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

Be_<i>n</i>H_<i>n</i> rings, isoelectronic with boron rings, offer a promising platform for designing unusual bonding patterns and unique geometries. Here, we report the first Be_<i>n</i>H_<i>n</i>-based inverse-sandwich complexes. Potential energy surface analyses of M₂Be₆H₆ (M = N, P, As, Sb, or Bi) show that Sb₂Be₆H₆ and Bi₂Be₆H₆ adopt inverse-sandwich geometries as global minima. The Sb(η⁶-Be₆H₆)Sb and Bi(η⁶-Be₆H₆)Bi structures feature a Be₆H₆ ring with <i>D</i>_6<i>h</i> symmetry sandwiched between two Sb or Bi atoms. Strong charge transfer from the Be₆H₆ 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.