Proteins with active sites consisting of metal centers bridged by oxo or hydroxo groups comprise a new subclass of metalloproteins. This class includes proteins that perform a variety of functions in biology-- dioxygen transport (hemerythrin), the conversion of ribonucleotides to deoxyribonucleotides (ribonucleotide reductase), phosphate ester hydrolysis (purple acid phosphatases), iron storage (ferritin), water oxidation (oxygen evolving complex of Photosystem II), disproportionation of peroxide (Mn-catalase), oxygen utilization (cytochrome c oxidase), and oxygen activation (methane monooxygenase). We propose to model the structures, spectroscopic properties, and reactivities of such sites using tripodal ligands to form (mu-oxo) or (mu-hydroxo) dimetal complexes and binucleating ligands with phenoxo or aloxo groups designed to bridge metal centers. Aspects to be modeled include dioxygen binding (hemerythrin and cytochrome oxidase), oxygen activation (methane monooxygenase and ribonucleotide reductase), mixed-valent states (hemerythrin, purple acid phosphatase, methane monooxygenase), integer spin states (methane monooxygenase, ribonucleotide reductase, cytochrome oxidase), and phosphate binding and phosphate ester hydrolysis in the purple acid phosphatases. The synthetic complexes will be characterized by x-ray crystallography when possible and by a variety of spectroscopic techniques such as NMR, EPR, UV- vis-NIR, Raman, Mossbauer, magnetism, and EXAFS. Peroxide (and dioxygen) complexes will be studied for their ability to oxygenate or oxidize substrates with an emphasis of obtaining mechanistic insight. Mixed-valent complexes will be investigated with regard to their electron delocalization and spin coupling properties. We will also exploit our ability to synthesize heterobimetallic complexes to investigate spin-spin interactions involving Fe(III) or Fe(II) that engender novel EPR and magnetic properties.