Nature of Interactions between Boron Clusters: Extended Delocalization and Retention of Aromaticity Post-Oxidation.

Polyhedral boron clusters are lauded as 3D aromatics that frequently form interconnected periodic networks resulting in boron-rich borides with metal and non-metals having high thermodynamic stability and hardness. This leads to the question of whether the spherical delocalization of electrons in these clusters is extended across the network as in organic aromatic networks. These borides also frequently show partial oxidation, having fewer electrons than what is mandated by electron counting rules, whose impact on their aromatic stability and geometry remains mysterious.

Anatomy of Classical Boron-Boron Bonding: Overlap and sp Dissonance.

Classical bonding is predominantly understood using the insipid sp hybridization for σ-bonds as well as π bonds and their delocalized variants. Because hybridization ignores intricate differences in the energy and size of valence atomic orbitals, its naïve application to classically bonded boron atoms leads to numerous surprises in bond strengths, frontier MOs/bands, and even geometry. Here we show that the sp dissonance caused by size mismatch between the valence s and p orbitals of boron plays a crucial role in its bonding, subtly distinct from that of carbon and silicon.