Transmetallation

Transmetallation is a chemical process in which a ligand, typically an organic group bound to a metal, transfers from one metal center to another. In organometallic chemistry it is a fundamental step in many catalytic cycles, enabling the movement of a reactive fragment from a donor metal to an acceptor metal so that subsequent bond-forming events can occur.

Transfers can occur via inner-sphere mechanisms, where the two metal centers share a common ligand or a bridge such as a halide, alkoxide, or sulfide, allowing direct exchange of the transferring group. Outer-sphere mechanisms involve ligand transfer without a direct M–M bond, often mediated by solvent or additives and sometimes through transient ion-pair or Lewis acid–base interactions. The rates and outcomes depend on the electronic and steric properties of the donor and acceptor metals, the nature of the transferring group, and the surrounding medium.

In cross-coupling chemistry, transmetallation transfers an organic fragment from a donor organometallic reagent to a catalytic metal such as palladium or nickel. Common archetypes include Suzuki–Miyaura coupling, in which an organoboron species transfers to palladium; Stille coupling, where an organotin donor is involved; and Negishi coupling, in which an organozinc transfers. The efficiency of the transmetallation step often governs the overall reaction rate and substrate scope, and competing pathways can arise with certain ligands, bases, solvents, or functional groups.

Transmetallation is also relevant in inorganic systems, such as metal clusters and multinuclear complexes, where ligand transfer between metal centers can influence reactivity and catalysis beyond cross-coupling. The term highlights exchange between different metals rather than intramolecular ligand rearrangements.