Transmetalation

Transmetalation is a process in organometallic and inorganic chemistry in which a ligand is transferred from one metal center to another. It involves exchange of ligands such as alkyl, aryl, vinyl, or halide groups between two metal complexes or sites, typically without a change in the overall oxidation state of the system. Transmetalation can occur in solution or in the solid state and may proceed via a four-centered transition state, via metal–metal exchange between close partners, or through associative and dissociative steps.

A central role of transmetalation is in catalytic cross-coupling cycles, where an organic group is transferred from a transmetallating reagent to a transition metal center to enable carbon–carbon bond formation. In Suzuki–Miyaura coupling, for example, an aryl group is transferred from an organoboron species to a palladium(II) complex in the presence of base, generating a Pd(II)-diaryl intermediate that undergoes reductive elimination to give the cross-coupled product. In Stille or Negishi couplings, aryl or alkyl ligands are transferred from organostannanes or organozincs to palladium or nickel centers, respectively.

Transmetalation is also used to prepare new organometallic reagents and to modify the ligand sphere of metal centers in bimetallic catalysts. It can occur between homogeneous complexes in solution or solid state, and in materials or surface contexts. Factors that influence rate and outcome include metal oxidation states, ligand identity, coordinating bases or halide ligands, and the relative softness or hardness of donor groups.

Understanding transmetalation is essential for designing efficient catalytic cycles and for synthetic routes that rely on ligand transfer between metals. Competing pathways such as reductive elimination, disproportionation, or side transmetalations can affect yields and selectivity.