DFT studies on the electronic properties of organometallic-polyoxomolybdate anions [Mo6O19−n](2−n)− (n = 1 or 2): Revealing bonding features of Cp∗–Mo

In this paper, the electronic properties of organometallic-polyoxomolybdate anions [Cp∗Mo6O18]− (Cp∗Mo6), [ Cp 2 ∗ Mo6O17]0 ( Cp 2 ∗ Mo6), and [Cp∗TiMo5O18]3− (Cp∗TiMo5) are investigated by density functional theory calculations, and several principles for structure–property relationship are obtained. The bonding energy calculations show that the cis-isomer of anion Cp 2 ∗ Mo6 is more stable than the trans-isomer due to large orbital mixing effect. The phenomena that central oxygen atom largely moves to Mo atom bearing Cp∗ fragment and extra surface charges likely extend to the trans-position terminal oxygen atoms in anions Cp∗Mo6, Cp 2 ∗ Mo6, and Cp∗TiMo5 are explained by trans-influence. Compared with [Mo6O19]2−, the energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital of anion Cp∗Mo6, as well as Cp 2 ∗ Mo6, is narrowed due to the raised HOMO which is mainly localized on Cp∗ fragment. Fragment analysis reveals that Cp∗–Mo bond characterizes two nonclassical σ bonds and two π bonds, while only π bonds contribute to the bond order. The static first hyperpolarizabilities of Cp∗Mo6 and Cp 2 ∗ Mo6 are smaller than that of [Mo6O18NPh]2− due to weak intramolecular charge transfer. The peculiar Cp∗–Mo bond would provoke the studies on novel POM-based organic–inorganic hybrid materials.