The new complex trans-[NCRu(py)(4)(CN)Ru(py)(4)NO](PF6)(3) (I) was synthesized. In acetonitrile solution, I shows an intense visible band (555 nm, is an element of = 5800 M-1 cm(-1)) and other absorptions below 350 rim, associated with dpi --> pi*(py) and pi(py) --> pi*(py) transitions. The visible band is presently assigned as a donor-acceptor charge transfer (DACT) transition from the remote Ru(II) to the delocalized {Ru-parallel to-NO+} moiety. Photoinduced release of NO is observed upon irradiation at the DACT band. Application of the Hush model reveals strong electronic coupling, with H-DA = similar to2000 cm(-1). The difference between the optical absorption energy and redox potentials for the donor and acceptor sites (Ru-parallel to\,Ru-parallel to, 1.40 V, and NO+/NO, 0.50 V, vs Ag/AgCl, 3 M KCl, respectively) (hnu - DeltaE(red)) is 1.33 eV, a large value which probably relates to the significant changes in distances and angles for the Ru-N-O moiety upon reduction. UV-vis absorptions, IR frequencies, and redox potentials are solvent-dependent. Controlled potential reduction (of NO) and oxidation (of Ru(II) associated with the dicyano-chromophore) of I afford stable species, [NCRuparallel to(py)(4)(CN)RU(py)(4)NO](2+) (I-red) and [NCRuparallel to\(py)(4)(CN)Ru(py)(4)NO](4+) (I-ox), respectively, which are characterized by UV-vis and IR spectroscopies. I-red shows an EPR spectrum characteristic of {Ru(II)-NO.} complexes. Compound I is electrophilically reactive in aqueous solution above pH 5: values of the equilibrium constant for the reaction [NCRu(py)(4)(CN)Ru(py)(4)NO](3+)+ 2 OH- reversible arrow [NCRu(py)(4)(CN)Ru(py)(4)NO2](+) + H2O, K = 3.2 +/- 1.4 x 10(15) M-2, and of the rate constant for the nucleophilic addition of OH-, k = 9.2 +/- 0.2 x 10(3) M-1 s(-1)(25 degreesC, / = 1 M), are obtained, with DeltaH(double dagger) = 90.7 +/-3.8 kJ mol(-1) and DeltaS(double dagger) = 135 +/- 13 J K-1 mol(-1). The oxidized complex, I-ox, shows an enhanced electrophilic reactivity toward OH-. This addition reaction is followed by irreversible processes, which most probably lead to disproportionation of bound nitrite and other products.