Redox Effects, The I Mechanism, and Rearrangements in Substitution

Because odd-electron species are more difficult to study and are often transients rather than stable compounds, their importance is only beginning to to be recognized.  

17e and 19e Specie As one might expect for a complex with an electron in an M—L ct orbital, 19e species tend to be more dissociatively labile than their 18e counterparts. This means that substitution of 18e species may be catalyzed by reduction. For example Fe(CO)5 can be substituted with electrochemical catalysis as shown in Eq. 4.43, where [Fe(CO)5] is the chain carrier in the catalytic cycle  

Alternatively, a trace of a free radical can abstract a le ligand from the metal, and the substitution be catalyzed by a chain reaction such as is shown in Eq. 4.46. The last step regenerates the chain carrier (CO) M  

Note that Eqs. 4.43-4.46 all involve 17e/l9e interconversions, while the previous examples of A and D mechanisms in diamagnetic molecules (e.g., Eqs. 4.28 and 4.32) involved 16e/18e interconversions. 

This stabilization of what would otherwise be coordinatively unsaturated intermediates can accelerate substitution reactions. In addition, species that appear from their stoichiometry to be coordinatively unsaturated interme- 

---