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Oxidative addition is one of the organometallic metal complex reactions in which usually a neutral ligand adds to a center of metal complex and then oxidizes the metal, typically by 2 electrons. This results in a change from a dⁿ to a d^n-2 or in some cases d^n-1 formal configuration of the metal. The reaction may be represented by

Adding to the transition metal complex, two electrons undergo transferring from the metal to the compound, thus leading to the cleavage of A-B bond. Therefore, both oxidation state and coordination number of the metal increase by 2. However, the reaction where the metal oxidation state and coordination number increase by 1 can also occur and be represented as follows:

History

Oxidative Addition processes have a relatively short history.

In 1959, an early example of an oxidative-addition reaction was noted by Bernard L. Shaw and J. Chatt. They prepared derivatives of Platinum(II) complexes using a variety of tertiary phosphines and triethylarsin as strong-field splitting ligands. However, the study of this class of the reactions was not striking until Vaska’s complex, IrCl(CO)(PPh₃)₂, and Wilkinson’s complex, RhCl(PPh₃)₃, were discovered in 1962 and 1965 respectively.

In 1962, Lauri Vaska with his coworker, Di Luzio reported the investigation of hydrogen addition in transition metal hydride complex. They found that the addition of hydrogen to trans- chlorocarbonylbis(triphenylphosphine)iridium(I), IrCl(CO)(PPh₃)₂ known as Vaska’s complex, yielded dihydridoiridium(III) complex, IrH₂Cl(PPh₃)₂. The reaction underwent from a square planar, 16 electron d⁸ metal complex with +I oxidation state to an octahedral 18 electron d⁶ complex with +III oxidation state.

Then, Vaska’s complex has become chemically versatile Ir compound since it can occur a number of oxidative addition reactions with other small molecules as illustrated below.

In 1965, Geoffrey Wilkinson discovered Tris(triphenylphosphinechlororhodium(I), RhCl(PPh₃)₃ later noted as ‘Wilkinson’s catalyst.’ It is one of the most practical organometal catalyst that itself undergoes oxidative addition with hydrogen and then catalyzes the hydrogenation of alkene.

Since then, the oxidative addition reactions have been recognized. More works have been studied and made this type of organometallic reactions becomes futile and brighter than the past development.

Mechanisms of Oxidative Addition

Concerted

Concerted oxidative additions occur through an associative mechanism in which a three centered σ complex is formed followed by intramolecular ligand bond cleavage to form the oxidized complex.

This mechanism is often observed for the activation of homonuclear diatomic molecules such as H₂ and N₂ as well as for highly strained ring systems such as cyclopropanes. Many C–H activation reactions also follow a concerted mechanism through the formation of an M–(C–H) agostic complex.

A well known example of this occurs in the addition of H₂ to Vaska’s complex. Formation of a trigonal bipyramidal dihydrogen complex is followed by cleavage of the H–H bond, due to electron back donation into the H–H σ* orbital.

SN₂ Type

SN₂ type oxidative addition is analogous to the well know substitution reaction in organic chemistry. Nucleophillic attack by the metal center at the least electronegative atom in the ligand substrate leads to cleavage of the L–X bond, to form an M–L+ species. This is followed by fast coordination of the free cation to the metal center. This mechanism is often observed in the addition of polarized substrates, like alkyl halides, to the metal center.

A classic example of this type of reaction is the addition of methyl iodide to Vaska’s complex.

Ionic

The ionic mechanism of oxidative addition is similar to the SN₂ type in that it involves the stepwise addition of two distinct ligand fragments. The key difference being that ionic mechanisms involve substrates which are dissociated in solution prior to any interactions with the metal center (eg. addition of HCl in aqueous solution).

Radical

In addition to SN₂ type reactions alkyl halides and other similar substrates can add to a metal center via a radical mechanism. However, there has been controversy concerning the validity of experiments used in the detection of radical intermediates.

Reactions which are generally accepted to proceed by a radical mechanism are known however. One example was proposed by Lednor and co-workers.

Applications

Impact on Organometallic Chemistry

Oxidative addition is an integral part of many mechanisms in organometallic chemistry. This is because oxidative addition of C-H forms M-C bonds and enables further reaction of the organic group on the metal (such as insertion). Without oxidative addition many reactions would not occur because C-H bonds are generally unreactive due to their high C-H bond energy (approximately 470 kJ/mol) and their low polarity. This is why the addition to a metal is said to activate the C-H bond. Oxidative addition can also occur with H₂, this enables the activation of the relatively strong H-H bond towards reaction by first coordinating it to a metal as two hydride ligands which can then undergo further reaction. The initial step in oxidative addition of H₂ can be seen as the coordination of the intact molecule in an η² fashion. This is supported by the fact that η²- H₂ complexes are oftentimes in equilibrium in solution with dihydride complexes as shown by variable-temperature nuclear magnetic resonance (NMR). Water can also be activated by oxidative addition.

Group 8,9, and 10 complexes having d⁸ metals also undergo oxidative addition, and the tendency to become oxidized to d⁶ increases as you move down a group or across a period. There are also known cases where early transition metals can undergo oxidative addition going from d⁶ to d⁴ . For example the reaction of: W(CO)₄(bipy) + SnCl₄→W(CO)₃(bipy)(SnCl₃(Cl)

Important Applications of Oxidative Addition in Industry

Oxidative addition plays an integral role in many commercial used processes. These include methanol carbonylation, hydrogenation of an alkene with an acetamido functional group, which is an important reaction in asymmetric hydrogenation. Oxidative addition is also an important component in the first step in the hydrocyanation of butadiene for the manufacturing of adiponitrile. During homogeneous catalytic reactions, bonds are generally broken by oxidative addition reactions and then new bonds are formed by reductive elimination and insertion reactions.

Many reactions are important steps in commercial homogenous catalytic processes.

This reaction is a step in methanol carbonylation:

This reaction shows the hydrogenation of an alkene with an acetamido functional group:

This is the first step in the hydrocyanation of butadiene for the manufacture of adiponitrile.

Many of the reactions that are common to main-group elements can be viewed as oxidative additions, including the addition of F₂ to BrF₃ and PF₃ to form BrF₄ and PF₄. Wilkinson's catalyst catalyzes the hydrogenation of alkenes, the mechanism of which involves the initial dissociation of one or two triphenylphosphine ligands to give 14 or 12-electron complexes, respectively, followed by oxidative addition of H2 to the metal.