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 (PES) of Tp*Mo(E)(tdt) complexes (where E = O, S, NO; Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate; tdt = 3,4-toluenedithiolate) is essentially independent of the nature of E, even though the formal oxidation state of the Mo center ranges from +2 to +5. The PES data for the tdt complexes contrast with the results for analogous complexes with alkoxide ligands, which show large variations in first ionization energy (Westcott, B. L.; Enemark, J. H. lnorg. Chern. 1997, 36, 5404-5405). For the tdt complexes the relative intensities of the two lowest energy ionizations do not substantially change as the excitation source is varied among Ne I, He I, and He II radiation, even though the atomic photoionization cross sections for Mo 4d and S 3p orbitals change dramatically over this energy region. These results all point to substantial covalency 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 bound axial ligands, and that this is an important role of ene-dithiolate (dithiolene) coordination in the molybdenum centers of enzymes.