examination of this reaction by lH NMR spectroscopy indicates that the eis isomer of Os(NCCH3)4Cl2 is produced. The photolysis of [(η6-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)5Cl]+, and Os(NCCH3)4Cl2) 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.