The nature of supermolecular bonds: Investigating hydrocarbon linked beryllium solvated electron precursors

Beryllium ammonia complexes Be(NH₃)₄ are known to bear two diffuse electrons in the periphery of a Be(NH₃)₄ ²⁺ skeleton. The replacement of one ammonia with a methyl group forms CH₃Be(NH₃)₃ with one peripheral electron, which is shown to maintain the hydrogenic-type shell model observed for Li(NH₃)₄. Two CH₃Be(NH₃)₃ monomers are together linked by aliphatic chains to form strongly bound beryllium ammonia complexes, (NH₃)₃Be(CH₂)_nBe(NH₃)₃, n = 1-6, with one electron around each beryllium ammonia center. In the case of a linear carbon chain, this system can be seen as the analog of two hydrogen atoms approaching each other at specific distances (determined by n). We show that the two electrons occupy diffuse s-type orbitals and can couple exactly as in H₂ in either a triplet or singlet state. For long hydrocarbon chains, the singlet is an open-shell singlet nearly degenerate with the triplet spin state, which transforms to a closed-shell singlet for n = 1 imitating the σ-covalent bond of H₂. The biradical character of the system is analyzed, and the singlet-triplet splitting is estimated as a function of n based on multi-reference calculations. Finally, we consider the case of bent hydrocarbon chains, which allows the closer proximity of the two diffuse electrons for larger chains and the formation of a direct covalent bond between the two diffuse electrons, which happens for two Li(NH₃)₄ complexes converting the open-shell to closed-shell singlets. The energy cost for bending the hydrocarbon chain is nearly compensated by the formation of the weak covalent bond rendering bent and linear structures nearly isoenergetic.