A series of oxo-bridged diosmium complexes with tpa ligand (tpa = tris(2-pyridylmethyl)amine) are synthesized. The hydrolytic reaction of the mononuclear osmium complex [(OsCl2)-Cl-III(tpa)]PF6 in aqueous solution containing a sodium carboxylate,yields a mu-oxo-mu-carboxylato-diosmium-(III) complex, [Os-2(III)(mu-O)(mu-RCOO)(tpa)(2)] (PF6)(3) (R = C3H7 (1), CH3 (2), or C6H5 (3)). One-electron oxidation of 1 with (NH4)(2)Ce-IV(NO3)(6) gives a mixed-valent [(OsOsIV)-Os-III(mu-O)(mu-C3H7COO)(tpa)(2)](PF6)(4) complex (4). A mixed-valent di-mu-oxo-diosmium complex, [(OsOsIV)-Os-III(mu-O)(2)(tpa)(2)](PF6)(3) (5), is also synthesized from 1 in an aerobic alkaline solution (pH 13.5). All the complexes exhibit strong absorption bands in a visible-near-infrared region based on interactions of the osmium d pi and oxygen p pi orbitals of the Os-O-Os moiety. The X-ray crystallographic analysis of 1, 3, and 4 shows that the osmium centers take a pseudo-octahedral geometry in the mu-oxo-mu-carboxylato-diosmium core. The mixed-valent osmium(III)osmium(IV) complex 4 has a shorter osmium-oxo bond and a larger osmium-oxo-osmium angle as compared with those of the diosmium(III) complex 1 having the same bridging carboxylate. Crystal structure of 5 reveals that the two osmium ions are bridged by two oxo groups to give an Os-2(mu-O)(2) core with the significantly short osmium-osmium distance (2.51784(7) angstrom), which is indicative of a direct osmium-osmium bond formation with the bond order of 1.5 (sigma(2)pi(2)delta(2)delta*(2)pi*(1) configuration). In the electrochemical studies, the mu-oxo-mu-carboxylato-diosmium(III) complexes exhibit two reversible (OsOsIII)-Os-III/(OsOsIV)-Os-III and (OsOsIV)-Os-III/(OsOsIV)-Os-IV oxidation couples and one irreversible redox wave for the (OsOsIII)-Os-III/(OsOsIII)-Os-II couple in CH3CN. The irreversible reductive process becomes reversible in CH3CN/H2O (1:1 Britton-Robinson buffer; pH 5-11), where the {1H(+)/2e(-)} transfer process is indicated by the plot of the redox potentials against the pH values of the solution of 1. Thus, the mu-oxo-mu-butyrato-diosmium(III) center undergoes proton-coupled electron transfer to yield a mu-hydroxo-mu-butyrato-diosmisum(II) species. The di(mu-oxo) complex 5 exhibits one reversible (OsOsIV)-Os-III/(OsOsIV)-Os-IV oxidation process and one reversible (OsOsIV)-Os-III/(OsOsIII)-Os-III reduction process in CH3CN. The comproportionation constants K-com of the (OsOsIV)-Os-III states for the present diosmium complexes are on the order of 10(19). The values are significantly larger when compared with those of similar oxo-bridged dimetal complexes of ruthenium and rhenium.