Question

A transition metal M forms 4 homoleptic octahedral coordination compounds, A, B, C, and D of the type [MX\(_6\)]\(^{2-}\) with monodentate ligands a, b, c, and d respectively. These compounds absorb red, blue, yellow, and blue-green light respectively. Which one of the options shows the correct order of decreasing ligand strength?

• A. B \( > \) D \( > \) C \( > \) A
• B. D \( > \) C \( > \) B \( > \) A
• C. A \( > \) C \( > \) D \( > \) B
• D. A \( > \) B \( > \) C \( > \) D

Hint:

The stronger the ligand, the greater the splitting of the d-orbitals and the higher the energy of the light absorbed. Ligands that cause greater splitting absorb light with shorter wavelengths.

Answer 1

The Correct Option is A

Step 1: Understand the relationship between light absorption and ligand strength.
The color of light absorbed by a coordination complex is related to the energy gap between the metal's d-orbitals. The larger the ligand field splitting (\( \Delta_0 \)), the greater the energy gap, and thus the light absorbed by the complex will have a shorter wavelength (higher energy). Therefore, ligands that cause a larger field splitting will absorb light of higher energy.

Step 2: Analyze the absorption wavelengths.
- Red light has the longest wavelength and the lowest energy, meaning it corresponds to the smallest field splitting.
- Blue light corresponds to higher energy and a larger field splitting.
- Yellow light corresponds to a field splitting larger than blue, but smaller than blue-green.
- Blue-green light corresponds to the largest field splitting.

Step 3: Determine the relative strength of the ligands.
Based on the absorption of different wavelengths: - Ligand B, which absorbs blue light, causes the largest splitting.
- Ligand D absorbs yellow light and causes moderate splitting.
- Ligand C absorbs blue-green light and causes a slightly smaller splitting.
- Ligand A absorbs red light, indicating it causes the smallest splitting.
Thus, the order of ligand strength (from strongest to weakest) is: B \( > \) D \( > \) C \( > \) A. This corresponds to option (A).

Step 4: Conclusion.
The correct order of decreasing ligand strength is (A) B \( > \) D \( > \) C \( > \) A.