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Aiming at highly efficient molecular catalysts for water oxidation, a mononuclear ruthenium complex Ru^II(hqc)(pic)₃ (1; H₂hqc = 8-hydroxyquinoline-2-carboxylic acid and pic = 4-picoline) containing negatively charged carboxylate and phenolate donor groups has been designed and synthesized. As a comparison, two reference complexes, Ru^II(pdc)(pic)₃ (2; H₂pdc = 2,6-pyridine-dicarboxylic acid) and Ru^II(tpy)(pic)₃ (3; tpy = 2,2′:6′,2″-terpyridine), have also been prepared. All three complexes are fully characterized by NMR, mass spectrometry (MS), and X-ray crystallography. Complex 1 showed a high efficiency toward catalytic water oxidation either driven by chemical oxidant (Ce^IV in a pH 1 solution) with a initial turnover number of 0.32 s^–1, which is several orders of magnitude higher than that of related mononuclear ruthenium catalysts reported in the literature, or driven by visible light in a three …