Sigma-bond metathesis


In organometallic chemistry, sigma-bond metathesis is a chemical reaction wherein a metal-ligand sigma bond undergoes metathesis with the sigma bond in some reagent. The reaction is illustrated by the exchange of lutetium methyl complex with a hydrocarbon :
This reactivity was first observed by Patricia Watson, a researcher at duPont.
The reaction is mainly observed for complexes of metals with d0 configuration, e.g. complexes of Sc, Zr, Nb, Ta, etc. f-Element complexes also participate, regardless of the number of f-electrons. The reaction is thought to proceed via cycloaddition. Indeed, the rate of the reaction is characterized by a highly negative entropy of activation, indicating an ordered transition state. For metals unsuited for redox, sigma bond metathesis provides a pathway for introducing substituents.
The reaction attracted much attention because hydrocarbons are normally unreactive substrates, whereas some sigma-bond metatheses are facile. Unfortunately the reaction does not readily allow the introduction of functional groups. It has been suggested that dehydrocoupling reactions proceed via sigma-bond metathesis.