organometallic chemistry Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...

, sigma-bond metathesis is a

chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

wherein a metal-ligand

sigma bond In chemistry, sigma bonds (σ bonds) or sigma overlap are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals along the internuclear axis. Sigma bonding is most simply defined for diat ...

undergoes metathesis (exchange of parts) with the sigma bond in some reagent. The reaction is illustrated by the exchange of

lutetium Lutetium is a chemical element; it has symbol Lu and atomic number 71. It is a silvery white metal, which resists corrosion in dry air, but not in moist air. Lutetium is the last element in the lanthanide series, and it is traditionally counted am ...

(III) methyl complex with a hydrocarbon (R-H): :(C₅Me₅)₂Lu-CH₃ + R-H → (C₅Me₅)₂Lu-R + CH₄ This reactivity was first observed by Patricia Watson, a researcher at duPont.{{cite journal, author=Watson, Patricia, title=Methane exchange reactions of lanthanide and early-transition-metal methyl complexes, journal=Journal of the American Chemical Society, year=1983, volume=32, pages=6491–6493, doi=10.1021/ja00359a023 The reaction is mainly observed for complexes of metals with d⁰ configuration, e.g. complexes of Sc(III), Zr(IV), Nb(V), Ta(V), etc. f-Element complexes also participate, regardless of the number of f-electrons. The reaction is thought to proceed via

cycloaddition In organic chemistry, a cycloaddition is a chemical reaction in which "two or more Unsaturated hydrocarbon, unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of th ...

. Indeed, the rate of the reaction is characterized by a highly negative

entropy of activation In chemical kinetics, the entropy of activation of a reaction is one of the two parameters (along with the enthalpy of activation) that are typically obtained from the temperature dependence of a reaction rate constant, when these data are analyzed ...

, indicating an ordered

transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked w ...

. For metals unsuited for

redox Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...

, 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.

References

Organometallic chemistry

See also

References