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

The order of absorption of light (wavelength) is determined by the ligand field strength, where stronger field ligands tend to absorb light at shorter wavelengths, while weaker field ligands absorb light at longer wavelengths.
1. A. \([ \text{Co(NH}_3\text{)}_6]^{3+}\): Ammonia (NH₃) is a moderate field ligand. 
Thus, this complex absorbs light at a medium wavelength.
2. B. \([ \text{Co(CN)}_6]^{3-}\): Cyanide (CN⁻) is a strong field ligand, leading to a large splitting of d-orbitals and absorption of light at a shorter wavelength (higher energy).
3. C. \([ \text{Cu(H}_2\text{O)}_4]^{2+}\): Water (H₂O) is a weak field ligand, resulting in small d-orbital splitting and absorption at longer wavelengths (lower energy).
4. D. \([ \text{Ti(H}_2\text{O)}_6]^{3+}\): Titanium in the +3 oxidation state (with water ligands) is a relatively weak field complex, leading to light absorption at longer wavelengths compared to ammonia and cyanide complexes.

Thus, the correct order of the wavelength of light absorbed is: \[ C<A<D<B \] 
Therefore, the correct answer is (3) C $<$ A $<$ D $<$ B.