Migratory insertion

A migratory insertion is a type of reaction in organometallic chemistry wherein two ligands on a metal complex combine. It is a subset of reactions that very closely resembles the insertion reactions, and both are differentiated by the mechanism that leads to the resulting stereochemistry of the products. However, often the two are used interchangeably because the mechanism is sometimes unknown. Therefore, migratory insertion reactions or insertion reactions, for short, are defined not by the mechanism but by the overall regiochemistry wherein one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.: A migratory insertion is a type of reaction in organometallic chemistry wherein two ligands on a metal complex combine. It is a subset of reactions that very closely resembles the insertion reactions, and both are differentiated by the mechanism that leads to the resulting stereochemistry of the products. However, often the two are used interchangeably because the mechanism is sometimes unknown. Therefore, migratory insertion reactions or insertion reactions, for short, are defined not by the mechanism but by the overall regiochemistry wherein one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.: In the migratory insertion, a ligand that is viewed as an anion (X) ligand in and a ligand that is viewed as neutral couple, generating a new anionic ligand. The anion and neutral ligands that react are adjacent. If the precursor complex is coordinatively saturated, migratory insertion often result in a coordinatively unsaturated product. A new (neutral) ligand can then react with the metal leading to a further insertion. The process can occur many times on a single metal, as in olefin polymerization. The anionic ligand can be: H− (hydride), R− (alkyl), acyl, Ar− (aryl), or OR− (alkoxide). The ability of these groups to migrate is called their migratory aptitude. The neutral ligand can be CO, alkene, alkyne, or in some cases, even carbene. Diverse reactions apply to the migratory insertion. One mechanism involves the attack of the anionic ligand on the electrophilic part of the neutral ligand (the anionic ligand migrates to the neutral ligand). The other mechanism involves the neutral ligand inserts itself between the metal and the anionic ligand. The insertion of carbon monoxide into a metal-carbon bond to form an acyl group is the basis of carbonylation reactions, which provides many commercially useful products. 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. Decarbonylation of aldehydes, the reverse of CO insertion, is a well-recognized reaction: