This thesis deals with the development of complexes that are active catalysts for H₂O oxidation. Promoting proton-coupled electron transfer has been a highly important feature in the development of these catalysts.

The first part deals with the modification of ligand frameworks for the development of a ruthenium complex capable of withstanding the highly oxidizing conditions required for H₂O oxidation. 

The second part of the thesis describes the development of two single-site ruthenium(III) complexes, housing two meridionally coordinating tridentate benzimidazole ligands. Studies on these complexes revealed that they can mediate H₂O oxidation, both by the use of a chemical oxidant and photochemically, and that the ligand frameworks were important in promoting proton-coupled electron transfer events.

In the third part, systematic modifications are introduced into one of the catalysts developed in the second part of the thesis. All of the complexes were shown to be active water oxidation catalysts (WOCs), and kinetic studies confirmed that all catalysts displayed a first-order dependence on catalyst concentration, thereby validating that H₂O oxidation occurs on a single metal site. By using linear free-energy relationships it was possible to elucidate the unusual behavior exerted by the ligand framework during the catalytic cycle.

The fourth part concerns the development of a ruthenium(III) WOC, containing a tetradentate bioinspired ligand architecture, and its deactivation pathway during H₂O oxidation catalysis. This revealed an unexplored, and perhaps general, deactivation pathway for ruthenium-based WOCs. Evidence was also found that the ruthenium WOC reaches a high-valent ruthenium(VI) state which is the active species in H₂O oxidation.

Finally, the fifth and last part deals with the development of a dinuclear manganese complex. Utilizing a bioinspired, highly functionalized ligand, enabled the formation of the first homogeneous manganese-based WOC capable of promoting catalytic H₂O oxidation with one-electron oxidants.