What is sigma bond metathesis?

Sigma bond metathesis is basically an "exchange" of a sigma bond that typically occurs with the transition metals on the first few columns (the "early" transition metals) that are in their highest oxidation state.

It is important to note that this does not always happen in one step.

EXAMPLE 1: METALLOCENE SUBSTITUTION

An example is this process, involving #(eta^5-stackrel(color(red)(-1))("C"_5"H"_5))_2stackrel(color(blue)(+3))("Sc")stackrel(color(green)(-1))("CH"_3)# and, let's say, some alkane #"R"-"H"#:

By this paper, we can determine the transition state to look roughly like this:

#(eta^5-"C"_5"H"_5)_2"Sc""CH"_3# is the 15-electron complex called bis(pentahaptocyclopentadienyl)methylscandium(III), and demonstrates the ability of scandium to coordinate with all five carbons (pentahapto) on cyclopentadiene.

#R# can be any typical #R# group, like hydrogen, alkyl, etc. Note that the oxidation state of scandium did not change. Thus, the process is neither oxidative nor reductive.

EXAMPLE 2: TERMINATION OF CHAIN GROWTH IN OLEFIN POLYMERIZATION

Another cool example is this:

The first step (including the transition state) is the reverse of 1,2-insertion, called #\mathbf(beta)#-hydride elimination. That changed the hapticity of the alkyl group so that it now binds dihapto (via two atoms) as an alkene.

(As a side note, 1,2-insertion is the second step shown here, which had followed a ligand association step.)

The second step is a ligand dissociation of the alkene from the complex.

Overall, this was a sigma bond metathesis that exchanged a sigma bond with an alkyl group for a sigma bond with an #"H"#, and added a #pi# bond onto the dissociated alkyl group.