What is the Difference Between Oxidative Addition and Reductive Elimination

Oxidative addition and reductive elimination are two reactions found in the field of organometallic chemistry. Organometallic chemistry is a field that mainly focuses on organometallic compounds, which are molecules containing at least one metal-carbon bond. This can involve metals like alkali metals, transition metals, and even some metalloids such as silicon or boron.

What is the difference between oxidative addition and reductive elimination? Oxidative addition increases the coordination number of the metal center, whereas reductive elimination decreases the coordination number of the metal center.

Key Areas Covered

1. What is Oxidative Addition  
      – Definition, Features, Role
2. What is Reductive Elimination
      – Definition, Features, Role
3. Similarities Between Oxidative Addition and Reductive Elimination
      – Outline of Common Features
4. Difference Between Oxidative Addition and Reductive Elimination
      – Comparison of Key Differences
5. FAQ: Oxidative Addition and Reductive Elimination
      – Answers to Frequently Asked Questions

Key Terms

Oxidative Addition, Reductive Elimination

What is Oxidative Addition

Oxidative addition is a reaction where a metal complex gains two electrons and increases its coordination number (the number of ligands bonded to the metal) by two. Here, the metal center becomes electron rich and gets two partner atoms. This is frequently found in organometallic chemistry. Here the metal complex’s oxidation state increases by 2. The metal gains electrons, making it more electron rich.

The coordination number, which is the number of ligands bonded to the metal, also increases by 2. This means the metal has more space for new bonding partners. To achieve this electron gain, the metal complex cleaves a single bond in a molecule, typically between two very electronegative atoms (like hydrogen and a halogen). By breaking this bond, the metal grabs some electron density from each fragment and forms new bonds with them.

What is Reductive Elimination

Reduction elimination involves a metal center shedding some electrons and forming a new covalent bond between two ligands already attached to it. Reductive elimination is the opposite of oxidative elimination in organic chemistry.

The metal center’s oxidation state decreases by 2, by losing electron density and becoming less electron rich. The coordination number also decreases by 2, as the metal lets go of two ligands to form a new bond between them.

The key feature is the formation of a new covalent bond between two ligands that were previously bonded to the same metal center. These ligands must be in a specific geometric arrangement (often cis to each other) for the reaction to occur. This new bond formation results in the release of two separate molecules from the metal complex.

Reductive elimination often serves as the final step in a catalytic cycle. After a series of transformations on a substrate molecule, the metal complex expels the product through this reaction.

Similarities Between Oxidative Addition and Reductive Elimination

1. Both reactions involve organometallic complexes, where a central metal atom is surrounded by ligand molecules.
2. Both processes cause a change in the coordination number of the metal center.

Difference Between Oxidative Addition and Reductive Elimination

Definition

• Oxidative addition is a reaction where a metal complex gains two electrons and increases its coordination number by two. Reduction elimination, on the other hand, is a reaction that involves a metal center shedding some electrons and forming a new covalent bond between two ligands already attached to it.

Oxidation State

• Oxidative addition increases the metal center’s oxidation state by 2 while reductive elimination decreases the metal center’s oxidation state by 2.

Electron Density

• The metal gains electron density in oxidative addition, while the metal loses electron density in reductive elimination.

Bonds

• Oxidative addition breaks a single bond in a molecule, typically between two electronegative atoms, while reductive elimination forms a new covalent bond between two ligands already attached to the metal center.

Ligands

• Oxidative addition introduces two new ligands from the broken bond to the metal center, whereas reductive elimination eliminates two ligands from the metal complex, forming a new bond between them.

In Catalytic Cycle

• Oxidative addition is often the first step in a catalytic cycle, activating the substrate molecule. Meanwhile, reductive elimination is often the final step in a catalytic cycle, releasing the product molecule.

Conclusion

Oxidative addition and reductive elimination are two contrasting processes. The main difference between oxidative addition and reductive elimination is that oxidative addition increases the coordination number of the metal center whereas reductive elimination decreases the coordination number of the metal center.

FAQ: Oxidative Addition and Reductive Elimination

1. What is meant by oxidative addition?

Oxidative addition is a reaction in organometallic chemistry where a metal center increases its oxidation state by two while forming new bonds with a molecule. It often involves breaking a single bond into two new bonds with the metal.

2. Why does oxidative addition increase down the group?

Down a group, metals tend to have lower ionization energy and larger atomic radii. These make oxidative addition generally more favorable down a group in the periodic table.

3. What favours oxidative addition?

Several factors favor oxidative addition. These include:

1. Low oxidation state of the metal center
2. The presence of good sigma donors in the incoming molecule
3. Absence of steric hindrance around the metal center

4. What is an example of oxidation addition?

A classic example of oxidative addition is the reaction between hydrogen (H₂) and chloromethane (CH₃Cl) with a catalyst like PtCl₂. The platinum metal center in PtCl₂ increases its oxidation state by 2 while breaking the H-H and C-Cl bonds, forming new Pt-H and Pt-Cl bonds.

5. What are the factors affecting reductive elimination?

Reductive elimination rates in organometallic chemistry refer to the process where two ligands are removed from a metal center to form a new bond between them. Reductive elimination rates depend on several factors like metal properties, steric bulk, ligand characteristics, coordination number, and geometry.

Reference:

1. “Oxidative Addition.” Wikipedia. Wikipedia Foundation.
2. “Reductive Elimination.” Science Direct.

Image Courtesy:

1. “Oxidative addition-reductive elimination general scheme” By anica pinongcos – Own work (CC0) via Commons Wikimedia
2. “General Reductive Elimination” By Jgurak – Own work (CC BY-SA 4.0) via Commons Wikimedia