Electron Exchange Rate Factors
The rate of outer-sphere electron-exchange reactions is significantly influenced by the structural reorganization energy ($\lambda$). This energy is required to distort the complex ion's geometry from its equilibrium configuration in one oxidation state to match the equilibrium configuration of the other oxidation state. Higher reorganization energy corresponds to a slower electron-exchange rate.
Comparative Analysis of Complex Ions
We compare the structural differences between the oxidized and reduced forms of the complex ions presented in the options:
- $[Co(NH_3)_6]^{3+/2+}$: Cobalt(III) and Cobalt(II) ions have different ionic radii and coordination geometries. The strong coordination of ammonia ligands, particularly to Co(III), and the change in electronic configuration (e.g., d6 to d7) result in substantial structural rearrangement. This leads to a high reorganization energy and thus a slow electron-exchange rate. The kinetic inertness associated with Co(III) amine complexes further contributes to this slowness.
- $[Fe(H_2O)_6]^{3+/2+}$: The iron couple with water ligands generally exhibits a faster exchange rate, indicating less structural reorganization is required.
- $[Ru(NH_3)_6]^{3+/2+}$: Ruthenium complexes often display different kinetic profiles compared to cobalt. This pair typically undergoes relatively facile electron exchange.
- $[Mn(CN)_6]^{3-/4-}$: While involving strong ligands, the Co(III)/Co(II) ammine system is a classic example known for its slow exchange due to significant structural changes.
Identifying the Slowest Reaction Pair
Based on the principle of reorganization energy, the pair exhibiting the slowest outer-sphere electron-exchange reaction at $25 ^\circ C$ is the one with the greatest structural differences between its two oxidation states. The complex pair $[Co(NH_3)_6]^{3+}$ and $[Co(NH_3)_6]^{2+}$ requires the largest structural adjustment, leading to the highest activation barrier and the slowest reaction kinetics among the given choices.
Important Questions from Electron Transfer Reactions
Consider the following statements for the self‐exchange electron transfer reaction in [Cr(H2O)6]2+/3+
a. σ* orbitals are only involved in electron transfer
b. It involves large inner‐sphere reorganization energy
c. It involves no change in M–L bond lengths
d. Rate of self‐exchange electron transfer is fast
The correct statements are
In 3‐iron ferredoxins, the number of sulfide bridges and cysteinyl ligands, respectively, are:
The second order rate constants for the outer sphere self- exchange electron transfer reactions for [Ru(NH3)6]2+/[Ru(NH3)6]3+ and [Co(NH3)6]2+/[Co(NH3)6]3+ are 9.2 × 102 M-1 sec-1 and ≤ 10-9 M-1 sec-1, respectively.
The correct rationale for the above data is
In the following electron transfer reactions, the one in which the bridging ligand comes from the reductant is
Of the following assertions regarding the mechanism of electron transfer,
A. An outer-sphere mechanism involves electron transfer from the reductant to the oxidant, with the coordination shells or spheres of each staying intact.
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.
C. In the inner-sphere mechanism, an oxidant possesses at least one ligand capable of binding simultaneously to two metal ions.
D. In the inner-sphere mechanism, ligands of reductant are substitutionally inert.
The correct statements are
