Nitrosylruthenium derivatives having NO+ and NO. states have been synthesized in a stepwise manner starting from [Ru II(trpy)(pdt)(Cl)](ClO4) {[1](ClO4)} → [RuII(trpy)(pdt)-(CH3CN)](ClO4)2 {[2](ClO4)2] → [RuII(trpy)(pdt)(NO 2)](C1O4) ([3](ClO4)} → [Ru II(trpy)(pdt)(NO+)](ClO4)3 {[4](ClO4)3} → [RuII(trpy)(pdt)(NO .)](CLO4)2 {[4](ClO4)2} [trpy = 2,2′:6′,2″-terpyridine, pdt = 5,6-diphenyl-3-pyridyl- as-triazine]. The molecular identity of [1](ClO4) and [2](ClO 4)2 and the subsequent stereoretentive transformation of [1](ClO4) → [2]-(ClO4)2 have been authenticated by single-crystal X-ray structures. Electrochemical and spectral features are investigated as a function of the monodentate ligands (Cl -, CH3CN, NO2-, NO+, NO.). The kinetic and thermodynamic aspects of the reaction of the moderately strong electrophilic {RuII-NO+} center [ν(NO): 1944 cm-1] in [4]3+ with a nucleophile such as OH- have been studied. The nitrosyl species [RuII(trpy) (pdt)-(NO+)]3+ ([4]3+) can be selectively reduced to [RuII(trpy)(pdt)-(NO.)]2+ ([4] 2+) electrochemically as well as chemically by hydrazine hydrate. On exposure to light an acetonitrile solution of [RuII(trpy)(pdt) (NO.)]2+ ([4]2+) undergoes slow photocleavage of the RuII-NO bond over a period of 4 h, resulting in the corresponding solvated species [RuII(trpy)(pdt)(CH 3CN)]2+ ([2]2+). The rate of photolability of the RuII-NO bond in [4]2+ has been monitored spectrophotometrically. © Wiley-VCH Verlag GmbH & Co. KGaA, 2007.