Difference Between Inner Sphere and Outer Sphere Mechanism

The key difference between inner sphere and outer sphere mechanism is that inner sphere mechanism occurs between complexes via a binding ligand whereas outer sphere mechanism occurs between complexes that do not undergo substitution.

Inner sphere mechanism and outer sphere mechanism are two different types of electron transfer in coordination complexes. The inner sphere mechanism occurs via a covalent bond or linkage while the outer sphere mechanism occurs between two separate species.

CONTENTS

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

What is Inner Sphere Mechanism?

Inner sphere mechanism is the most common type of electron transfer in coordination complexes. It is a type of redox chemical reaction. This electron transfers proceeds through a covalent bond that exists between the oxidant and the reductant of the redox reaction.

In this inner sphere mechanism, a ligand acts as a bridge between the metal ions of the oxidant and the reductant. However, the presence of large ligands inhibits the inner-sphere mechanism. It is because they prevent the formation of the bridging intermediates. Therefore, this mechanism can be found very rarely in biological systems, since there are many bulky groups of proteins present where redox reactions take place.

Moreover, the ligand that participates in the formation of a bridge is called the bridging ligand. It should be a chemical species that can convey electrons. Typically, these ligands have more than one lone electron pairs. Therefore, it can serve as an electron donor. i.e. halides, hydroxide, thiocyanate are some bridging ligands. Furthermore, the formation of a bridging complex is a reversible process. An alternative pathway for inner-sphere mechanism is the outer sphere electron transfer which occurs through un-linked chemical species.

What is Outer Sphere Mechanism?

Outer sphere mechanism is a type of electron transfer which occurs between separate chemical species. Here, the two chemical species involved in the electron transfer exists separate and intact before, during and after the electron transfer process. Since the two species are separate, the electrons are forced to move through space from one species to the other.

There are two common examples where the outer-sphere mechanism takes place:

1. Self-exchange: the electron transfer occurs between two identical chemical species which have different oxidation states. Ex: the degenerate reaction between tetrahedral ions of permanganate and manganate.
2. Iron-sulfur proteins: the basic mechanism for the function of these iron-sulfur proteins. The electron transfer occurs rapidly in these structures because of the small structural difference between them.

What is the Difference Between Inner Sphere and Outer Sphere Mechanism?

Inner sphere and outer sphere mechanisms are two different types of electron transfer mechanisms. The key difference between inner sphere and outer sphere mechanism is that the inner sphere mechanism occurs between complexes via a binding ligand, whereas outer-sphere mechanism occurs between complexes that do not undergo substitution. That means; the outer sphere mechanism occurs between chemical species which are separate and intact before, during and after the electron transfer. Therefore, bridging ligands are not involved in outer-sphere mechanism, instead, they transfer electrons via forcing the electrons to move through the space. Moreover, the outer-sphere mechanism is an alternative pathway for the inner-sphere mechanism.

Below is a side by side comparison of the difference between inner sphere and outer sphere mechanism.

Summary – Inner Sphere vs Outer Sphere Mechanism

Inner sphere and outer sphere mechanisms are two different types of electron transfer mechanisms. The key difference between inner sphere and outer sphere mechanism is that inner sphere mechanism occurs between complexes via a binding ligand whereas outer sphere mechanism occurs between complexes that do not undergo substitution.

Reference:

1. “RO9. Electron Transfer: Inner Sphere – Chemistry Libretexts”. Libretexts.Org, 2019, Available here.
2. “Inner Sphere Electron Transfer”. Wikipedia.Org, 2019, Available here.
3. “Coordination Compound – Oxidation-Reduction”. Encyclopedia Britannica, 2019, Available here.

Image Courtesy:

1. “Creutz-Taube-Ion” By Nothingserious – Own work (Public Domain) via Commons Wikimedia
2. “FdRedox” By Smokefoot – Own work (Public Domain) via Commons Wikimedia