Mechanism
CO inserts into a metal-alkyl bond via migratory insertion. The key concept is that both the CO and the alkyl groups are ligands on the same metal. For example, the reaction of 13CO with Mn(CO)5CH3 exclusively form Mn(CO)4(13CO)COCH3. The alkyl group migrates intramolecularly to an adjacent CO ligand within the coordination sphere of the Mn(I) centre. Subsequent to the migration, the metal binds free CO (see figure below).
CO insertion does not always involve migration. Treatment of CpFe(L)(CO)CH3 with 13CO yields a mix of both alkyl migration products and products formed by true insertion of bound carbonyls into the methyl group. Product distribution is influenced by the choice of solvent.
Alkyl derivatives of square planar complexes undergo CO insertions particularly readily. Insertion reactions on square planar complexes are of particular interest because of their industrial applications. Since square planar complexes are often coordinatively unsaturated, they are susceptible to formation of 5-coordinate adducts, which undergo migratory insertion readily. In most cases the in-plane migration pathway is preferred, but, unlike the nucleophilic pathway, it is inhibited by an excess of CO.
Read more about this topic: Migratory Insertion, CO Insertion
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