Delocalized Bonding in Li2X2 Rings: Probing the Limits of the Covalent and Ionic Bonding Models

In spite of the highly ionic character of most lithium-element bonds, the bonding within Li2X2 rings presents similarities with that found in analogous transition metal systems. They obey simple framework electron counting rules that allow us to predict whether they will form a regular ring or a squeezed one with short Li-Li or X-X distances. A combined computational and structural database analysis discloses the orbital conditions that determine the framework electron counting rules. These systems probe the borderline between the covalent and ionic bonding models since, paradoxically, a non-negligible covalent contribution of the two Li atoms (formally Li+ ions in the ionic model) to the Li-X framework bonding favors them approaching each other within bonding distance.

Automated summary

This study reveals the bonding mechanism in Li2X2 rings, introduces simple electron counting rules, and illustrates the borderline between the covalent and ionic bonding models.