Acylmetal

An acylmetal complex is a compound in which an acyl group, R–CO–, is bound to a metal center. The acyl fragment is typically attached through the carbonyl carbon, giving a metal–carbonyl bond described as M–C(O)R. Such complexes are common in organometallic chemistry and frequently participate in catalytic cycles that transform carbonyl-containing fragments into value-added products.

Typical formation routes include oxidative addition of acyl chlorides to low-valent metal centers (for example Pd(0), Ni(0) or Rh(I) complexes) to afford metal(II) acyl species, and migratory insertion of CO into a metal–alkyl bond followed by subsequent steps. Acyl ligands can also be generated by transacylation from other acyl donors. In many cases the acyl ligand is formally an anionic two-electron donor, and the metal–acyl bond can participate in migratory insertion, reductive elimination, or cooperative activation of substrates.

Reactivity and applications: Acylmetal complexes enable key transformations such as hydroacylation of alkenes, carbonylation, and cross-coupling processes that form ketones or aldehydes. In hydroacylation, an acyl–metal fragment adds across a C=C bond to furnish a ketone after subsequent steps. Palladium, rhodium, nickel and other metals have been used to access acyl complexes that mediate these reactions. Stability and ligand environment strongly influence reactivity; bulky phosphines or N-heterocyclic carbenes can stabilize reactive acyl species and steer selectivity.

Examples and scope: Rhodium and nickel systems are among the most studied for acyl chemistry, with reported acyl intermediates in catalytic cycles. Acyl complexes of ruthenium, iron, cobalt, and other transition metals also occur, expanding the utility of acylation and carbonylation processes in synthesis and industrial chemistry.

See also: acylation, carbonylation, hydroacylation, migratory insertion, organometallic chemistry.