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Characterization of agostic interactions in theory and computation

Agostic interactions are covalent intramolecular interactions between an electron deficient metal and a sigma-bond in close geometrical proximity to the metal atom. While the classic cases involve CH sigma-bonds close to early transition metals like titanium, many more agostic systems have been proposed which contain CH, SiH, BH, CC and SiC sigma-bonds coordinated to a wide range of metal atoms. Recent computational studies of a multitude of agostic interactions are reviewed in this contribution. It is highlighted how several difficulties with the theoretical description of the phenomenon arise because of the relative weakness of this interaction. The methodology used to compute and interpret agostic interactions is presented and different approaches such as atoms in molecules (AIM), natural bonding orbitals (NBO) or the electron localization function (ELF) are compared and put into context. A brief overview of the history and terminology of agostic interactions is given in the introduction and fundamental differences between alpha, beta; and other agostic interactions are explained.