During the last years, artificial photosynthesis research has been in the spotlight due to the increasing need for clean and sustainable energy sources. A long-standing challenge has been the study and understanding of light-driven water splitting and the development of efficient catalysts for this process. Water oxidation steps are the bottleneck in this machinery, due to the large energy penalties involved. Because of their catalytic performance, Ruthenium (Ru) complexes are model systems to gain insights into the water oxidation mechanism. Experimental studies recently showed that extension of the π-system from [RuII(tpy)(bpy)(Cl)]1+ to [RuII(tpy)(dppz)(Cl)]1+, strongly and unexpectedly stabilizes the chloro-ligand towards water ligand exchange. Experiments also showed significantly different TON for the respective Ru-H2O catalysts in the water oxidation reaction. Herein, we present a theoretical study of these complexes. Computed reaction barriers for the ligand exchange reaction, after conformational searches over the transition states, are in line with experimental trends of the half-life time of the respective reactants. The analysis of the reaction barriers across the catalytic cycle indicates that the observed differences in TOF are a consequence of the Ru-Cl bond stability. According to our results, the water-nucleophilic attack (WNA) is rate-limiting and the theoretical kinetic rates showed unprecedented agreement with previously published experimental data. On this basis, we suggest that the oxygen release should no longer be considered the slowest step during catalysis, and efforts to improve catalyst performance should focus on decreasing the WNA reaction barrier. Catalyst deactivation has been related to the weakening of the axial Ru-N bond when increasing the Ru oxidation state. We propose the Wiberg bond indices as a descriptor for the rational design of substitution patterns that can increase catalyst stability and hence, the TON. Our results constitute a step forward to the understanding of the water oxidation mechanism and our computational protocol is suggested for future studies to obtain theoretical reaction rates comparable to those from experiments.