Water oxidation is a key reaction in natural photosynthesis and in many schemes for artificial photosynthesis. Although metal complexes capable of oxidizing water based on Ru, Mn, and Ir are known, a significant question is whether or not dimeric or higher order structures are required for water oxidation. We report here single-site catalytic water oxidation by the monomeric complexes [Ru(tpy)(bpm)(OH2)](2+) and [Ru(tpy)(bpz)(OH2)](2+) (tpy is 2,2':6',2 ''-terpyridine; bpm is 2,2'-bipyrimidine; bpz is 2,2'-bipyrazine) by a well-defined mechanism involving Ru-v square O.