In
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 IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
wherein a metal-ligand
sigma bond
In chemistry, sigma bonds (σ bonds) are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most simply defined for diatomic molecules using the language and tools of s ...
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 with the 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):
:(C
5Me
5)
2Lu-CH
3 + R-H → (C
5Me
5)
2Lu-R + CH
4
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 d
0 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
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, 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 wi ...
. 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
Organometallic chemistry