Beryllium bonding is actually a subclass of secondary bonding. Similar to the case of halogen bonding, the sigma- and pi-holes on the Be atom of the monomers give in zeroth approximation the direction of electrophilic attack favorable to the formation of beryllium bonds. The nature of beryllium bonding is purely electrostatic so that the symmetry-adapted perturbation theory energy decomposition perfectly explains the relevance of the polarization and dispersion contribution on the formation of the beryllium bond.
