Ruthenium(II)-acetonitrile complexes having eta(3)-tris(2-pyridylmethyl)amine (TPA) with an uncoordinated pyridine ring and diimine such as 2,2'-bipyrimidine (bpy) and 2,2'-bipyrimidine (bpm), [Ru(II)(eta(3)-TPA)(diimine)(CH(3)CN)](2+), reacted with m-chloroperbenzoic acid to afford corresponding Ru(II)-acetonitrile complexes having an uncoordinated pyridine-N-oxide aim, [Ru(II) (eta(3)-TPA-O)(diimine)(CH(3)CN)](2+), with retention of the coordination environment. Photoirradiation of the acetonitrile complexes having amine and the eta(3)-TPA with the uncoordinated pyridine-N-oxide arm afforded a mixture of [Ru(II)(TPA)(diimine)](2+), intermediate spin (S = 1) Ru(IV)-oxo complex with uncoordinated pyridine arm, and intermediate spin Ru(IV)-oxo complex with uncoordinated pyridine-N-oxide arm. A R(II) complex bearing an oxygen bound pyridine-N-oxide as a ligand and bpm as a diimine ligand- was also obtained, and its crystal structure was determined by X-ray crystallography. Femtosecond laser flash photolysis of the isolated O-coordinated Ru(II)-pyridine-N-oxide Complex has been investigated to reveal the photodynamics. The Ru(IV)-oxo complex With an uncoordinated pyridine Moiety was alternatively prepared by reaction Of the corresponding acetonitrile complex with 2,6,-dichloropyridine-N-oxide (Cl(2)py-O) to identify the Ru(IV)-oxo species. The formation of Ru(IV)-oxo complexes was concluded to proceed via intermolecular oxygen atom transfer from the uncoordinated pyridine-N-oxide to a Ru(II) center on the basis of the :results of the reaction with Cl(2)py-O and the concentration dependence of the consumption of the starting Ru (II) complexes having the uncoordinated pyridine-N-oxide moiety. Oxygenation reactions of organic substrates by [Ru(II)(eta(3)-TPA-O)(diimine)(CH(3)CN)](2+) were examined under irradiation (at 420 +/- 5 nm) and showed selective allylic oxygenation of cyclohexene to give cyclohexen-1-ol and cyclohexen-1-one and cumene oxygenation to afford cumyl alcohol and acetophenone.