Oxidative addition
Encyclopedia
Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry
. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre. The oxidative addition process is often employed in catalytic cycle
s, in conjunction with its reverse reaction, reductive elimination.
satisfy this requirement, but even high oxidation state metals undergo oxidative addition, as illustrated by the oxidation of Pt(II) with chlorine:
In classical organometallic chemistry
, the formal oxidation state of the metal and the electron count of the complex both increase by two. One-electron changes are also possible and in fact some oxidative addition reactions proceed via series of 1e changes. Although oxidative additions can occur with the insertion of a metal into many different substrates, oxidative additions are most commonly seen with H-H, H-X, and C-X bonds because these substrates are relevant to commercial applications.
Oxidative addition requires that the metal complex have a vacant coordination site. For this reason, oxidative additions are common for four- and five coordinate complexes.
Reductive elimination is the reverse of oxidative addition. 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 mutually adjacent on the metal's coordination sphere
. Reductive elimination is the key product-releasing step of several reactions that form C-H and C-C bonds.
pathways. Such substrates lack pi-bonds, consequently a three-centered σ complex is invoked, followed by intramolecular ligand bond cleavage to form the oxidized complex. The resulting ligands will be mutually cis, although subsequent isomerization may occur.
This mechanism applies to the addition of homonuclear diatomic molecules
such as H2. Many C–H activation reactions also follow a concerted mechanism through the formation of an M–(C–H) agostic complex.
Representative is the reaction of hydrogen
with Vaska's complex
, trans-IrCl(CO)[P(C6H5)3]2. In this transformation, iridium changes its formal oxidation state
from +I to +III. 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 whereas the product is a six-coordinate 18 electron complex.
Formation of a trigonal bipyramidal di-hydrogen intermediate is followed by cleavage of the H–H bond, due to electron back donation into the H–H σ* orbital.
This system is also in chemical equilibrium
, with the reverse reaction proceeding by the elimination of hydrogen gas with simultaneous reduction of the metal center.
Since the H-H bond cleavage requires electron back donation into the H–H σ* orbital, electron-rich metals favor this reaction. The concerted mechanism produces a cis dihydride, the stereochemistry of the other oxidative addition pathways do not usually produce cis adducts.
. Nucleophillic attack by the metal center at the less electronegative atom in the substrate leads to cleavage of the R–X bond, to form an [M–R]+ species. This step is followed by rapid coordination of the anion to the cationic metal center. For example, reaction of a square planar complex with methyl iodide:
This mechanism is often assumed in the addition of polar and electrophilic substrates, such as alkyl halides and halogens.
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.
Initiation
Propagation
(i.e., in solution) and heterogeneous catalysis
. such as the Monsanto process
and alkene
hydrogenation
using Wilkinson's catalyst
.
Organometallic chemistry
Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...
. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre. The oxidative addition process is often employed in catalytic cycle
Catalytic cycle
A catalytic cycle in chemistry is a term for a multistep reaction mechanism that involves a catalyst . The catalytic cycle is the main method for describing the role of catalysts in biochemistry, organometallic chemistry, materials science, etc. Often such cycles show the conversion of a...
s, in conjunction with its reverse reaction, reductive elimination.
In transition metals
For transition metals, oxidative reaction results in the decrease in the dn to a configuration with fewer electrons, often 2e fewer. Oxidative addition is favored for metals that are basic or easily oxidized. Metals with a relatively low oxidation stateOxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
satisfy this requirement, but even high oxidation state metals undergo oxidative addition, as illustrated by the oxidation of Pt(II) with chlorine:
- [PtCl4]2- + Cl2 → [PtCl6]2-
In classical organometallic chemistry
Organometallic chemistry
Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...
, the formal oxidation state of the metal and the electron count of the complex both increase by two. One-electron changes are also possible and in fact some oxidative addition reactions proceed via series of 1e changes. Although oxidative additions can occur with the insertion of a metal into many different substrates, oxidative additions are most commonly seen with H-H, H-X, and C-X bonds because these substrates are relevant to commercial applications.
Oxidative addition requires that the metal complex have a vacant coordination site. For this reason, oxidative additions are common for four- and five coordinate complexes.
Reductive elimination is the reverse of oxidative addition. 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 mutually adjacent on the metal's coordination sphere
Coordination sphere
In coordination chemistry, the coordination sphere refers to a central atom or ion and an array of molecules or anions, the ligands, around.Molecules that are attached noncovalently to the ligands are called the second coordination sphere....
. Reductive elimination is the key product-releasing step of several reactions that form C-H and C-C bonds.
