The Nature of Transannular Interactions in E₄N₄ and E₈²⁺ (E = S, Se)

The electronic structures of tetrachalcogen tetranitrides, E₄N₄, and octachalcogen dications, E₈²⁺, and the nature of their intramolecular E···E interactions (E = S, Se) was studied with high-level theoretical methods. The results reveal that the singlet ground states of both systems have a surprisingly large correlation contribution which functions to weaken and therefore lengthen the cross-ring E–E bond. The observed correlation effects are primarily static in E₄N₄, whereas in E₈²⁺ the dynamic part largely governs the total correlation contribution. The presented description of bonding is the first that gives an all-inclusive picture of the origin of cross-ring interactions in E₄N₄ and E₈²⁺; not only does it succeed in reproducing all experimental structures but it also offers a solid explanation for the sporadic performance of different computational methods that has been reported in previous studies. Furthermore, the theoretical data demonstrate that E···E bonds in E₄N₄ and E₈²⁺ are unique and fundamentally different from, for example, dispersion that plays a major role in weak intermolecular chalcogen···chalcogen contacts.