Question

The stability sequence of divalent cations in the formation of chelates is:
(A). \( \text{Cu}^{2+} \)
(B). \( \text{Ni}^{2+} \)
(C). \( \text{Co}^{2+} \)
(D). \( \text{Zn}^{2+} \)
Choose the correct answer from the options given below:

• A. (A), (B), (C), (D).
• B. (A), (C), (D), (B).
• C. (B), (A), (D), (C).
• D. (C), (B), (D), (A).

Hint:

To easily remember the Irving-Williams series:
Copper (\( \text{Cu}^{2+} \)) always forms the most stable complexes among the common divalent micronutrient cations, followed by Nickel (\( \text{Ni}^{2+} \)).
This hierarchy is driven by crystal field stabilization energy and ionic radius contraction.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Chelation is the formation of stable complexes between organic ligands and metal cations.
In soil science, chelating agents (organic matter, fulvic acids, humic acids, synthetic EDTA) bind transition metals, affecting their solubility, mobility, and plant availability.
The stability of these divalent metal-chelate complexes follows a highly predictable thermodynamic sequence known as the Irving-Williams series.

Step 2: Detailed Explanation:
The Irving-Williams stability series describes the relative stability of complexes formed by transition metal divalent ions.
For the first-row transition metal ions, the stability of divalent complexes increases to a maximum at copper, regardless of the nature of the co-ordinating ligand.
The general order of stability of these divalent metal-chelate complexes is:
\[ \text{Mn}^{2+} \text{Zn}^{2+} \]
According to this series, the relative order of stability for the given cations is:
\[ \text{Cu}^{2+} > \text{Ni}^{2+} > \text{Co}^{2+} > \text{Zn}^{2+} \]
Comparing this decreasing stability order with the given list:
- \( \text{Cu}^{2+} \) is (A)
- \( \text{Ni}^{2+} \) is (B)
- \( \text{Co}^{2+} \) is (C)
- \( \text{Zn}^{2+} \) is (D)
Thus, the correct decreasing stability sequence is (A), (B), (C), (D).
This matches Option (A).

Step 3: Final Answer:
The correct stability sequence is (A), (B), (C), (D).