Question

The correct order of the wavelength of light absorbed by the following complexes is:
A. $[ \text{Co(NH}_3\text{)}_6]^{3+}$
B. $[ \text{Co(CN)}_6]^{3-}$
C. $[ \text{Cu(H}_2\text{O)}_4]^{2+}$
D. $[ \text{Ti(H}_2\text{O)}_6]^{3+}$
Choose the correct answer from the options given below:

• A. B $<$ A $<$ C $<$ D
• B. C $<$ D $<$ A $<$ B
• C. C $<$ A $<$ D $<$ B
• D. B $<$ D $<$ A $<$ C

Hint:

The wavelength of light absorbed by a complex is inversely proportional to the ligand field strength. Strong field ligands (e.g., CN⁻) cause larger d-orbital splitting and absorb light at shorter wavelengths.

Answer 1

The Correct Option is C

Ligand field strength dictates light absorption wavelength. Stronger field ligands absorb at shorter wavelengths, and weaker field ligands absorb at longer wavelengths.
1. A. \([ \text{Co(NH}_3\text{)}_6]^{3+}\): Ammonia (NH₃) is a moderate field ligand, resulting in medium wavelength light absorption.
2. B. \([ \text{Co(CN)}_6]^{3-}\): Cyanide (CN⁻) is a strong field ligand, causing significant d-orbital splitting and absorbing light at shorter wavelengths (higher energy).
3. C. \([ \text{Cu(H}_2\text{O)}_4]^{2+}\): Water (H₂O) is a weak field ligand, leading to minimal d-orbital splitting and absorption at longer wavelengths (lower energy).
4. D. \([ \text{Ti(H}_2\text{O)}_6]^{3+}\): This complex with titanium in the +3 oxidation state and water ligands is relatively weak field, absorbing light at longer wavelengths compared to ammonia and cyanide complexes.

The order of light absorption wavelength is: \[ C<A<D<B \]
The correct answer is (3) C $<$ A $<$ D $<$ B.