Agostic interactions from a computational perspective: One name, many interpretations

The agostic bond defines an intramolecular interaction where a a bond is geometrically close to an electron deficient centre (often a transition metal). The computational studies on this energetically weak interaction are reviewed and discussed. Various types of a bonds have been considered (C-H, C-C, Si-H, Si-C, B-H). It is suggested that a C-X bond in which X carries a lone pair should preferably not be viewed as agostic. The factors that contribute to his occurrence are discussed. In particular the agostic interaction is very sensitive to steric effects. Explanations based on molecular orbital analysis, electron delocalization and topological analysis of the electron density are presented.