The complexes of osmium with tacn (1,4,7-triazacyclononane) and Me(3)tacn (1,4,7-trimethyl-1,4,7-triazacyclononane), [LOs (eta(6)-C6H6)](PF6)(2) (L = tacn) and LOsCl(3) (L = tacn, Me(3)tacn), have been prepared by substitution of L on [Os(eta(6)-C6H6)Cl-2](2) or [Os2Cl8](2-), respectively. Reaction of LOsCl(3) with neat triflic acid leads to partial replacement of chloride and formation of the binuclear Os(III)-Os(III) complexes [LOs(mu-Cl-3)OsL](PF6)(3) (L = tacn, Me(3)tacn). The binuclear nature was established by NMR spectroscopy and elemental analysis and, for L = tacn, a partially refined X-ray crystal structure which shows the Os-Os separation to be 2.667 Angstrom, indicative of significant metal-metal bonding. Reduction of [LOs(mu-Cl-3)OsL](3+) over zinc amalgam in either aqueous or non-aqueous solution yields the intensely colored Os(II)-Os(III) mixed-valence ions [LOs(mu-Cl-3)OsL](2+). Electrochemical measurements on [LOs(mu-Cl-3)OsL](3+) in CH3CN reveal the reversible formation of the mixed valence ions. These are further reduced at lower potential to the Os(II)-Os(II) binuclear species, reversibly for L = Me(3)tacn. (Me(3)tacn)OsCl3 is oxidized by persulfate ion to give [(Me(3)tacn)OsCl3](+); zinc amalgam reduction in an aqueous solution at high concentration produces the binuclear complex [(Me(3)tacn)Os(mu-Cl-3)Os(Me(3)tacn)](3+) or, at low concentration, a solution containing an air sensitive osmium(II) species. Addition of BPh(4)(-) results in the eta(6)-arene zwitterion [(Me(3)tacn)Os(eta(6)-C(6)H(5)BPh(3))](+), which was characterized by X-ray diffraction on the BPh(4)(-) salt. The compound crystallizes in the triclinic space group P1 with a = 11.829(2) Angstrom, b = 12.480(3) Angstrom, c = 17.155(4) Angstrom, alpha = 84.42(2)degrees, beta = 83.52(2)degrees, gamma = 71.45(2)degrees, V = 2380(2) Angstrom(3), Z = 2, and R = 7.62%, and R(w) = 7.39%.