HE-I VALENCE PHOTOELECTRON-SPECTRA OF OXOMOLYBDENUM(V) COMPLEXES CONTAINING DIOLATO OR ALKOXIDE LIGANDS

The He I valence photoelectron spectra (PES) are reported for several monooxo-molybdenum(V) compounds with the general formula of LMoO[O(CH2)nO] and LMoO(OR)2 (L = hydrotris(3,5-dimethyl-1-pyrazolyl)borate; n = 2-4; R = Me, Et, nPr). The spectra show that the size of the metal-chelate ring in these diolato complexes has a substantial effect on the HOMO ionization. As the chelate ring size in the diolato complexes increases, the HOMO shifts (ca 0.21-0.24 eV) to lower ionization energy. The diolato complexes have their HOMO's at a higher ionization energy than analogous open chain bis-alkoxide complexes possessing the same number of carbon atoms. The unconstrained bis-alkoxide complexes show much smaller shifts ( ≤ 0.11 eV) in the ionization potential associated with the HOMO upon the addition of a CH2 unit to each alkoxide group. These gas-phase results are similar to the results observed by electrochemistry in acetonitrile solution, where both the diolato and bis-alkoxide complexes become easier to oxidize upon increasing the number of CH2 groups, as indicated by their half-wave potentials. The pπ orbital ionizations of the oxygen atoms in the alkoxide functions are also shifted (ca 0.12 eV) to lower ionization energy upon adding CH2 residues to the alkoxide chain. © 1990.