Sigma-bond metathesis

In organometallic chemistry, sigma-bond metathesis is a chemical reaction.^[1] At UC San Diego, professor Don Tilley's work on early transition metal-silicon compounds helped discover this new reaction mechanism. Early examples were provided by studies on (C₅Me₅)₂Lu-CH₃, which undergoes degenerate exchange with methane as revealed using CD₄. This lutetium(III) complex also exchanges with other hydrocarbons:

(C₅Me₅)₂Lu-CH₃ + R-H → (C₅Me₅)₂Lu-R + CH₄

Hydrocarbons are normally unreactive substrates, but sigma-bond metathesis is facile. Unfortunately the reaction does not readily allow the introduction of functional groups. It has been suggested that many dehydrocoupling reactions proceed via sigma-bond metathesis.

The reaction is mainly observed for complexes of metals with d⁰ configuration, e.g. complexes of Sc(III), Zr(IV), Nb(IV), Ta(V), 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 without requiring oxidative addition.