Molybdenum plays an essential role in a number of metalloenzymes, including nitrogenase and xanthine oxidase. Although X-ray absorption studies provide the first direct evidence as to the nature of the Mo-binding site in nitrogenase, the detailed coordination sphere about Mo has not been elucidated, nor has the relationship of the Mo-coordination in nitrogenase to that of xanthine oxidase, where common Mo- terminal-oxo coordination appears to be implicated. The accumulated data on the biological systems and on model compounds, however, supports sulfur coordination for both enzyme types. A major objective of the proposed research is to provide new chemical and structural information about molybdenum-sulfur complexes. Synthetic efforts will be directed toward the preparation of mercapto-and sulfido- complexes of molybdenum, incorporating various donor groups and displaying varying ligand geometries and charges. The systematic effects of donor type and geometry will be investigated by the usual spectroscopic techniques, S-ray crystallography, and electrochemistry, with particualr emphasis on the correlation of structural and redox trends. The research will also concentrate on the nature of the Mo-nitrogen multiple bond, a structural situation analogous to possible intermediate steps in dinitrogen fixation. The structural systematics of Mo-hydrazido, imido, and nitrido coordination are of particular interest. The scope of the research will be broadened to include the investigation of mixed metal clusters of Mo-Fe as possible models for the metallo-prosthetic group in nitrogenase.