A ligation of Ru(tpy)Cl-3 (tpy = 2,2':6',2 ''-terpyridine) with 2-(2-pyridyl)-1,8-naphthyridine) (pynp) in the presence of LiCl gave distal-[Ru(tpy)(pynp)Cl](+) (d-1Cl) selectively, whereas the ligation gave proximal-[Ru(tpy)(pynp)OH2](2+) (p-1H(2)O) selectively in the absence of halide ions. (The proximal/distal isomers were defined by the structural configuration between the 1,8-naphthyridine moiety and the aquo or chloro ligand.) An aquation reaction of d-1Cl quantitatively afforded distal-[Ru(tpy)(pynp)OH2](2+) (d-1H(2)O) in water, and d-1H(2)O is quantitatively photoisomerized to p-1H(2)O. The mechanism of the photoisomerization was investigated by transient absorption spectroscopy and quantum chemical calculations. The temperature dependence of the transient absorption spectral change suggests existence of the thermally activated process from the (MLCT)-M-3 state with the activation energy (Delta E = 49 kJ mol(-1)), which is close to that (41.7 kJ mol(-1)) of the overall photoisomerization reaction. However, quantum chemical calculations suggest another activation process involving the conformational change of the pentacoordinated distal structure to the proximal structure. Quantum chemical calculations provide redox potentials and pK(a) values for proton coupled electron transfer reactions from Ru-II-OH2 to Ru-IV=O in good agreement with experiments and provide an explanation for mechanistic differences between d-1H(2)O and p-1H(2)O with respect to water oxidation. The calculations show that water nucleophilic attack (WNA) on d-[Ru-V-O](3+) (the ruthenyl oxo species derived from d-1H(2)O, calculated Delta G double dagger of 87.9 kJ/mol) is favored over p-[Ru-V-O](3+) (calculated Delta G double dagger of 104.6 kJ/mol) for O-O bond formation. Examination of the lowest unoccupied molecular orbitals in d- and p-[Ru-V-O](3+) indicates that more orbital amplitude is concentrated on the [Ru-O] unit in the case of d-[Ru-V-O](3+) than in the case of p-[Ru-V-O](3+), where some of the amplitude is instead delocalized over the pynp ligand, making this isomer less electrophilic.