On the nature of chemical bonding in the all-metal aromatic [Sb3Au3Sb3]3− sandwich complex

Abstract

In a recent communication, an all-metal aromatic sandwich [Sb₃Au₃Sb₃]³⁻ was synthesized and characterized. We report herein a density-functional theory (DFT) study on the chemical bonding of this unique cluster, which makes use of a number of computational tools, including the canonical molecular orbital (CMO), adaptive natural density partitioning (AdNDP), Wiberg bond index, and orbital composition analyses. The 24-electron, triangular prismatic sandwich is intrinsically electron-deficient, being held together via six Sb–Sb, three Au–Au, and six Sb–Au links. A standard, qualitative bonding analysis suggests that all CMOs are primarily located on the three Sb₃/Au₃/Sb₃ layers, three Au 6s based CMOs are fully occupied, and the three extra charges are equally shared by the two cyclo-Sb₃ ligands. This bonding picture is referred to as the zeroth order model, in which the cluster can be formally formulated as [Sb₃^1.5+Au₃³⁻Sb₃^1.5+]³⁻ or [Sb₃⁰Au₃³⁻Sb₃⁰]. However, the system is far more complex and covalent than the above picture. Seventeen CMOs out of 33 in total involve remarkable Sb → Au electron donation and Sb ← Au back-donation, which are characteristic of covalent bonding and effectively redistribute electrons from the Sb₃ and Au₃ layers to the interlayer edges. This effect collectively leads to three Sb–Au–Sb three-center two-electron (3c–2e) σ bonds as revealed in the AdNDP analyses, despite the fact that not a single such bond can be identified from the CMOs. Orbital composition analyses for the 17 CMOs allow a quantitative understanding of how electron donation and back-donation redistribute the charges within the system from the formal Sb₃⁰/Au₃³⁻ charge states in the zeroth order model to the effective Sb₃^1.5−/Au₃⁰ charge states, the latter being revealed from the natural bond orbital analysis.