EVALUATION OF MOLYBDENUM-SULFUR INTERACTIONS IN MOLYBDOENZYME MODEL COMPLEXES BY GAS-PHASE PHOTOELECTRON-SPECTROSCOPY - THE ELECTRONIC BUFFER EFFECT

The first ionization energy in the gas-phase photoelectron spectra (PE S) of TpMo(E)(tdt) complexes (where E = O, S, NO; Tp* = hydrotris(3,5 -dimethyl-1-pyrazolyl)borate; tdt = 3,4-toluenedithiolate) is essentia lly independent of the nature of E, even though the formal oxidation s tate of the Mo center ranges from +2 to +5. The PES data for the tdt c omplexes contrast with the results for analogous complexes with alkoxi de ligands, which show large variations in first ionization energy (We stcott, B. L.; Enemark, J. H. lnorg. Chern. 1997, 36, 5404-5405). For the tdt complexes the relative intensities of the two lowest energy io nizations do not substantially change as the excitation source is vari ed among Ne I, He I, and He II radiation, even though the atomic photo ionization cross sections for Mo 4d and S 3p orbitals change dramatica lly over this energy region. These results all point to substantial co valency in the Mo-S bonds. It is proposed that the S atoms of the tdt ligand act as an ''electronic buffer'' to the effects of strongly boun d axial ligands, and that this is an important role of ene-dithiolate (dithiolene) coordination in the molybdenum centers of enzymes.