Sigma-bond metathesis
In organometallic chemistry, sigma-bond metathesis is a chemical reaction. 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:
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.
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