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 Al₄Li₄²⁺, π-equalization is the driving force for the square symmetric structure.