Question

Which of the following complex ion absorbs yellow colour in the visible region?

• A. \([Co(NH_3)_6]^{+3}\)
• B. \([Co(CN)_6]^{3-}\)
• C. \([Cu(H_2O)_4]^{+2}\)
• D. \([Co(NH_3)_5(H_2O)]^{+3}\)
• E. The complex \([CoCl(NH_3)_5]^{+2}\) absorbs yellow light.

Hint:

Memory aid: "VIBGYOR".
Absorb Yellow (Y) \(\rightarrow\) Complementary is Violet (V).
Absorb Blue (B) \(\rightarrow\) Complementary is Orange (O).
The presence of a weak ligand like \(Cl^-\) decreases the crystal field splitting, shifting absorption towards longer wavelengths (like yellow).

Answer 1

Answer: (E)

Step 1: Understanding the Concept:
The color of a coordination complex is determined by the absorption of light corresponding to d-d transitions. The color we perceive is the complementary color of the light absorbed.

Step 2: Detailed Explanation:
1. Complementary colors can be visualized using a color wheel:
- Yellow absorbed \(\leftrightarrow\) Violet observed.
- Blue absorbed \(\leftrightarrow\) Orange observed.
- Red absorbed \(\leftrightarrow\) Green observed.
2. \([Co(NH_3)_6]^{3+}\) is yellow-orange (absorbs blue-violet).
3. \([CoCl(NH_3)_5]^{2+}\) is violet in color. Since it appears violet, it must be absorbing light in the yellow region of the spectrum.
4. This shift in absorption depends on the ligand field strength (\(Cl^-\) is a weaker ligand than \(NH_3\)).

Step 3: Final Answer:
The complex \([CoCl(NH_3)_5]^{+2}\) absorbs yellow light.