Reaction of chloranilic acid (H(2)ca) with [Os(bpy)(2)Br-2] (bpy = 2,2'-bipyridine) affords a dinuclear complex of type [{Os(bpy)(2)}(2)(ca)](2+), isolated as the perchlorate salt. A similar reaction of H(2)ca with [Os(PPh3)(2)(pap)Br-2] (pap = 2-(phenylazo)pyridine) affords a dinuclear complex of type [{Os(PPh3)(2)(pap)}(2)(ca)](2+) (isolated as the perchlorate salt) and a mononuclear complex of type [Os(PPh3)(2)(pap)(ca)]. Reaction of H(2)ca with [Os(PPh3)(2)(CO)(2)(HCOO)(2)] gives a dinuclear complex of type [{Os(PPh3)(2)(CO)(2)}(2)(r-ca)], where r-ca is the two electron reduced form of the chloranilate ligand. The structures of the [{Os(PPh3)(2)(pap)}(2)(ca)](CIO4)(2), [Os(PPh3)(2)(pap)(ca)], and [{Os(PPh3)(2)(CO)(2)}2(r-ca)] complexes have been determined by X-ray crystallography. In the [{Os)(bpy)(2)}(2)(ca)](2+) and [{Os(PPh3)(2)(pap)}(2)(ca)](2+) complexes, the chloranilate dianion is serving as a tetradentate bridging ligand. In the [{Os(PPh3)(2)(pap)(ca)] complex, the chloranilate dianion is serving as a bidentate chelating ligand. In the [{Os(PPh3)(2)(CO)(2)}(2)(r-ca)] complex, the reduced form of the chloranilate ligand (r-ca(4-)) is serving as a tetradentate bridging ligand. All the four complexes are diamagnetic and show intense metal-to-ligand charge-transfer transitions in the visible region. The [Os(PPh3)(2)(pap)(ca)] complex shows an Os(II)-Os(III) oxidation, followed by an Os(III)-Os(IV) oxidation on the positive side of a standard calomel electrode. The three dinuclear complexes show two successive oxidations on the positive side of SCE. The mixed-valent Os-II-Os-III species have been generated in the case of the two chloranilate-bridged complexes by coulometric oxidation of the homovalent Os-II-Os-III species. The mixed-valent Os-II-Os-III species show intense intervalence charge-transfer transitions in the near-IR region.