Reaction of 2-(arylazo)phenols (H(2)ap-R; R = OCH3, CH3, H, Cl, and NO2) with [Os(PPh3)(2)(CO)(2)(HCOO)(2)] affords a family of organometallic complexes of osmium(II) of type [Os(PPh3)(2)(CO)(ap-R)] where the 2-(arylazo)phenolate ligand is coordinated to the metal center as a tridentate C,N,O-donor. Structure of the [Os(PPh3)(2)(CO)(ap-H)] complex has been determined by X-ray crystallography. All the [Os(PPh3)(2)(CO)(ap-R)] complexes are diamagnetic and show characteristic H-1 NMR signals and intense MLCT transitions in the visible region. They also show emission in the visible region at ambient temperature. Cyclic voltammetry on the [Os(PPh3)2(CO)(ap-R)] complexes shows a reversible Os(II)-Os(III) oxidation within 0.39-0.73 V vs SCE, followed by a reversible Os(III)-Os(IV) oxidation within 1.06-1.61 V vs SCE. Coulometric oxidation of the [Os(PPh3)(2)(CO)(ap-R)] complexes generates the [Os-III-(PPh3)(2)(CO)(ap-R)](+) complexes, which have been isolated as the hexafluorophosphate salts. The [Os-III(PPh3)(2)(CO)(ap-R)]PF6 complexes are one-electron paramagnetic and show axial ESR spectra. In solution they behave as 1:1 electrolytes and show intense LMCT transitions in the visible region. The [Os-III(PPh3)(2)(CO)(ap-R)]PF6 complexes have been observed to serve as mild one-electron oxidants in a nonaqueous medium.