Inorganic and Organometallic Reaction Mechanisms: A Comprehensive Guide for Chemists

Inorganic and organometallic reactions are fundamental processes that underlie many areas of chemistry, from catalysis to materials science. Understanding how these reactions work requires a knowledge of the mechanisms that govern them. In this article, we will provide an overview of the main types of inorganic and organometallic reaction mechanisms, and illustrate them with examples from the literature.

What are inorganic and organometallic reactions?

Inorganic reactions are those that involve elements or compounds that are not carbon-based, except for carbon monoxide, carbon dioxide, carbonates, cyanides, and a few others. Organometallic reactions are those that involve compounds that contain a metal-carbon bond, such as metal alkyls, metal carbonyls, metal carbenes, and metal clusters. In both cases, the reactions can involve coordination complexes, which are molecules or ions that have a central metal atom or ion surrounded by ligands. Ligands are atoms or groups of atoms that donate one or more pairs of electrons to the metal to form a bond.

What are the main types of inorganic and organometallic reaction mechanisms?

There are many types of inorganic and organometallic reaction mechanisms, but we can classify them into two main categories: those that involve gain or loss of ligands, and those that involve modification of ligands. The former include ligand substitution, oxidative addition, reductive elimination, and nucleophilic displacement. The latter include migratory insertion, β-hydride elimination, and transmetalation. We will briefly describe each type and give some examples below.

Ligand substitution

Ligand substitution is a reaction in which one ligand is replaced by another ligand at the metal center of a complex. The coordination number (the number of ligands attached to the metal) does not change in this reaction. Ligand substitution can occur via two main pathways: associative or dissociative. In the associative pathway, the incoming ligand first adds to the metal center, forming an intermediate with a higher coordination number. Then, one of the original ligands leaves the complex, restoring the original coordination number. In the dissociative pathway, one of the original ligands first leaves the complex, forming an intermediate with a lower coordination number. Then, the incoming ligand replaces the vacant site on the metal center.

For example, consider the reaction of [Co(NH₃)₅Cl] with water in acidic solution:

[Co(NH₃)₅Cl] + H₂O → [Co(NH₃)₅(H₂O)] + Cl

This reaction involves the substitution of a chloro ligand by a water ligand at the cobalt center. The coordination number of cobalt remains 6 throughout the reaction. The mechanism of this reaction is dissociative, as shown below:

[Co(NH₃)₅Cl] → [Co(NH₃)₅] + Cl

[Co(NH₃)₅] + H₂O → [Co(NH₃)₅(H₂O)]

The rate-determining step (the slowest step) is the first one, where the chloro ligand leaves the complex. The second step is fast and reversible.

Oxidative addition

Oxidative addition is a reaction in which one or more ligands add to the metal center of a complex and oxidize it. This addition can occur in a cis- or trans-fashion. A cis-addition means that two ligands are added so that

Reductive elimination

Reductive elimination is a reaction in which the oxidation state of the metal center decreases while forming a new covalent bond between two ligands. It is the microscopic reverse of oxidative addition, and is often the product-forming step in many catalytic processes. Since oxidative addition and reductive elimination are reverse reactions, the same mechanisms apply for both processes, and the product equilibrium depends on the thermodynamics of both directions. [1] [2]

Reductive elimination can involve a two-electron change at a single metal center (mononuclear) or a one-electron change at each of two metal centers (binuclear, dinuclear, or bimetallic). [1] [2] For mononuclear reductive elimination, the oxidation state of the metal decreases by two, while the d-electron count of the metal increases by two. This pathway is common for d₈ metals Ni (II), Pd (II), and Au (III) and d₆ metals Pt (IV), Pd (IV), Ir (III), and Rh (III). Additionally, mononuclear reductive elimination requires that the groups being eliminated must be cis to one another on the metal center. [3] For binuclear reductive elimination, the oxidation state of each metal decreases by one, while the d-electron count of each metal increases by one. This type of reactivity is generally seen with first row metals, which prefer a one-unit change in oxidation state, but has been observed in both second and third row metals. [4]

As with oxidative addition, several mechanisms are possible with reductive elimination. The prominent mechanism is a concerted pathway, meaning that it is a nonpolar, three-centered transition state with retention of stereochemistry. In addition, an S_N2 mechanism, which proceeds with inversion of stereochemistry, or a radical mechanism, which proceeds with obliteration of stereochemistry, are other possible pathways for reductive elimination. [1]

For example, consider the reaction of [Pd(Me)₂(PPh₃)₂] with iodine:

[Pd(Me)₂(PPh₃)₂] + I₂ → MeI + [PdI(PPh₃)₂] + MeI

This reaction involves the oxidative addition of I₂ to Pd (0) followed by the reductive elimination of MeI from Pd (II). The mechanism of the reductive elimination step is concerted, as shown below:

[Pd(Me)₂(PPh₃)₂] → [Pd(Me)(Me)(PPh₃)₂] + MeI

The rate-determining step (the slowest step) is the reductive elimination step, where the methyl groups form a new C−C bond and leave the palladium center. The transition state involves a bending of the Pd−C bonds and a partial overlap of the C−C σ* orbital with the Pd d orbital.

