Be-Be Bond in Action: Lessons from the Beryllium-Ammonia Complexes [Be(NH₃)_0-4]₂^0,2

High-level electronic structure calculations are performed to elucidate the Be-Be chemical bond in the (NH₃)Be-Be(NH₃) species for n = 0-4. We show that the Be₂ bond is explained as a resonance between two Lewis structures, where one beryllium atom donates an electron pair to the second one, and vice versa. The presence of ammonia ligands enhances the stability of this bond considerably. The ∼2.5 kcal/mol binding energy of Be₂ becomes ∼30 kcal/mol for [Be(NH₃)_1-3]₂ because of their more polarizable electron pairs. The larger Be(NH₃)₄ complex has been classified as a solvated electron precursor in the past and has an electron pair in the periphery of a Be(NH₃)₄²⁺ core occupying a diffuse s-type orbital. The analogy of Be(NH₃)₄ to Be reflects into the electronic structure of their dimers. The two systems have identical bonding patterns and low-lying electronic states. The ground state binding energy of [Be(NH₃)₄]₂ is 3 times larger than Be₂, and its excitation energies are considerably lower by a factor of 3. We also studied the dimers of the cationic Be(NH₃)⁺ species, and we found that the Coulombic repulsion is counterbalanced by the formation of a single covalent bond in the cases of n = 1, 2 forming stable dicationic [(NH₃)Be-Be(NH₃)]²⁺ systems, unlike Be₂²⁺. We believe that our numerical results will allow the identification and characterization of these exotic species and their solid state (beryllium liquid metals) analogues in future experiments.