Cyclic species, transannular interactions

The formation of a weak a-bond by overlap of two singly occupied 7i -orbitals is a characteristic feature of certain 7i-electron-rich inorganic systems. This structural feature can occur via an intramolecular process resulting in a transannular interaction that converts a monocyclic ring into a bicyclic system. Alternatively, it may involve an intermolecular contact in which two cyclic monomers are weakly associated to give a dimeric species. 

A cyclic species has an annular form, and a transannular interaction is one between atoms across a ring. 

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