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The active site of sulfite oxidase has been investigated by X-ray absorption spectroscopy at the molybdenum K-edge at 4 K. We have investigated all three accessible molybdenum oxidation states, Mo(IV), Mo(V), and Mo(VI), allowing comparison with the Mo(V) electron paramagnetic resonance data for the first time. Quantitative analysis of the extended X-ray absorption fine structure indicates that the Mo(VI) oxidation state possesses two terminal oxo (Mo = O) and approximately three thiolate-like (Mo-S-) ligands and is unaffected by changes in pH and chloride concentration. The Mo(IV) and Mo(V) oxidation states, however, each have a single oxo ligand plus one Mo-O- (or Mo-N less than) bond, most probably Mo--OH, and two to three thiolate-like ligands. Both reduced forms appear to gain a single chloride ligand under conditions of low pH and high chloride concentration.

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Spectra at the molybdenum L2 and L3 edges have been recorded by use of synchrotron radiation and analyzed in terms of ligand field theory. Four distinct p -» d transitions were observed in the derivative spectra of molybdenum oxychloride complexes. Comparison with optical data for the same compounds, as well as for Tc analogues, showed that L2 3-edge spectra qualitatively reflect the unfilled Mo d-level splittings. A semiempirical correlation scheme, using Racah parameters to correct for exchange and Coulomb interactions, predicted optical splittings with an accuracy of better than 5%. This capability was used to reject certain interpretations of the Mo042~, MoOCl4(H20)~, and MoOClf spectra. Single-crystal spectra for [N(Et)4][MoOCl4(H20)] helped confirm the assignments. Chemical effects on Mo L-edge spectra were surveyed for LMoOXY compounds, where L represents hydrotris(3,5-dimethyl-1 -pyrazolyl)borate and X and Y are various ligands. Spectral sensitivity to oxidation state, terminal oxo vs terminal sulfido ligands, and different halide ions are also compared. Preliminary spectral analysis of several molybdenum enzymes is presented. L3-edge splittings of 1.72 and 1.40 eV were observed for nitrogenase and active xanthine oxidase, respectively. Oxidized sulfite oxidase gave L3-edge splittings of 1.05, 2.14, and 3.05 eV. L-edge spectroscopy is a useful technique for studying molybdenum in small molecules, enzymes, and catalysts, especially if such materials are available in oriented forms.Molybdenum K-edge X-ray absorption has been used for the study of molybdenum enzymes,1 4aqueous solutions,2 amorphous materials,3,4 and catalysts.5 Although the experimental conditions at 20 keV are ideal for EXAFS, utilization of the K-edge XANES