L-EDGE SPECTROSCOPY OF MOLYBDENUM COMPOUNDS AND ENZYMES

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 MoO4 2-, MoOCl4(H2O)-, and MoOCl5 - spectra. Single-crystal spectra for [N(Et)4][MoOCl4(H2O)] 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. © 1990, American Chemical Society. All rights reserved.