Advanced Inorganic Chemistry/Oxidative Addition/Reductive Elimination

Oxidative addition and Reductive elimination are reaction pairs that involve a change in the oxidation state and coordination number of the metal. Oxidative addition is the increase in the oxidation state and coordination number of the metal. Meanwhile, reductive elimination is the decreases in the oxidation state and coordination number of the metal. These two reactions occur through sigma bond and can be described schematically by the following:

Oxidative Addition
Oxidative addition is the reaction where the oxidation state of the metal center increases by two. This reaction cannot occur if the metal center of the complex doesn't have accessible two units higher than the initial oxidation state. Metal center in a complex is known to act as Lewis acid. However, it both behaves as a Lewis acid and a Lewis base under oxidative addition. The metal center of the complexes is a Lewis acid when it takes electrons from the sigma bond (σ) and it's a Lewis base when it donates electrons from the sigma antibonding (σ*).

3 Main Oxidative Addition Mechanisms

 * Concerted Oxidative Addition Mechanism


 * Non-polarized substrates, such as H and C-H and Si-H bond, undergo concerted oxidative addition.
 * Under concerted oxidative addition mechanism, ligands end up in cis position although the more stable product is in trans positions due to isomerization.
 * Reaction of the Vaska’s comple,trans-IrCl(CO)[P(C6H5)3]2, with dihydrogen is one of the example of the concerted oxidative addition mechanism. In this example, the dihydrogen coordinates with the Iridium. Afterwards, the two hydrogens ends up in cis position with each other and the CO in the complex was pushed towards the trans position with the hydride because of the trans effect.


 * Non-concerted Oxidative Addition Mechanism
 * Non-concerted oxidative addition mechanism is like nucleophilic displacement (SN2) reaction.
 * Polarized substrates, such as methyl, allyl, and benzyl halides, undergo non-concerted oxidative addition mechanism.
 * Another way to identify that a reaction undergo non-concerted mechanism is by identifying the substrate if it’s optically active.


 * Radical Mechanism
 * Alkyl halides can react with the metal center of a complex through radical reaction.
 * Other byproducts can form through the radical reaction.
 * Radical reactions are sensitive to dioxygen due to it’s a paramagnetic property.
 * There are two types of radical mechanism: Non-chain and Chain radical mechanism.

Reductive Elimination
Reductive Elimination is the opposite of the oxidative addition where the oxidation state of the metal center of the complex decrease by two units. Unlike oxidative addition, reduction elimination only has one mechanism which is the counter part of the concerted oxidative addition mechanism. Reductive elimination is an intramolecular reaction and favored by low electron density of the metal center. Ligands must be in a cis position in order to undergo reductive elimination.

Application
The biggest application of the oxidative addition and reductive elimination is the cross coupling reaction. One of the most important reactions that allow the formation of a new bond (usually carbon) with the help of a metal catalyst.

Catalytic Cycle
Below shows a generic Pd-catalyzed cross-coupling cycle. Aside from Palladium, Nickel, Iron, Cobalt and Copper can also be work as a catalyst. In the given catalytic cycle below, the Pd(0) was generated from a palladium precatalyst. Afterwards it went through oxidative addition followed by transmetallation where metal (M) could be Sn, Zn, B, and Zr. Before reduction elimination could occur, isomerization must conducted first for the ligands to be in cis positions.
 * Catalytic cycle.jpg

Examples of C-C Cross Coupling Reaction

 * Cross Coupling Reactions.jpg