With the inspiration from an oxygen evolving complex (OEC) in Photosystern II (PSII), a mononuclear Ru(II) complex with a tetradentate ligand containing two carboxylate groups has been synthesized and structurally characterized. This Ru(II) complex showed efficient catalytic properties toward water oxidation by the chemical oxidant cerium(IV) ammonium nitrate. During the process of catalytic water oxidation, Ru(III) and Ru(IV) species have been successfully isolated as intermediates. To our surprise, X-ray crystallography together with HR-MS revealed that the Ru(IV) species is a seven-coordinate Ru(IV) dimer complex containing a [HOHOH](-) bridging ligand. This bridging ligand has a short O center dot center dot center dot O distance and is hydrogen bonded to two water molecules. The discovery of this very uncommon seven-coordinate Ru(IV) dimer together with a hydrogen bonding network may contribute to a deeper understanding of the mechanism for catalytic water oxidation. It will also provide new possibilities for the design of more efficient catalysts for water oxidation, which is the key step for solar energy conversion into hydrogen by tight-driven water splitting, the ultimate challenge in artificial photosynthesis.