Oxygen evolution by water oxidation is an important reaction in the photosynthesis of green plants which supports almost all the lives on the earth. The electrons abstracted from water are used to reduce carbon dioxide after their energy is elevated by solar energy in the photosystems II and I. Since oxygen evolution is a four-electron process, one-electron process must be coupled to achieve four-electron oxidation of two molecules of  water (2H₂O →O₂ + 4H⁺ + 4e‾). Because of this difficult requisite, not many artificial water oxidation systems have been known. Metal oxides such as RuO₂ and MnO₂ have been reported to catalyze oxygen evolution by one-electron oxidant. Di-, tri-, and poly-nuclear metal complexes can catalyze four-electron oxidation of water. Even the mononuclear complex (Ru(NH₃)₅(H₂O)³⁺) has been found recently to catalyze water oxidation in a homogeneous aqueous solution probably through an interaction of the intermediates which would enable four-electron oxidation of two molecules of water. Di-μ-oxo Mn complexes, which have been studied as photosynthetic water oxidation complex models and act as one-electron oxidant, could not oxidize water under homogeneous conditions, but did oxidize water to evolve oxygen under heterogeneous conditions.