Stability of T_d-Distorted Be₈X₁₂²⁺ (X = H, F) Clusters with Multicenter Bonding and Aromaticity

Beryllium bonding is intriguing because of the Be 2s²2p⁰ valence electron configuration. Here, we explore the stability of Be₈ clusters of Be₈H₁₂²⁺ and Be₈F₁₂²⁺. Extensive quantum-chemical studies show that these clusters have T_d geometries, high thermal stability, aromaticity, and non-nuclear attractors at the center. The origin of the distortion from O_h to T_d geometry is explained by the extended Hückel molecular orbital model, and the aromaticity of these clusters is verified via nucleus-independent chemical shifts, iso-chemical shielding surface, and magnetically induced current density. Be₈F₁₂²⁺ is found to possess two-electron aromaticity, which is mainly contributed by a four-center two-electron (4c-2e) delocalized bond among four inner beryllium atoms and the "pseudo-atom". Apart from two-electron aromaticity dominated by a delocalized 20c-2e bond, Be₈H₁₂²⁺ also exhibits weak aromaticity from skeleton orbitals as well, which correspond to 12 equiv 3c-2e Be-H-Be bonds and make Be₈H₁₂²⁺ more aromatic than Be₈F₁₂²⁺. Our results demonstrate cubic aromatic characteristics in T_d-symmetric clusters, thereby generalizing applicability of the concept beyond traditional O_h structures to T_d geometries.