Migratory insertion builds ligand structure

Two ligands can also react together to produce a new complex that still has the composite ligand attached to the metal, ready for further modifi cation. This process involves migration of one of the ligands from the metal to the other ligand, and insertion of one of the ligands into the other metal–ligand bond. It is known as migratory insertion. The insertion process is reversible and, as the metal effectively loses a ligand in the process, the overall insertion may be driven by the addition of extra external ligands (L) to produce a coordinatively saturated complex. As with reductive elimination, a cis arrangement of the ligands is required and the migrating group (X) retains its stereochemistry (if it has any) during the migration.

Wilkinson’s catalyst is used in homogeneous hydrogenation of alkenes. The catalyst is soluble in many organic solvents such as EtOH, chloroform, or some hydrocarbons. The alkene complexes with the metal and migratory insertion forms an alkyl metal complex by hydrogen transfer. The next step, reductive elimination, usually follows rapidly to give the alkane and a complex that adds a hydrogen molecule to regenerate the catalyst.

Migratory insertion is the principal way of building up the chain of an organic ligand before elimination. The group to be inserted must be unsaturated in order to accommodate the additional bonds and common examples include carbon monoxide, alkenes, and alkynes, producing metal–acyl, metal–alkyl, and metal–alkenyl complexes, respectively. In each case the insertion is driven by additional external ligands, which may be an increased pressure of carbon monoxide in the case of carbonylation or simply excess phosphine for alkene and alkyne insertions. In principle, the chain extension process can be repeated indefinitely to produce polymers by Ziegler–Natta polymerization.

An example of the carbonylation process is the reaction of the tetracarbonyl ferrate dianion [Fe (CO)₄ ²⁻] with alkyl halides. This reagent is made by dissolving metal reduction of the 18-electron Fe(0) compound Fe(CO)₅. Addition of two electrons would give an unstable 20-electron species but the loss of one of the ligands with its two electrons restores the stable 18-electron structure.

This iron anion is a good soft nucleophile for alkyl halides and can be used twice over to produce fi rst a monoanion with one alkyl group and then a neutral complex with two alkyl groups and four CO ligands. Each of these complexes has 18 electrons. If extra CO is added by increasing the pressure, CO inserts into one Fe–C bond to form an iron acyl complex. Finally, reductive elimination couples the acyl group to the other alkyl group in a conceptually simple ketone synthesis. It does not matter which Fe–C bond accepts the CO molecule: the same unsymmetrical ketone is produced at the end.

Any good two-electron ligand will cause the CO insertion: Ph₃P is often used instead of an increased CO pressure. The phosphine adds to the iron and pushes out the poorest ligand (one of the alkyl groups) on to a CO ligand in a process of ligand migration. We can represent this as the mechanism below, although the phosphine addition and alkyl migration may well be concerted to avoid the formation of a 20-electron complex as an intermediate.

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مواضيع ذات صلة

The Suzuki coupling couples boronic acids to halides

Hydropalladation–dehydropalladation

Palladium is the most widely used metal in homogeneous catalysis

Insertion reactions are reversible

Carbon monoxide incorporation extends the carbon chain

Reductive elimination removes metal atoms and forms new single bonds

Oxidative addition inserts metal atoms into single bonds

Bonding and reactions in transition metal complexes

Ligands can be attached in many different ways

The 18-electron rule

Transition metals extend the range of organic reactions

The Ritter reaction and the Beckmann fragmentation