Oxidative addition

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Oxidative addition and reductive elimination are two important classes of reactions in organometallic chemistry. Their relationship is shown below where y represents the number of ligands on the metal and n is the oxidation state of the metal.

In oxidative addition, a metal complex with vacant coordination sites and a relatively low oxidation state is oxidized by the insertion of the metal into a covalent bond (X-Y). Both the formal oxidation state of the metal, n, and the electron count of the complex increase by two. Although oxidative additions can occur with the insertion of a metal into many different covalent bonds, they are most commonly seen with H-H and carbon(sp³)-halogen bonds. Carbon that is sp² hybridized, as in the case of a vinyl group, can also undergo oxidative addition. This process proceeds with retention of configuration about the double bond. The reverse of oxidative addition is reductive elimination. Reductive elimination is favored when the newly formed X-Y bond is strong. For reductive elimination to occur the two groups (X and Y) should be adjacent to each other in the metal's coordination sphere.

An example of oxidative addition is the reaction of Vaska's complex, trans-IrCl(CO)[P(C₆H₅)₃]₂, with hydrogen. In this transformation, the metal oxidation state changes from Ir(I) to Ir(III) because the product is described as Ir³⁺ bound to three anions: Cl^-, and two hydride, H^-, ligands. As shown below, the metal complex initially has 16 valence electrons and a coordination number of four. After the addition of hydrogen, the complex has 18 electrons and a coordination number of six. The reaction can proceed in the opposite direction. In this case hydrogen gas would be lost and the metal complex would be reduced. This backwards reaction is an example of reductive elimination.

Just as a metal oxidatively inserts itself into a H-H bond, it can also oxidatively add to C-H bonds. This process is called C-H bond activation and is an active research area because of its potential value in converting petroleum-derived hydrocarbons into more complex products.

Oxidative addition and reductive elimination are seen in many catalytic cycles such as the monsanto process and alkene hydrogenation using Wilkinson's catalyst.