Structures of Mo_x W _(3-x) O₆ (x=0-3) anion and neutral clusters determined by anion photoelectron spectroscopy and density functional theory calculations

The structures of Mo₃ O₆, Mo₂ WO ₆, MoW₂ O₆, and W₃ O₆ and their associated anions were studied using a combination of anion photoelectron (PE) spectroscopy and density functional theory calculations. The 3.49 eV photon energy anion PE spectra of all four species showed broad electronic bands with origins near 2.8 eV. Calculations predict that low-spin, cyclic structures are the lowest energy isomers for both the anion and neutral species. The lowest energy neutral structures for all four species are analogous, C _3v (Mo₃ O₆ and W₃ O₆) or C_s (mixed clusters) symmetry structures in which all three metal atoms are in formally equivalent oxidation states, with singlet ground electronic states. The lowest energy isomers predicted for Mo₃ O ^6- and W³ O^6- are the same with doublet electronic states. The lowest energy structures calculated for the mixed anions are lower symmetry, with the tungsten centers in higher oxidation states than the molybdenum centers. However, C_s symmetry structures are competitive, and appear to be the primary contributors to the observed spectra. Spectral simulations based on calculated spectroscopic parameters validate the assignments. This series of clusters is strikingly different from the Mo ₂ O₄ / MoWO₄ / W₂ O₄ anion and neutral series described recently [Mayhall, J. Chem. Phys. 130, 124313 (2009)]. While the average oxidation state is the same for both series, the structures determined for the Mo₂ O₄ / MoWO₄ / W₂ O₄ anions and neutrals were dissimilar and lower symmetry, and high spin states were energetically favored. This difference is attributed to the large stabilizing effect of electronic delocalization in the more symmetric trimetallic cyclic structures that is not available in the bimetallic species.