Electron Transfer Reactions Overview

Electron transfer reactions are fundamental processes in chemistry where one species loses electrons (is oxidized, acting as the reductant) and another species gains electrons (is reduced, acting as the oxidant). These reactions can occur via two main mechanisms: outer-sphere and inner-sphere electron transfer.

Inner-Sphere Electron Transfer

In the inner-sphere mechanism, the two reacting metal complexes become directly linked by a common ligand, called a bridging ligand. This bridge facilitates the transfer of an electron from the reductant to the oxidant. The bridging ligand must be able to coordinate simultaneously to both metal centers.

A key aspect of the inner-sphere mechanism is the formation of a precursor complex where the two reactants are connected by the bridging ligand. After electron transfer occurs through the bridge, this complex breaks apart, forming the products.

The bridging ligand can originate from either the oxidant complex or the reductant complex. The question asks to identify the reaction where the bridging ligand comes from the reductant.

Analyzing the Electron Transfer Reactions

Let's examine each given electron transfer reaction to determine the roles of the reactants and the potential origin of a bridging ligand in an inner-sphere mechanism.

1. [IrCl6]2- + [Cr(OH2)6]2+ → products

   • Reductant: [Cr(OH2)6]2+ (Cr is in +2 oxidation state, can be oxidized to +3).
   • Oxidant: [IrCl6]2- (Ir is in +4 oxidation state, can be reduced to +3 or +2).
   • Potential Bridging Ligand: Cl⁻ from the oxidant or H₂O from the reductant. Chloride is a common bridging ligand. If inner-sphere, a Cl⁻ from [IrCl6]2- would likely bridge to Cr²⁺. The bridging ligand would come from the oxidant.

2. [Co(NH3)5Cl]2+ + [Cr(OH2)6]2+ → products

   • Reductant: [Cr(OH2)6]2+ (Cr²⁺ → Cr³⁺).
   • Oxidant: [Co(NH3)5Cl]2+ (Co³⁺ → Co²⁺).
   • Potential Bridging Ligand: Cl⁻ from the oxidant or H₂O from the reductant. This reaction is a classic example of inner-sphere electron transfer involving a chloride bridge. The chloride ligand comes from the cobalt(III) complex, which is the oxidant. The bridging ligand comes from the oxidant.

3. [Fe(CN)6]4- + [IrCl6]2- → products

   • Reductant: [Fe(CN)6]4- (Fe²⁺ → Fe³⁺).
   • Oxidant: [IrCl6]2- (Ir⁴⁺ → Ir³⁺).
   • Potential Bridging Ligand: CN⁻ from the reductant or Cl⁻ from the oxidant. Both complexes are relatively inert, and this reaction can proceed via an outer-sphere mechanism. However, if an inner-sphere pathway is significant, either CN⁻ or Cl⁻ could potentially bridge. If a CN⁻ ligand bridges, it comes from the reductant. If a Cl⁻ ligand bridges, it comes from the oxidant.

4. [CrO4]2- + [Fe(CN)6]4- → products

   • Reductant: [Fe(CN)6]4- (Fe²⁺ → Fe³⁺).
   • Oxidant: [CrO4]2- (Cr⁶⁺ → lower oxidation states, e.g., Cr³⁺).
   • Potential Bridging Ligand: CN⁻ from the reductant or O²⁻ from the oxidant. Electron transfer reactions involving chromate (Cr(VI)) can occur via inner-sphere mechanisms. In the reaction with hexacyanoferrate(II), a cyanide ligand from the [Fe(CN)6]4- complex can attack a chromium center in [CrO4]2-, forming a bridged intermediate like [(NC)5Fe^{III}-CN-Cr^{VI}O3]5-. Electron transfer then occurs across the cyanide bridge. In this specific pathway, the bridging ligand (CN⁻) comes from the reductant ([Fe(CN)6]4-).

Comparing the options, options 1 and 2 typically involve bridging ligands from the oxidant. Option 3 can be outer-sphere, or inner-sphere potentially from either reactant's ligands. Option 4, the reaction between chromate and hexacyanoferrate(II), is known to proceed, at least partially, via an inner-sphere mechanism where a cyanide ligand from the hexacyanoferrate(II) (the reductant) bridges to the chromate (the oxidant).

Therefore, the electron transfer reaction in which the bridging ligand comes from the reductant is [CrO4]2- + [Fe(CN)6]4- → products.