On the Nature of β-Agostic Interactions: A Comparison Between the Molecular Orbital and Charge Density Picture

Abstract

The phenomenon and nature of agostic interactions are reviewed in light of combined molecular orbital and charge density studies. As an introduction a historical perspective is given, illustrating the successes and short falls of the various bonding concepts developed during the past 45 years since the discovery of the phenomenon in transition metal complexes. The finding that β-agostic species might represent stable intermediates along the β-elimination reaction coordinate classifies them as suitable benchmark systems to study the microscopic origin of C–H bond activation processes. We outline the salient electronic parameters that control and quantify the extent of agostic interactions on the basis of physically observable charge density properties. Despite the focus on charge density studies, we also complement these studies with arguments based on molecular orbital theory and an irrefutable body of crystallographic, kinetic, and spectroscopic evidence.KeywordsAgostic interactionBond activationCharge density studiesComputational studiesDFTElectron delocalizationMolecular orbitalsMultipolar refinementsNegative hyperconjugationTopological analysisX-ray analysis