Question

The following complex ions absorb in the ultraviolet-visible region of light. Which one of these shows violet colour?
[\(\text{CoCl}(\text{NH}_3)_5\)]\(^{2+}\), [\(\text{Co}(\text{H}_2\text{O})(\text{NH}_3)_5\)]\(^{3+}\), [\(\text{Co}(\text{NH}_3)_6\)]\(^{3+}\), and [\(\text{Co}(\text{CN})_6\)]\(^{3-}\)

• A. [\(\text{CoCl}(\text{NH}_3)_5\)]\(^{2+}\)
• B. [\(\text{Co}(\text{H}_2\text{O})(\text{NH}_3)_5\)]\(^{3+}\)
• C. [\(\text{Co}(\text{NH}_3)_6\)]\(^{3+}\)
• D. [\(\text{Co}(\text{CN})_6\)]\(^{3-}\)

Hint:

Remember the relationship from the standard NCERT table for Cobalt(III) complexes:
- \(\text{Cl}^-\) (weakest ligand here) \(\to\) smallest \(\Delta_o\) \(\to\) longest absorbed wavelength (yellow-green) \(\to\) complementary observed color is violet.
This simple logical link saves you from memorizing the entire table.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks us to identify which of the listed Cobalt(III) coordination complexes displays a violet color when it absorbs light in the UV-visible region.

Step 2: Key Formula or Approach:
The color exhibited by a coordination complex is the complementary color of the wavelength of light it absorbs.
1. The wavelength of light absorbed (\(\lambda_{\text{abs}}\)) is inversely proportional to the crystal field splitting energy (\(\Delta_o\)):
\[ \Delta_o = \frac{hc}{\lambda_{\text{abs}}} \]
2. The value of \(\Delta_o\) depends on the strength of the ligands surrounding the metal ion, as defined by the spectrochemical series:
\[ \text{Cl}^- < \text{H}_2\text{O} < \text{NH}_3 < \text{CN}^- \]

Step 3: Detailed Explanation:
Let's analyze the absorption properties and complementary colors of the complexes based on ligand field strength:
- \(\text{CN}^-\) is an extremely strong field ligand, resulting in a very large \(\Delta_o\). The complex [\(\text{Co}(\text{CN})_6\)]\(^{3-}\) absorbs high-energy ultraviolet light, and thus appears pale yellow or virtually colorless in the visible range.
- \(\text{NH}_3\) is a strong ligand, giving a relatively high \(\Delta_o\) value. [\(\text{Co}(\text{NH}_3)_6\)]\(^{3+}\) absorbs blue-violet light (\(\lambda \approx 475\text{ nm}\)), showing its complementary color: yellow-orange.
- In [\(\text{Co}(\text{H}_2\text{O})(\text{NH}_3)_5\)]\(^{3+}\), replacing one \(\text{NH}_3\) with the weaker \(\text{H}_2\text{O}\) ligand decreases \(\Delta_o\) slightly. It absorbs blue-green light (\(\lambda \approx 500\text{ nm}\)) and displays its complementary color: red-pink.
- In [\(\text{CoCl}(\text{NH}_3)_5\)]\(^{2+}\), the weak field ligand \(\text{Cl}^-\) decreases \(\Delta_o\) further. It absorbs yellow-green light (\(\lambda \approx 535\text{ nm}\)), and thus displays its complementary color: violet.

Step 4: Final Answer:
Based on the spectrochemical series and complementary color relationships, [\(\text{CoCl}(\text{NH}_3)_5\)]\(^{2+}\) is the complex that exhibits a violet color, which corresponds to option (A).