Rationalization of the π−σ (Anti)aromaticity in All Metal Molecular Clusters

A σ-π separation analysis of the energies in Al4Li4 reveals that the system is more π-antiaromatic than the σ-aromaticity in it. This is true also for C4H4 and Ga4Li4. Unlike C4H4 that has a very large component of π-antiaromaticity, for these all-metal clusters, these energy scales are comparable though π-antiaromaticity is the major driving force for the distortion of the molecules from the square (σ-aromatic) structure to the rectangular (π-antiaromatic) architecture. For the dianion Al4Li4(2)(-), the σ-equalization prevails over the π-distortion in Al4Li4, and for the dication Al4Li4(2+), π-equalization is the driving force for the square symmetric structure.