The research objectives are to synthesize and fully characterize new types of metal-thiolate complexes for the purpose of obtaining fundamental information about the structural, spectroscopic and reactivity properties of proteins containing metal-cysteine interactions. The metal-cysteine ligation mode occurs in a wide range of metalloproteins; these proteins are of vast biological and medical significance. The approach is to use sterically encumbered thiolates to create new types of complexes which mimic some of the more unusual properties exhibited by metal-cysteine centers in proteins. The rationale behind this approach comes from the thought that proteins act as sterically hindered ligands. Interestingly, there are numerous examples where monomeric, high-valent metal-cysteine centers are stabilized in proteins under conditions where simple organic thiolates would interact with metals to form low-valent, oligomeric species. Results obtained in our laboratory have demonstrated that sterically encumbered thiolates react with metals: (1) to form monomeric species, and (2) to stabilize metals in high oxidation states. Using this approach, we have prepared synthetic analogs for: (1) the oxidized form of rubredoxin, [Fe(SR)4]1-; (2) the oxidized form of high potential iron-sulfur proteins, [Fe4S4(SR)4]1-; (3) the molybdenum containing oxidases, [O=Mo(VI)(SR)4]; (4) the Ni(III) center in hydrogenases and carbon monoxide dehydrogenase [Ni(SR)4]1-. Additional compounds have been acquired for which there is, as yet, no direct biological analogy: [Fe4S4(SR)4]0, [Ru(SR)4(CO)], [Co(SR)4]1-, etc. We intend to exploit the chemistry of these compounds and to extend this approach to new challenges. Chemico-physical studies of these models compounds will be used to achieve a fundamental understanding of the relationship between the spectroscopic and electronic properties of the metal center. Moreover, the spectroscopic parameters determined for a variety of related metal-thiolate model compounds should be useful to biochemists in their efforts to identify and distinguish different types of metal-cysteine coordination modes. A number of spectroscopic techniques are used to characterize the compounds including: X-ray crystallography, electrochemical, magnetic susceptibility electronic, vibrational, NMR, ESR, MCD, EXAFS, and Mossbauer measurements.