E2(η6–Be6H6) (E = Si–Pb): Doubly Aromatic Inverse-Sandwich Complexes

Abstract

Abstract We report E2(η6-Be6H6) (E═Si, Ge, Sn, Pb), a new family of doubly aromatic inverse sandwich clusters in which a tetrel diatomic is sandwiched between a beryllium hydride ring. All four members adopt a singlet D6h global minimum, separated from the nearest competing isomer by 31.3, 29.1, 19.8, and 8.4 kcal·mol–1 for E═Si, Ge, Sn, and Pb, respectively, with large HOMO–LUMO gaps of 4.3–4.8 eV and dynamic stability confirmed up to at least 500 K. The electronic structure is strongly polarized: the tetrel atoms carry large negative charges (−2.1 to −1.8 |e|) and interact with the Be6H6 ring through predominantly ionic but non-negligible covalent E–Be contacts, while the ring is sustained by six Be–H–Be three-center two-electron bonds and the E2 fragment retains significant E–E bonding character. A delocalized 6σ/6π bonding pattern spans the full inverse sandwich framework, and diatropic σ and π ring currents─with total ring-current strengths of 15.6–19.5 nA·T–1─confirm σ/π double Hückel aromaticity throughout the series. These results extend the Be6H6-based inverse sandwich family to group 14 and show that the aromatic electron count in these systems is governed by the electron-accepting capacity of the axial fragment.