Understanding Water Exchange Rates in Complex Ions
The rate at which water molecules (ligands) exchange in coordination complexes depends primarily on the properties of the central metal ion. Key factors include the ion's charge and its electronic configuration, which influences ligand field stabilization energy (LFSE).
Factors Affecting Exchange Rates
- Metal Ion Charge: A higher positive charge on the metal ion generally increases the attraction for ligands, making dissociation harder and slowing down the exchange rate.
- Electronic Configuration & LFSE: Configurations with high LFSE, such as d3 in an octahedral field like $[Cr(H_2O)_6]^{3+}$, tend to form kinetically inert complexes (slow exchange). Configurations with lower LFSE or specific electronic arrangements often form labile complexes (fast exchange).
Analysis of Specific Complex Ions
- $[Cr(H_2O)_6]^{3+}$: The $Cr^{3+}$ ion has a d3 configuration and a +3 charge. This combination results in a high LFSE and significant kinetic inertness, leading to a very slow water exchange rate.
- $[Co(H_2O)_6]^{2+}$: The $Co^{2+}$ ion has a d7 configuration and a +2 charge. It generally exhibits relatively low LFSE in an octahedral field and is considered kinetically labile, resulting in a fast water exchange rate.
- $[V(H_2O)_6]^{2+}$: The $V^{2+}$ ion has a d3 configuration but only a +2 charge. While d3 provides some LFSE, the +2 charge and overall electronic factors make it kinetically labile, with an exchange rate typically faster than inert complexes like $[Cr(H_2O)_6]^{3+}$.
Determining the Exchange Rate Order
Based on the analysis of kinetic inertness and lability:
- $[Cr(H_2O)_6]^{3+}$ is the most inert (slowest exchange).
- Both $[Co(H_2O)_6]^{2+}$ and $[V(H_2O)_6]^{2+}$ are labile.
- Comparing the labile ions, experimental data and established trends often indicate that $[Co(H_2O)_6]^{2+}$ exchanges water slightly faster than $[V(H_2O)_6]^{2+}$.
Therefore, the order of water exchange rates from fastest to slowest is:
$[Co(H_2O)_6]^{2+} > [V(H_2O)_6]^{2+} > [Cr(H_2O)_6]^{3+}$
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
