σ-Aromaticity Planar Pentacoordinate Beryllium Atoms.

Six-valence-electron planar pentacoordinate beryllium (ppBe) is explored herein as a global minimum, which is only constructed by s-block metals in BeM (M = Cu, Ag, Au). The bonding in ppBe can be regarded as the excited-stated Be with a 2p2p electronic configuration, forming electron sharing with doublet M motifs followed by two sets of Be(p) → [M] σ donations and one Be(s) ← [M] σ back-donation. Thus, the σ aromaticity originating from three delocalized σ orbitals gives rise to the whole stability of the high -symmetry ppBe and strongly enriches s-block planar hypercoordinate bonding.