Bonding and Aromaticity in Electron-Rich Boron and Aluminum Clusters.

In this work bonding and aromaticity of triply bonded atoms of group 13 elements (M≡M, M = B and Al) in recently characterized BAl, NaAl, and NaAl are studied. Here, I show that although molecular orbital-based analyses characterize triple bonds, the electropositive nature of group 13 elements gives these bonds unique characteristics. The bond orders derived from the delocalization index, topology of the electron density, and local characteristics of (3, -1) critical points, as defined within the context of quantum theory of atoms in molecules, do not conform with those of ordinary triple bonds. In NaAl and NaAl clusters non-nuclear attractors form between the electropositive Al atoms acting like pseudo atoms. The bond between boron atoms in BAl is more similar to an ordinary triple covalent bond benefiting from the exchange-correlation component of the interatomic interaction energy as defined via interacting quantum atom theory. However, extreme electrostatic repulsion between negatively charged boron atoms attenuates this bond. Finally, current density analysis suggests that BAl is a magnetic aromatic system, nearly 50% more aromatic compared to benzene.