Sb(η ⁶ -Be ₆ H ₆ )Sb and Bi(η ⁶ -Be ₆ H ₆ )Bi: Double Aromatic Inverse-Sandwich Complexes

Be n H n rings, isoelectronic with boron rings, offer a promising platform for designing unusual bonding patterns and unique geometries. Here, we report the first Be n H n -based inverse-sandwich complexes. Potential energy surface analyses of M 2 Be 6 H 6 (M = N, P, As, Sb, or Bi) show that Sb 2 Be 6 H 6 and Bi 2 Be 6 H 6 adopt inverse-sandwich geometries as global minima. The Sb(η 6 -Be 6 H 6 )Sb and Bi(η 6 -Be 6 H 6 )Bi structures feature a Be 6 H 6 ring with D 6 h symmetry sandwiched between two Sb or Bi atoms. Strong charge transfer from the Be 6 H 6 ring to the metals provides electrostatic stabilization, while three delocalized 14-center two-electron (14c-2e) σ bonds and three eight-center two-electron (8c-2e) π bonds confer covalent stabilization. Together, these interactions generate dual Hückel aromaticity, accounting for the exceptional stability of these complexes.