Difference Between Inner and Outer Sphere Mechanism

The key difference between inner and outer sphere mechanism is that the inner sphere electron transfer occurs between complexes via bridging ligands, whereas the outer sphere electron transfer mechanism occurs between complexes that do not undergo substitution.

Inner sphere and outer sphere electron transfer mechanisms are two limiting mechanisms of electron transfer. These mechanisms describe the redox reactions of coordination complexes.

CONTENTS

1. Overview and Key Difference
2. What is Inner Sphere Mechanism
3. What is Outer Sphere Mechanism
4. Side by Side Comparison – Inner vs Outer Sphere Mechanism in Tabular Form
5. Summary

What is Inner Sphere Mechanism?

Inner sphere mechanism of electron transfer is a redox chemical reaction that occurs via a covalent linkage between oxidant and reductant reactant of the reaction. Here, a ligand bridges the oxidant and reductant reactants during the reaction. However large ligands inhibit this reaction. This is because these large ligands can prevent the reaction from forming the crucial bridged intermediate. Therefore, this type of electron transfer mechanisms is rare in biological systems. Usually, this mechanism is useful in describing reactions of transition metal complexes.

What is Outer Sphere Mechanism?

Outer sphere mechanism of electron transfer is a chemical reaction in which reactants and products exist separate from each other before, during and after the event of electron transfer. Unlike in the inner sphere mechanism, there is no bridging between reactants in outer sphere mechanism. Therefore, this electron transfer occurs without any disruption of the coordination complex.

Figure 01: Redox Reactions for Fe₄S₄ Clusters

In this mechanism, the electrons are forced to move from one redox centre to another through space. Further, the outer sphere mechanism of electron transfer is the basis of the biological function of iron-sulfur proteins.

What is the Difference Between Inner and Outer Sphere Mechanism?

Inner sphere and outer sphere mechanism of electron transfer describe the redox reactions of coordination complexes. The key difference between inner and outer sphere mechanism is that the inner sphere electron transfer occurs between complexes via bridging ligands, whereas the outer sphere electron transfer mechanism occurs between complexes that do not undergo substitution. Inner sphere mechanism of electron transfer is rare in biological systems, but the outer sphere mechanism is common.

The below infographic provides more details on the difference between inner and outer sphere mechanism.

Summary – Inner vs Outer Sphere Mechanism

Inner sphere and outer sphere mechanism of electron transfer describe the redox reactions of coordination complexes. The key difference between inner and outer sphere mechanism is that the inner sphere electron transfer occurs between complexes via bridging ligands whereas outer sphere electron transfer mechanism occurs between complexes that do not undergo substitution.

Reference:

1. “Oxidation-Reduction.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., Available here.

Image Courtesy:

1. “FdRedox” By Smokefoot – Own work, (Public Domain) via Commons Wikimedia

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