The ruthenium oxyl radical complex, [Ru-II(trpy)(Bu(2)SQ)O.-] (trpy = 2,2':6',2"-terpyridine, Bu-2-SQ = 3,5-di-tert-butyl-1,2-benzosemiquinone) was prepared for the first time by the double deprotonation of the aqua ligand of [Ru-III(trpy)(Bu(2)SQ)(OH2)](ClO4)(2), [Ru-III(trpy)(BU2SQ)(OH2)](ClO4)(2) is reversibly converted to [Ru-III(trpy)(Bu(2)SQ)(OH-)](+) upon dissociation of the aqua proton (pK(a) 5.5). Deprotonation of the hydroxo proton gave rise to intramolecular electron transfer from the resultant O2- to Ru-dioxolene. The resultant [Ru-II(trpy)(Bu(2)SQ)O.-] showed antiferromagnetic behavior with a Ru-II-semiquinone moiety and oxyl radical, the latter of which was characterized by a spin trapping technique. The most characteristic structural feature of [Ru-II(trpy)(Bu(2)SQ)O.-] is a long Ru-O bond length (2.042(6) Angstrom) as the first terminal metal-O bond with a single bond length. To elucidate the substituent effect of a quinone ligand, [Ru-III(trpy)(Bu(2)SQ)(OH2)](ClO4)(2) (4CISQ = 4-chloro-1,2-benzosemiquinone) was prepared and we compared the deprotonation behavior of the. aqua ligand with that of [Ru-III(trpy)(Bu(2)SQ)(OH2)](ClO4)(2). Deprotonation of the aqua ligand of [Ru-III(trpy)(4CISQ)(OH2)](ClO4)(2) induced intramolecular electron transfer from OH- to the [Ru-III-(4CISQ)] moiety affording [Ru-II(trpy)(4CISQ)(OH.)](+), which then probably changed to [Ru-II(trpy)(4CISQ)O.-]. The antiferromagnetic interactions. (J values) between Ru-II-semiquinone and the oxyl radical for [Ru-II(trpy)(Bu(2)SQ)O.-] and for [Ru-II(trpy)(4CISQ)O.-] were 2J = -0.67 cm(-1) and -1.97 cm(-1), respectively.