A new powerful and oxidatively rugged pyrazolate-based water oxidation catalyst of formula {[Ru-II(py-SO3)(2)(H2O)](2)(mu-Mebbp)}(-), 1(H2O)(2)(-), has been prepared and thoroughly characterized spectroscopically and electrochemically. This new catalyst has been conceived based on a specific ligand tailoring design, so that its performance has been systematically improved. It was also demonstrated how subtle ligand modifications cause a change in the O-O bond formation mechanism, thus revealing the close activation energy barriers associated with each pathway.