Electronic structure, stability, and aromaticity of M 2 B 6 (M = Mg, Ca, Sr, and Ba): an interplay between spin pairing and electron delocalization.

It has been shown in previous studies that the Be 2 B 6 complex exhibits a triplet ground state with double aromaticity. In this work, the stability, electronic structure, and aromaticity of the homologous series M 2 B 6 (M = Mg, Ca, Sr and Ba) were examined and compared to those of Be 2 B 6 . At the CCSD(T)/def2-TZVP//B3LYP/def2-TZVP level of theory, the target molecules were found to be more stable in the singlet than in the triplet spin state. Magnetically induced current densities and multicentre delocalization index (MCI) were employed to assess the aromatic character of the studied complexes. Both employed methods agree that M 2 B 6 (M = Mg, Ca, Sr and Ba) are π aromatic and σ nonaromatic in the singlet ground state, and double aromatic in the triplet state. It was demonstrated that the electron counting rules of aromaticity cannot be used to correctly predict the aromaticity and relative stability of the examined molecules in different spin states.