Consider the following pairs of complexes $[CoF(NH_3)_5]^{2+}$ and $[Cr(OH_2)_6]^{2+}$ $[Co(NH_3)_5 (OH_2)]^{3+}$ and $[Cr(OH_2)_6]^{2+}$ $[Co(NH_3)_6]^{3+}$ and $[Cr(OH_2)_6]^{2+}$ $[Co(NH_3)_5]^{2+}$ and $[Cr(OH_2)_6]^{2+}$ The electron transfer rate will be fastest in the pair
$[CoI(NH_3)_5]^{2+}$ and $[Cr(OH_2)_6]^{2+}$
Electron Transfer Rate Factors
The rate of electron transfer between coordination complexes depends significantly on the properties of the ligands involved, particularly their ability to facilitate the transfer mechanism.
Ligand Influence on Electron Transfer
- Bridging Ability: Ligands that can effectively connect the two metal centers (act as bridges) generally enhance electron transfer rates.
- Polarizability: Larger, more polarizable ligands can better stabilize the transition state complex formed during electron transfer, thus increasing the rate.
Comparing Ligand Effects in Cobalt Complexes
The question compares several Cobalt complexes with the common Chromium complex, $[Cr(OH_2)_6]^{2+}$. The key difference lies in the Cobalt complex's ligands:
- $[CoF(NH_3)_5]^{2+}$: Contains fluoride ($F^-$). Fluoride is a small ligand with low polarizability and poor bridging ability.
- $[Co(NH_3)_5(H_2O)]^{3+}$: Contains a water ($H_2O$) ligand. Water is a polar molecule but a relatively weak bridging ligand. This complex also has a higher overall charge ($3+$).
- $[Co(NH_3)_6]^{3+}$: Contains only ammonia ($NH_3$) ligands. This complex has a high charge ($3+$) and lacks a readily available bridging or labile ligand for facile electron transfer.
- $[CoI(NH_3)_5]^{2+}$: Contains iodide ($I^-$). Iodide is a large, highly polarizable anion and an excellent bridging ligand.
Determining the Fastest Electron Transfer Rate
Electron transfer is generally fastest when a highly effective bridging and polarizable ligand is present. Comparing the ligands, iodide ($I^-$) offers the most significant advantage for facilitating electron transfer due to its size and polarizability.
Therefore, the pair $[CoI(NH_3)_5]^{2+}$ and $[Cr(OH_2)_6]^{2+}$ will exhibit the fastest electron transfer rate among the given options.
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
