When Does Electronic Delocalization Become a Driving Force of Molecular Shape and Stability? The “Aromatic” Sextet

The conceptual grasp of electronic delocalization vs. localization is reexamined computationally by using “aromatic” 6-electron 6-center (X₆) model species as archetypal systems. It is shown that the character and weights of the resonance structures, which contribute to the “aromatic” sextet, yield no differentiating information regarding the stabilities or geometries of the X₆ species. A qualitative model is utilized to understand the physical basis behind the quantitative results. It is shown that the X₆ systems must “pay a price” for electronic reorganization, while undergoing delocalization. The “price” depends on the identity of X in X₆. Thus, whenever the X-X two-electron bond is strong (e.g., H-H), the price for electronic reorganization is too high. Such systems (e.g., H₆) will prefer a geometry that prohibits delocalization of the electronic sextet. Among these systems is the suspended πSystem, Li₆(π), that lacks a σ skeleton and shows no propensity for π delocalization. Only in a few cases will the price of electronic reorganization be low enough to afford delocalization of the electronic sextet. Such systems possess weak X-X two-electron bonds and will consequently tend to cluster in a regular hexagonal X₆ structure—where electronic delocalization takes place. In view of these trends, a question mark is cast over the propensity of the π System of benzene to remain delocalized without the buttressing effect of the σ framework.