Migratory Insertion

Introduction

Migratory insertion is a fundamental organometallic reaction where a cis-positioned anionic ligand (X) and neutral unsaturated ligand (Y) combine to form a new anionic ligand (XY), creating a vacant coordination site at the metal center (Figure 12.1.9). The reaction typically involves migration of the anionic component (e.g., hydride, alkyl) to the neutral ligand (e.g., CO, alkene), though the reverse migration can also occur. This process is reversible, establishing an equilibrium between insertion and its reverse reaction—deinsertion (or elimination). The equilibrium can be shifted toward insertion by adding excess neutral ligand or a different ligand to block the vacant site. Common neutral ligands (Y) include CO, alkenes, alkynes, carbenes, O₂, CO₂, and nitriles, while typical migrating anionic ligands (X) are hydride, alkyl, aryl, alkoxy, and amido groups. These reactions are crucial in catalytic cycles, such as olefin polymerization and carbonylation, where controlled insertion and elimination steps enable substrate activation and product formation. The reversibility and ligand-dependent nature of migratory insertions make them highly valuable in synthetic and industrial applications.

Carbonyl Migratory Insertion

One of the most common insertions are carbonyl insertions into alyl groups like methyl groups. The carbonyl insertion produces an acyl group. To drive the reaciotn, tehr eaction can be carried out itn eh rpesenc of a neutral ligand L like free CO, which occupies the vanct site.

Olefin Insertion and β-Hydride Elimination

The propensity for β-hydride elimination frequently destabilizes transition metal alkyl complexes containing β-hydrogens (Figure 12.1.12). For instance, platinum dialkyl complexes with β-hydrogens exhibit thermal instability because phosphine ligand dissociation creates the necessary vacant site for elimination. Subsequent β-hydride elimination produces volatile alkene products that can leave the coordination sphere, driving the reaction forward. In contrast, methyl complexes (lacking β-hydrogens) remain stable as they cannot undergo this elimination pathway

Instability of Transition Metal Complexes Caused by β-Hydride Elimination

Migratory insertion of olefins into metal-hydride bonds represents a fundamental organometallic transformation where a coordinated olefin inserts into an M-H bond to form a metal-alkyl complex, generating a vacant coordination site. This equilibrium process can be driven toward alkyl formation by introducing strong-field ligands like CO to occupy the vacancy. The reverse reaction, β-hydride elimination, involves cleavage of a C-H bond at the β-carbon (second from the metal center) and requires precise syn-coplanar alignment between the M-C and Cβ-H bonds. This elimination pathway frequently destabilizes transition metal alkyl complexes containing β-hydrogens, as seen in platinum dialkyl systems where phosphine dissociation enables elimination to form volatile alkenes. In contrast, methyl complexes remain stable due to the absence of β-hydrogens.

Carbene Migratory Insertion

Carbene ligands also participate in migratory insertion, with metal-carbene (M=CH₂) species inserting into M-alkyl bonds to form extended alkyl chains that remain susceptible to subsequent β-hydride elimination. These competing insertion and elimination processes play critical roles in catalytic cycles, with their relative rates dictating reaction outcomes in industrially important processes like olefin polymerization and hydroformylation. The balance between these transformations depends sensitively on metal identity, ligand environment, and reaction conditions, highlighting the delicate interplay between stability and reactivity in organometallic systems.