15: Intramolecular Beryllium Bonds. Further Insights into Resonance Assistance Phenomena

Beryllium bonds are acid–base closed-shell interactions in which the Lewis acid is a BeXY derivative. These molecular linkers share common characteristics with hydrogen bonds (HBs), though they produce strong distortions of the electron density distribution of the Lewis base participating in the interaction. The characteristics of intramolecular beryllium bonds (IMBeBs) in which a basic site interacts with a BeH group of the same molecule have been analyzed through DFT and high-level ab initio calculations. IMBeBs are stronger than intramolecular HBs in analogous environments, and also stronger in unsaturated compounds. However, this larger strength does not arise from resonance assisted phenomena, but from a larger basicity of the basic site and a larger Lewis acidity of the BeH group when belonging to an unsaturated moiety. Hence, it is the high strength of the beryllium bond that triggers an enhancement of the resonance within the system, and not the resonance stabilization of the system that renders the IMBeB stronger. The dimerization of malonaldehyde-like structures is also analyzed. These dimers are stabilized by Be–H–Be bonds similar to the ones responsible for the stability of diborane. The substitution of H by halogen atoms, alkyl and phenyl groups in these bridges is also investigated.