The mononuclear ruthenium(II) complex [Ru]2+ (Ru = Ru(dpp)(pic)2, where dpp is the tetradentate 2,9-dipyrid-2'-yl-1,10-phenanthroline ligand and pic is 4-picoline) reported by Thummel's group (Inorg. Chem. 2008, 47, 1835-1848) that contains no water molecule in its primary coordination shell is evaluated as a catalyst for water oxidation in artificial photosynthesis. A detailed theoretical characterization of the energetics, thermochemistry, and spectroscopic properties of intermediates allowed us to interpret new electrochemical and spectroscopic experimental data, and propose a mechanism for the water oxidation process that involves an unprecedented sequence of seven-coordinate ruthenium complexes as intermediates. This analysis provides insights into a mechanism that generates four electrons and four protons in the solution and a gas-phase oxygen molecule at different pH values. On the basis of the calculations and corroborated substantially by experiments, the catalytic cycle goes through [²RuIII3+ and ²RuVO]3+ to [¹RuIV(OOH)]3+ then [²RuIII···³O2]3+ at pH 0, and through [³RuIV(O)]2+, [²RuV(O)]3+, and [¹RuIV(OO)]2+ at pH 9 before reaching the same [²RuIII(···³O2)]3+ species, from which the liberation of the weakly bound O2 might require an additional oxidation to form [³RuIV(O)]2+ to initiate further cycles involving all seven-coordinate species. [ABSTRACT FROM AUTHOR]