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Mononuclear Ru-based water oxidation catalysts containing anionic ancillary ligands have shown promising catalytic efficiency and intriguing properties. However, their insolubility in water restricts a detailed mechanism investigation. In order to overcome this disadvantage, complexes [Ru^II(bpc)(bpy)OH₂]⁺ (1⁺, bpc = 2,2′-bipyridine-6-carboxylate, bpy = 2,2′-bipyridine) and [Ru^II(bpc)(pic)₃]⁺ (2⁺, pic = 4-picoline) were prepared and fully characterized, which features an anionic tridentate ligand and has enough solubility for spectroscopic study in water. Using Ce^IV as an electron acceptor, both complexes are able to catalyze O₂-evolving reaction with an impressive rate constant. On the basis of the electrochemical and kinetic studies, a water nucleophilic attack pathway was proposed as the dominant catalytic cycle of the catalytic water oxidation by 1⁺, within which several intermediates were detected by MS …