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.