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

The color observed in complex ions is due to the absorption of light in the visible region, which is associated with electronic transitions within the d-orbitals, known as d-d transitions. When a complex ion absorbs a particular wavelength of light, the complementary color is observed. To determine the color of the complexes mentioned in the question, we analyze the nature of the ligands and their influence on the absorption spectrum. 

1. Ligand Field Theory plays a crucial role in determining the color of complex ions. Different ligands generate different crystal field strengths on a central metal ion, altering the energy separation between the d-orbitals (known as the crystal field splitting, \(\Delta\)).
2. Generally, the observed color of a complex is complementary to the color of light it absorbs. According to the color wheel, if a complex absorbs yellow light, it will appear violet, because yellow and violet are complementary colors.
3. Let's analyze the complexes:
   • [\(\text{CoCl}(\text{NH}_3)_5\)]\(^{2+}\): This complex contains a mixture of strong and weak field ligands (\(\text{NH}_3\) strong, \(\text{Cl}^-\) weak). It tends to absorb light in the yellow region, which results in the complex appearing violet.
   • [\(\text{Co}(\text{H}_2\text{O})(\text{NH}_3)_5\)]\(^{3+}\): Contains strong field ligands (\(\text{NH}_3\)) and a neutral weak ligand (\(\text{H}_2\text{O}\)), leading to a different splitting and likely a different observed color.
   • [\(\text{Co}(\text{NH}_3)_6\)]\(^{3+}\): All strong field ligands, leading to a large crystal field splitting, absorbing in the different region than a mix of strong and weak field ligands.
   • [\(\text{Co}(\text{CN})_6\)]\(^{3-}\): \(\text{CN}^-\) is a strong field ligand, likely to absorb in the high-energy (short wavelength) region, which results in a different visible color.
4. Based on the nature of the ligands and their field strength, the complex [\(\text{CoCl}(\text{NH}_3)_5\)]\(^{2+}\) absorbs yellow light and therefore appears violet. This is because the combination of one weak and several strong field ligands adjusts its absorption into the correct region of the spectrum.

Therefore, the correct answer is [\(\text{CoCl}(\text{NH}_3)_5\)]\(^{2+}\) as it shows a violet color.