Singlet and triplet states of the sandwich-type Be2B6 and Be2B7+ clusters. A test for the electron counting rules of aromaticity

Abstract

The double aromatic character of Be₂B₆ and Be₂B₇⁺ complexes in their triplet and singlet spin states was examined by means of their magnetically induced current densities, which were calculated at the B3LYP/def2-TZVP level of theory using the diamagnetic-zero variant of the continuous transformation of origin of current density (CTOCD-DZ) method. The studied complexes exhibit double aromaticity (π aromatic and σ aromatic) in their triplet spin states. The found aromatic characteristics are in line with the predictions of Hückel and Baird's rules, which can be separately applied for cyclically delocalized closed-shell and open-shell electronic subsystems in the examined molecules. By employing these electron counting rules of aromaticity, the obtained regularities can be generalized to all complexes of Be₂B^x−6_x having a planar x-membered boron ring sandwiched between two Be atoms.