Agostic bonds in cyclometalation

Cyclometalation reactions proceed very easily with one step reaction between metal compounds and substrates having a heteroatom such as O, S, N, P and As. However, under mild reaction conditions, many agostic compounds which are intermediates in these cyclometalation reactions, can be isolated. The metal compounds used for the formation of these agostic intermediates are both transition metal and main group metal compounds. The substrates are nitrogen-containing compounds, phosphorus-containing compounds, oxygen-containing compounds and sulfur-containing compounds. These agostic intermediates are mainly δ-C–H agostic compounds, some are γ-C–H agostic compounds and very few are ɛ-C–H-agostic compounds. The agostic intermediates are prepared, usually, under mild reaction conditions in the cyclometalation reaction. These agostic compounds are also prepared from cyclometalation reaction products, e.g., by the protonation, irradiation, and elimination of ligand molecules by vacuum, inert gas current, dehydration with a molecular sieves 4A, etc. Some agostic compounds are utilized for preparation of stable catalysts, e.g., hydrogenation catalysts.