Quantitative investigation of bonding characteristics in ternary Zintl anions: Charge and energy analysis of [Sn2E152(ZnPh)]− (E15 = Sb, Bi) and [Sn2Sb5(ZnPh)2]3−

Abstract

The analysis of chemical bonding in Zintl anions and complexes thereof is mostly based on frontier molecular orbital (FMO) analysis. While this approach delivers remarkable insights, it falls short of providing quantitative measures for chemical bonding in these compounds. Here, we investigate the organozinc-ligated Zintl anions [Sn2E(15)2(ZnPh)](-) (E(15) = Sb, Bi) and [Sn2Sb5(ZnPh)2](3-) with charge and energy analysis methods. Partial charge analysis confirms that natural population analysis is more reliable than the Hirshfeld method for the diffuse charge density of the Zintl anions. In a subsequent step, the combined method energy decomposition analysis with natural orbitals for chemical valence is used to deliver quantitative results for the chemical bond between the organozinc fragment and the Zintl anionic cage. From this analysis, we conclude that the shared-electron description represents the chemical bonding in these compounds more appropriate. The bonding is characterized by a σ-type bond polarized toward the ZnPh fragment and a strong π-donation (15-20% of orbital interaction) into the p-orbitals at zinc. Electrostatic contributions, which are not considered in FMO analyses, make up around two-thirds of the attractive metal-ligand interaction and should not be neglected in the discussion of chemical bonding in these compounds. Usage of ligands with better σ- or π-accepting ability might thus serve to further stabilize the interesting class of compounds with multinary Zintl anions in the future.