The reaction of [Os(H)(Br)(CO)(PPh3)(3)], 5, with 2-(phenylazo)pyridine (pap) in boiling dry heptane has afforded the azo anion radical complex [Os(pap(.-))(Br)(CO)(PPh3)(2)], 6a, as the major product and [Os(pap)(H)(CO)(PPh3)(2)]-Br, 7, as a minor byproduct. Upon replacing pap by the better pi-acceptor azo-2,2'-bipyridine (abp) in the above synthesis, the radical complex [Os(abp(.-))(Br)(CO)(PPh3)(2)], 6b, becomes the sole product. It is proposed that 6 is formed via homolytic cleavage of the Os-II bond in 5; in the formation of 7, the Os-Br bond of 5 is heterolytically cleaved. The X-ray structures of 6b and 7(.)CH(2)Cl(2) have been determined. In 6b, the N-N length is 1.35(2) Angstrom, consistent with the anion radical description; in 7(.)CH(2)Cl(2) the length is 1.27(1) Angstrom. The spin-bearing extended Huckel HOMO in a model of 6 is found to be similar to 70% azo-pi* in character associated with a small metal contribution. An electronic band observed in the range 600-700 nm in solutions of 6 is assigned to the HOMO --> LUMO transition, the LUMO being 95% pyridine-pi* in character. One-electron paramagnetic 6 displays well-defined anisotropic EPR features hear g = 2.00. The anisotropy arises from the metal character of HOMO and is magnified by the large spin-orbit coupling in osmium. In a moisture-free environment 6 is indefinitely stable in the solid state, but in CH2Cl2-MeCN solution 6a is rapidly oxidized by air, affording [Os(pap)(Br)(CO)(PPh3)(2)](+), 6a(+), which has been isolated as the diamagnetic PF6- salt; 6b(+)PF(6)(-) has been similarly prepared. The voltammetric reduction potentials of the 6(+)/6 couple follow the order 6a(+)/6a < 6b(+)/6b, and the carbon monoxide stretching frequencies follow the order 6a < 6b and 6a+ < 6b(+). These trends are consistent with the pi-acidity order pap < abp. Crystal data are as follows: (6b, C47H38BrN4OOsP2) monoclinic, space group P2(1)/c (no. 14), a = 10.215(4) Angstrom, b = 17.634(7) Angstrom, c 22.473(8) Angstrom, beta = 97.67(3)degrees, Z = 4; (7(.)CH(2)Cl(2), C49H42BrCl2N3OOsP2) monoclinic, space group P2(1)/n (no. 14), a 15.323(7) Angstrom, b = 15.201(6) Angstrom, c = 19.542(7) Angstrom, beta = 92.51(3)degrees, Z = 4.