Carbon-Carbon Double Bonds with a Covalent σ and an Ionic p-Bond in Anti-Bredt Olefins

In the universe of chemical bonds, a spectrum stretches from the almost purely ionic to the perfectly covalent. Yet, amid this intricate zoo of possibilities, one elusive creature remains unseen: a mixed chemical double bond where a covalent σ-bond partners with an ionic p-framework. Using state-of-the-art first-principles computations, we introduce a series of molecules in which adjacent σ-bonded carbon atoms at the branching site and the one-carbon bridge of certain bicyclo-bridged olefins form a stable carbanion-carbocation pair instead of an anti-Bredt π-bond or a diradical. We propose the term ionic p-bond to describe this phenomenon, as the bond involves two neighboring, primarily p-based orbitals accommodating a cation and an anion in close proximity. The formation of stable ionic p-bonds relies on two key factors: the stabilization of the cation on the one-carbon bridge through strongly electron-donating substituents, and the stabilization of the anion via pyramidalization of the branching carbon. This stability is further reinforced by electron-withdrawing groups substituting the molecule's σ-skeleton.