Planar tetracoordinate beryllium in σ-aromatic Li₄Be and Na₄Be clusters: A missing member in first-octal row planar tetracoordinate family.

This article focuses on the theoretical discovery and characterization of planar tetracoordinate (pt) alkali (A) and alkaline-earth (Ae) metal centers, specifically identifying pt beryllium (ptBe) species within Li4Be and Na4Be clusters as the only energetically favored planar tetracoordinate structures among 50 candidates studied. The global-minimum ptBe structures feature a Be− center with a 2s¹2pₓ¹2p_y¹ electronic configuration forming three σ electron-sharing bonds with quartet Li4⁺/Na4⁺ rings, exhibiting σ aromaticity that contributes to their stability. These ptBe clusters maintain planarity and enhanced stability when encapsulated within [7] and [8]cycloparaphenylene nanoloops, stabilized predominantly by electrostatic interactions. The findings expand the understanding of planar hypercoordinate bonding beyond traditional carbon-centered systems to include alkali and alkaline-earth metals.