Allylmetal

Allylmetal is a term used in organometallic chemistry to describe compounds in which an allyl ligand, C3H5, is bound to a metal center. The allyl fragment commonly coordinates in an η3 fashion, engaging all three carbon atoms and the metal; in some cases an η1 binding mode occurs when geometry or ligands favor end-on attachment.

Metals typically associated with allylmetal species include palladium, nickel, platinum, copper, rhodium, ruthenium, iron, and other transition metals. The oxidation state varies with ligands and synthesis, often ranging from 0 to II. Complexes can be neutral or cationic, depending on the ligand set, which may include phosphines, amines, or halides.

Preparation methods include oxidative addition of allyl halides to low-valent metal fragments, transmetalation from allyl boron or allyl stannane reagents, and other substitution routes. Allylmetal complexes are commonly used as catalysts or intermediates in carbon–carbon and carbon–heteroatom bond-forming reactions.

Applications and reactivity: Allylmetal species are central to catalytic allylation, including enantioselective variants created with chiral ligands, and to reactions such as the Tsuji–Trost process. They can transfer the allyl group to nucleophiles such as enolates, aldehydes, and imines, and may undergo migratory insertion, beta-hydride elimination, and allyl migration during catalytic cycles. In some cases, they act as reservoirs of the allyl group in polymerisation or cross-coupling protocols.

Safety and handling: many allylmetal complexes are air- or moisture-sensitive and are handled under inert atmosphere in specialized glassware. They may be toxic or irritant; proper disposal and ventilation are advised.

Example: η3-allyl palladium or nickel complexes illustrate the class and its role in catalytic processes.