Transition metal acyl complexes

Transition metal acyl complexes describes organometallic complexes containing one or more acyl (RCO) ligands. Such compounds occur as transient intermediates in many industrially useful reactions, especially carbonylations. A special case are the transition metal formyl complexes.

Acyl complexes are usually low-spin and spin-paired.

Monometallic acyl complexes adopt one of two related structures, C-bonded and η²-C-O-bonded. These forms sometimes interconvert. For the purpose of electron-counting, C-bonded acyl ligands count as 1-electron ligands, akin to pseudohalides. η²-Acyl ligands count as 3-electron "L-X" ligands.

bridging acyl ligands are also well known, where the carbon bonds to one metal and the oxygen bonds to a second metal. One example is the bis(μ-acetyl) complex [(CO)₃Fe(C(O)CH₃)₂Fe(CO)₃]^2-.

Metal acyls are often generated by the reaction of low-valent metal centers with acyl chlorides. Illustrative is the oxidative addition of acetyl chloride to Vaska's complex, converting square planar Ir(I) to octahedral Ir(III):

Some acyl complexes can be produced from aldehydes by C-H oxidative addition. This reaction underpins hydroacylation.

In a related reaction, metal carbonyl anions are acylated by acyl chlorides:

Another important route to metal acyls entails insertion of CO into a metal alkyl bond. In this pathway, the alkyl ligand migrates to an adjacent CO ligand. This reaction is a step in the hydroformylation process.