Question

Which from the following cations forms the least stable complex with the same ligand?

• A. \( \text{Co}^{2+} \)
• B. \( \text{Fe}^{2+} \)
• C. \( \text{Cd}^{2+} \)
• D. \( \text{Cu}^{2+} \)

Hint:

In transition metal complexes, the size and charge of the metal ion significantly influence the stability of the complex. A higher charge-to-radius ratio typically leads to greater stability.

Answer 1

The Correct Option is C

Step 1: Consider the factors affecting stability of complexes.
The stability of a complex depends on the charge and size of the central metal ion, as well as its ability to accept electron pairs from the ligand. Metal ions with a high charge-to-radius ratio tend to form more stable complexes, while those with a low charge-to-radius ratio form less stable complexes.

Step 2: Compare the charge-to-radius ratios.
- \( \text{Co}^{2+} \) has a +2 charge and a relatively small radius, so it can form relatively stable complexes.
- \( \text{Fe}^{2+} \) also has a +2 charge but a larger radius than \( \text{Co}^{2+} \), so it forms less stable complexes compared to \( \text{Co}^{2+} \).
- \( \text{Cu}^{2+} \) has a +2 charge and a relatively small radius, so it forms stable complexes as well.
- \( \text{Cd}^{2+} \) has a +2 charge but a much larger radius compared to the other cations, making its complexes less stable.

Step 3: Final conclusion.
Thus, the least stable complex is formed by:
\[ \boxed{(3)\ \text{Cd}^{2+}} \]