Unusually Short Be−Be Distances with and without a Bond in Be₂F₂and in the Molecular Discuses Be₂B₈and Be₂B₇⁻

Quantum-chemical calculations at the CCSD(T)/cc-pVTZ level of theory show that beryllium subfluoride, Be₂F₂, has a bond dissociation energy of D_e=76.9 kcal mol⁻¹, which sets a record for the strongest Be−Be bond. The synthesis of this molecule should thus be possible in a low-temperature matrix. The discus-shaped species Be₂B₈and Be₂B₇⁻possess the shortest Be–Be distance for a molecule in the electronic ground state, but there is no Be−Be bond. The cyclic species Be₂B₈and Be₂B₇⁻exhibit double aromaticity with 6σ and 6π electrons, which strongly bind the Be₂fragment to the boron atoms. The very short interatomic distance between the beryllium atoms is due to the Be−B σ and π bonds, which operate like spokes in a wheel pressing the beryllium atoms together. The formation of the Be−B bonds has effectively removed the electronic charge of the valence space between the beryllium atoms. Along the Be–Be axis, there are two cage critical points adjacent to a ring critical point at the midpoint, but there is no bond critical point and no bond path.