The proposed research is designed to provide mechanistic insight into the water-splitting reaction that occurs at the oxygen-evolving complex (OEC) in photosystem II. Specifically, a research plan is presented that describes the preparation of novel trianionic, tris(arylphenoxide) ligands to accomplish the following: (1) the generation of high-valent metal oxo complexes to serve as OEC mimics, (2) investigate the metal oxo generation and stability as a function of secondary bonding environments; (3) probe the mechanism of O-O bond forming reactions relevant to the OEC and other functional models; and (4) make progress towards the ultimate goal of realizing a sustainable water-splitting catalyst. Controlling the secondary bonding environments will be critical for stabilizing high-valent metal oxo species, bringing two water molecules into a close proximity for coupling, and tuning the redox potential of the oxo and the pKa of incoming oxygen nucleophiles.The models described herein are unique in that they will facilitate both acid-base and radical- coupling O-O bond forming reaction mechanisms to be investigated. ? ?