The reactivity of sigma-arylruthenium cyclometallate [Ru(L-SB1)(PPh3)(2)Cl] [1; (LH2)-H-SB1 = 4-methyl-2-(4-nitrobenzylideneamino)phenol] with nitric oxide (NO) gave rise to nitrosylation at the metal center, ring nitration, and oxidative cyclization, affording benzoxazole derivative formation. The molecular structure of the resultant nitrosyl complex,[Ru(L-PB1)(PPh3)(2)(NO)Cl(ClO4) [2; (LH)-H-PB1 = 5-methyl-7-nitro-2-(4-nitrophenyl)benzoxazole]was determined, and a different sigma-arylruthenium cyclometallate was characterized in which the benzoxazole derivative was found to be coordinated to the metal center. The crystal structure and IR and NMR spectral data confirmed the formation of a diamagnetic {RuNO}(6) species with a S = 0 ground state and a {(RuNO+)-N-II}(6) description of the {RuNO}(6) moiety. Coordinated NO in the resultant complex 2 was photolabile under visible light and was transferred to reduced myoglobin.