Nucleophilic displacement

Nucleophilic displacement is a reaction in which a nucleophilic ligand displaces another ligand at the metal center of a complex. The coordination number and the oxidation state of the metal do not change in this reaction. Nucleophilic displacement can occur via two main pathways: S_N1 or S_N2. In the S_N1 pathway, the leaving ligand first dissociates from the metal center, forming an intermediate with a lower coordination number. Then, the nucleophilic ligand attacks unsaturated ligand such as CO or a 1,2-insertion of a η unsaturated ligand such as an alkene or an alkyne. In both cases, the total electron count of the complex decreases by two during the actual insertion event. [7]

Migratory insertion can occur via two main mechanisms: one in which the anionic ligand attacks the electrophilic part of the neutral ligand (the anionic ligand migrates to the neutral ligand), and one in which the neutral ligand inserts itself between the metal and the anionic ligand (the neutral ligand migrates to the metal). The former mechanism is more common for CO insertions, while the latter mechanism is more common for alkene and alkyne insertions. [6]

For example, consider the reaction of [Fe(CO)₄(CH₃)] with CO:

[Fe(CO)₄(CH₃)] + CO → [Fe(CO)₅] + CH₄

This reaction involves the migratory insertion of CO into the Fe−CH₃ bond followed by the reductive elimination of CH₄. The mechanism of the migratory insertion step is shown below:

[Fe(CO)₄(CH₃)] → [Fe(CO)₅(CH₃)]

[Fe(CO)₅(CH₃)] → [Fe(CO)₅] + CH₄

The first step involves a 1,1-insertion of CO into the Fe−CH₃ bond, where the carbon atom of CO attacks the hydrogen atom of CH₃. This forms a pentacarbonyl methyl complex with a vacant coordination site. The second step involves a reductive elimination of CH₄, where the methyl group forms a new C−H bond and leaves the iron center. The transition state involves a bending of the Fe−C bonds and a partial overlap of the C−H σ* orbital with the Fe d orbital.

Migratory insertion – Wikipedia”,”snippets”:[“In organometallic chemistry, a migratory insertion is a type of reaction wherein two ligands on a metal complex combine. It is a subset of reactions that very closely resembles the insertion reactions, and both are differentiated by the mechanism that leads to the resulting stereochemistry of the products.”],”url”:”https://en.wikipedia.org/wiki/Migratory_insertion”}]}

Beta-hydride elimination

Beta-hydride elimination is a reaction in which an alkyl group bonded to a metal center is converted into the corresponding metal-bonded hydride and an alkene. The alkyl group must have hydrogens on the beta-carbon, which is the carbon atom next to the one attached to the metal. For instance, butyl groups can undergo this reaction but methyl groups cannot. [8] Beta-hydride elimination is the microscopic reverse of olefin insertion into a M–H bond, and is often the product-forming step in many catalytic processes. [9]

Beta-hydride elimination requires a few conditions to occur: the complex must have an open coordination site and an accessible, empty orbital on the metal center; the M–C_α and C_α–C_β bonds must be able to align in a syn coplanar arrangement; and the metal must be able to accept a hydride ligand. [9] The mechanism of beta-hydride elimination involves a four-center transition state in which the hydride is transferred from the beta-carbon to the metal center, forming a new M–H bond and a new C_α–C_β double bond. The stereochemistry of the alkene product depends on the configuration of the alkyl group and the metal center. [8]

For example, consider the reaction of [Ir(CO)(PPh₃)₂(CH₂CH₂CH₃)] with CO:

[Ir(CO)(PPh₃)₂(CH₂CH₂CH₃)] + CO → [Ir(CO)₂(PPh₃)₂] + CH₂=CHCH₃

This reaction involves the beta-hydride elimination of propyl group followed by the coordination of CO to the iridium center. The mechanism of the beta-hydride elimination step is shown below:

[Ir(CO)(PPh₃)₂(CH₂CH₂CH₃)] → [Ir(CO)(PPh₃)(H)(CH=CHCH^*

[Ir(CO)(PPh₃)(H)(CH=CHCH^*

The first step involves a syn coplanar alignment of the Ir–C and C–C bonds, where the hydride migrates from the beta-carbon to the iridium center. This forms a pentacarbonyl propene complex with a vacant coordination site. The second step involves the coordination of CO to fill that site. The transition state involves a bending of the Ir–C bonds and a partial overlap of the C–H σ* orbital with the Ir d orbital.

Beta-hydride elimination – Wikipedia”,”snippets”:[“Beta-hydride elimination is a reaction in which an alkyl group bonded to a metal centre is converted into the corresponding metal-bonded hydride and an alkene. The alkyl must have hydrogens on the beta-carbon.”],”url”:”https://en.wikipedia.org/wiki/Beta-hydride_elimination”}]}

Conclusion

In this article, we have discussed some of the main types of inorganic and organometallic reaction mechanisms, such as ligand substitution, oxidative addition, reductive elimination, nucleophilic displacement, migratory insertion, and beta-hydride elimination. These mechanisms involve the formation or cleavage of metal-ligand bonds, the change of coordination number or oxidation state of the metal center, and the generation or consumption of vacant sites on the metal complex. These reactions are influenced by various factors, such as the nature and geometry of the ligands, the electronic and steric properties of the metal center, and the thermodynamics and kinetics of the processes. These reactions are also important for many applications in catalysis, synthesis, and materials science. Understanding these mechanisms can help us to design better catalysts and reagents for various chemical transformations.

Inorganic And Organometallic Reaction Mechanisms