The word agostic was first coined for organometallic chemistry by Malcolm Green and Maurice Brookhart to describe a weak bonding interaction between a transition metal and a C-H bond on a carbon atom that is held close to the metal center through an additional π- or σ-bond. It is derived from the Greek word for "to hold close to oneself". This interaction closely resembles the transition state of an oxidative addition or reductive elimination reaction. Agostic interactions are best demonstrated by crystallography, but can be inferred by the presence of a 1H NMR peak that is shifted upfield from that of a normal aryl or alkane. towards that of a hydride ligand. The coupling constant JCH is typically lowered to 70-100 Hz versus the 125 Hz expected for a normal sp3 carbon-hydrogen bond. The interaction energy is estimated at about 10–15 kcal/mol from experimental data and computational chemistry studies. This makes agostic interactions a little bit stronger than most hydrogen bonds.
Agostic bonds sometimes play a role in catalysis by increasing 'rigidity' in transition states. For instance, in Ziegler-Natta catalysis the highly electrophilic metal center has agostic interactions with the growing polymer chain.
- Maurice Brookhart, Malcolm L. H. Green, Carbon-hydrogen-transition metal bonds. Journal of Organometallic Chemistry, 250(1), 395-408 (1983).
- Kubas, G. J., "Metal Dihydrogen and σ-Bond Complexes", Kluwer Academic/Plenum Publishers: New York, 2001