All-metal aromatic clusters M4 2- (M = B, Al, and Ga). Are π-electrons distortive or not?

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The π-electrons in benzene, the quintessential aromatic molecule, were previously shown to be distortive, i.e., they prefer localized double bonds alternating with single bonds. It is the σ-electrons that force the double bonds to delocalize, leading to a regular, D6h geometry. Herein, we computationally investigate the double-bond localizing or delocalizing propensities of σ- and π-electrons in the archetypal all-metal aromatic cluster Al4 2- and its second- and fourth-period analogs B4 2- and Ga4 2-, using Kohn-Sham molecular orbital (MO) theory at BP86/TZ2P in combination with quantitative bond energy decomposition analyses (EDA). We compare the three all-metal aromatic clusters with the structurally related organic species C4H 4 2+, C4H4, and C4H 4 2-. Our analyses reveal that the π-electrons in the group-13 M4 2- molecules have a weak preference for localizing the double bonds. Instead, the σ-electrons enforce the regular D4h equilibrium geometry with delocalized double bonds ​
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