Probing the Electronic Structure of [MoO<i>S</i>₄]^- Centers Using Anionic Photoelectron Spectroscopy

Using photodetachment photoelectron spectroscopy (PES) in the gas phase, we investigated the electronic structure and chemical bonding of six anionic [Mo^VO]³⁺ complexes, [MoOX₄]^- (where X = Cl (<b>1</b>), SPh (<b>2</b>), and SPh-<i>p</i>-Cl (<b>3</b>)), [MoO(edt)₂]^- (<b>4</b>), [MoO(bdt)₂]^- (<b>5</b>), and [MoO(bdtCl₂)₂]^- (<b>6</b>) (where edt = ethane-1,2-dithiolate, bdt = benzene-1,2-dithiolate, and bdtCl₂ = 3,6-dichlorobenzene-1,2-dithiolate). The gas-phase PES data revealed a wealth of new electronic structure information about the [Mo^VO]³⁺ complexes. The energy separations between the highest occupied molecular orbital (HOMO) and HOMO−1 were observed to be dependent on the O−Mo−S−C(α) dihedral angles and ligand types, being relatively large for the monodentate ligands, 1.32 eV for Cl and 0.78 eV for SPh and SPhCl, compared to those of the bidentate dithiolate complexes, 0.47 eV for edt and 0.44 eV for bdt and bdtCl₂. The threshold PES feature in all six species is shown to have the same origin and is due to detaching the single unpaired electron in the HOMO, mainly of Mo 4d character. This result is consistent with previous theoretical calculations and is verified by comparison with the PES spectra of two d⁰ complexes, [VO(bdt)₂]^- and [VO(bdtCl₂)₂]^-. The observed PES features are interpreted on the basis of theoretical calculations and previous spectroscopic studies in the condensed phase.