Oxidative addition
Oxidative addition and reductive elimination are two important classes of reactions in organometallic chemistry [1][2][3].
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 and the electron count of the complex increase by two.[4] 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.
Reductive elimination is the reverse of oxidative addition.[5] 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. Reductive elimination is the key product releasing step of several C-C bond formation coupling reactions.[6]
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 +I to +III as the product is formally bound to three anions: one chloride and two hydride ligands. As shown below, the initial metal complex has 16 valence electrons and a coordination number of four whilst the product is a six-coordinate 18 electron complex. The system is a chemical equilibrium, with the reverse process proceeding by the elimination of hydrogen gas with simultaneous reduction of the metal centre.
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.
References
- ↑ The Organometallic Chemistry of Transition Metals by Robert Crabtree
- ↑ Inorganic Chemistry (3rd Edition) by Gary L. Miessler, Donald A. Tarr
- ↑ Inorganic Chemistry by D. F. Shriver, P. W. Atkins
- ↑ Compendium of Chemical Terminology, oxidative addition accessed 6 Feb 2007.
- ↑ Compendium of Chemical Terminology, reductive elimination accessed 6 Feb 2007.
- ↑ Organometallics, 2005, 24, 715. doi:10.1021/om0490841
nl:Oxidatieve additie ja:酸化的付加 pl:Addycja oksydatywna pt:Adição oxidativa zh:氧化加成