Mechanisms of oxidative addition
Oxidative additions proceed via many pathways that depend on the metal center and the substrates.Concerted pathway
Oxidative additions of nonpolar substrates such as hydrogen and hydrocarbons appear to proceed via concertedConcerted reaction
In chemistry, a concerted reaction is a chemical reaction in which all bond breaking and bond making occurs in a single step. Reactive intermediates or other unstable high energy intermediates are not involved. Concerted reaction rates tend not to depend on solvent polarity ruling out large buildup...
pathways. Such substrates lack pi-bonds, consequently a three-centered σ complex is invoked, followed by intramolecular ligand bond cleavage to form the oxidized complex. The resulting ligands will be mutually cis, although subsequent isomerization may occur.
This mechanism applies to the addition of homonuclear diatomic molecules
Homonuclear molecule
Homonuclear molecules, or homonuclear species, are molecules composed of only one type of element. Homonuclear molecules may consist of various numbers of atoms, depending on the element's properties. Some elements form molecules of more than one size. Noble gases rarely form bonds, so they only...
such as H2. Many C–H activation reactions also follow a concerted mechanism through the formation of an M–(C–H) agostic complex.
Representative is the reaction of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
with Vaska's complex
Vaska's complex
Vaska's complex is the trivial name for the chemical compound trans-chlorocarbonylbisiridium, which has the formula IrCl[P3]2. This square planar diamagnetic organometallic complex consists of a central iridium atom bound to two mutually trans triphenylphosphine ligands, carbon monoxide, and a...
, trans-IrCl(CO)[P(C6H5)3]2. In this transformation, iridium changes its formal oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
from +I to +III. The product is formally bound to three anions: one chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
and two hydride
Hydride
In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
ligands. As shown below, the initial metal complex has 16 valence electrons and a coordination number of four whereas the product is a six-coordinate 18 electron complex.
Formation of a trigonal bipyramidal di-hydrogen intermediate is followed by cleavage of the H–H bond, due to electron back donation into the H–H σ* orbital.
This system is also in chemical equilibrium
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
, with the reverse reaction proceeding by the elimination of hydrogen gas with simultaneous reduction of the metal center.
Since the H-H bond cleavage requires electron back donation into the H–H σ* orbital, electron-rich metals favor this reaction. The concerted mechanism produces a cis dihydride, the stereochemistry of the other oxidative addition pathways do not usually produce cis adducts.
SN2-type
Some oxidative additions proceed analogously to the well known bimolecular nucleophilic substitution reactions in organic chemistryOrganic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
. Nucleophillic attack by the metal center at the less electronegative atom in the substrate leads to cleavage of the R–X bond, to form an [M–R]+ species. This step is followed by rapid coordination of the anion to the cationic metal center. For example, reaction of a square planar complex with methyl iodide:
This mechanism is often assumed in the addition of polar and electrophilic substrates, such as alkyl halides and halogens.
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 (e.g. 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 radicalRadical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
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.
Initiation
- [(CH3)2C(CN)N]2 → 2 (CH3)2(CN)C• + N22(CN)C• + PhBr → (CH3)2(CN)CBr + Ph•
Propagation
- Ph• + [Pt(PPh3)2] → [Pt(PPh3)2Ph]•
- [Pt(PPh3)2Ph]• + PhBr → [Pt(PPh3)2PhBr] + Ph•
Applications
Oxidative addition and reductive elimination are invoked in many catalytic processes both in homogeneous catalysisHomogeneous catalysis
In chemistry, homogeneous catalysis is a sequence of reactions that involve a catalyst in the same phase as the reactants. Most commonly, a homogeneous catalyst is codissolved in a solvent with the reactants.-Acid catalysis:...
(i.e., in solution) and heterogeneous catalysis
Heterogeneous catalysis
In chemistry, heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid, liquid, vs gas, but also immiscible liquids, e.g. oil and water. The great majority of practical heterogeneous catalysts...
. such as the Monsanto process
Monsanto process
The Monsanto process is an important method for the manufacture of acetic acid by catalytic carbonylation of methanol. This process operates at a pressure of 30–60 atm and a temperature of 150–200 °C and gives a selectivity greater than 99%. It was developed 1960 by German BASF and...
and alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
hydrogenation
Hydrogenation
Hydrogenation, to treat with hydrogen, also a form of chemical reduction, is a chemical reaction between molecular hydrogen and another compound or element, usually in the presence of a catalyst. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically...
using Wilkinson's catalyst
Wilkinson's catalyst
Wilkinson's catalyst is the common name for chlorotrisrhodium, a coordination compound with the formula RhCl3 . It is named after the late organometallic chemist and 1973 Nobel Laureate, Sir Geoffrey Wilkinson who popularized its use.-Structure and basic properties:The compound is a square planar,...
.