Sigma-bond metathesis

In organometallic chemistry, sigma-bond metathesis is a chemical reaction wherein a metal-ligand sigma bond undergoes metathesis (exchange of parts) with the sigma bond in some reagent. The reaction is illustrated by the exchange of lutetium(III) methyl complex with a hydrocarbon (R-H):[1]

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

This reactivity was first observed by Patricia Watson, a researcher at duPont.[2]

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.

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.

See also
  • C-H activation

References
  1. Waterman, Rory (2013). "σ-Bond Metathesis: A 30-Year Retrospective". Organometallics. 32: 7249–7263. doi:10.1021/om400760k.
  2. Watson, Patricia (1983). "Methane exchange reactions of lanthanide and early-transition-metal methyl complexes". Journal of the American Chemical Society. 32: 6491–6493. doi:10.1021/ja00359a023.
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