Electron Transfer Mechanisms

Electron transfer reactions are fundamental in many chemical and biological processes. They can occur via different pathways, primarily categorized as outer-sphere and inner-sphere mechanisms.

Understanding Outer-Sphere Mechanism

In the outer-sphere electron transfer mechanism, the electron moves from the reductant to the oxidant without any direct bond formation or breaking between the coordination spheres of the two reacting species. The electron essentially tunnels through space or the surrounding medium.

• The coordination shells of both the reductant and the oxidant remain intact during the electron transfer step.
• This mechanism is common when both reactants are substitutionally inert, meaning their ligands do not easily exchange.

Let's evaluate assertion A:

• A. An outer-sphere mechanism involves electron transfer from the reductant to the oxidant, with the coordination shells or spheres of each staying intact. This statement accurately describes the outer-sphere mechanism. The electron transfers while the original coordination structures of the metal complexes are preserved.

Therefore, assertion A is correct.

Understanding Inner-Sphere Mechanism

The inner-sphere electron transfer mechanism involves the formation of a bridged intermediate complex where the reductant and oxidant are linked by a common ligand. The electron is transferred through this bridging ligand.

• A bridging ligand is shared between the two metal centers.
• This typically requires at least one of the reactants to be substitutionally labile (meaning its ligands can be easily replaced or shared) to form the bridged intermediate.
• The electron transfer occurs within this intermediate complex.

Let's evaluate assertions B, C, and D:

• B. In the inner-sphere mechanism, the reductant and oxidant share a ligand in their inner or primary coordination sphere which assists in electrons being transferred. This statement correctly describes the key feature of the inner-sphere mechanism: the formation of a bridged complex via a shared ligand, which facilitates electron transfer.
• C. In the inner-sphere mechanism, an oxidant possesses at least one ligand capable of binding simultaneously to two metal ions. For the formation of a bridged intermediate, at least one of the reacting species must have a ligand that can act as a bridge between the two metal centers. This ligand could come from either the oxidant or the reductant. The statement says the oxidant possesses such a ligand, which is a common scenario leading to the bridged species required for inner-sphere transfer.
• D. In the inner-sphere mechanism, ligands of reductant are substitutionally inert. This statement is generally incorrect. For the inner-sphere mechanism to proceed via a bridging ligand provided by the reductant, the reductant must be substitutionally labile to form the bridge. If the bridge is provided by the oxidant, the reductant can be labile or inert, but the statement claims inertness as a general characteristic, which is not true. Lability is often required for one or both reactants to form the bridged intermediate.

Based on the analysis:

• Assertion B is correct.
• Assertion C is correct.
• Assertion D is incorrect.

Summary of Correct Statements

Based on the detailed understanding of outer-sphere and inner-sphere electron transfer mechanisms, the correct assertions are A, B, and C.

Assertion A correctly describes the outer-sphere mechanism.

Assertions B and C correctly describe aspects of the inner-sphere mechanism necessary for forming the bridged intermediate.

Assertion D is incorrect because the reductant is often substitutionally labile in inner-sphere reactions, especially if it provides the bridging ligand.

Therefore, the correct statements are A, B and C.