Question

Consider the following complexes: 
(A) [Co(CN)$_6$]$^{3-}$             
(B) [Co(NH$_3$)$_5$H$_2$O]$^{3+}$ 
(C) [Co(H$_2$O)$_6$]$^{3+}$      
(D) [CoF$_6$]$^{3-}$ 
The wavelength absorbed by the above complexes are in the order: 
 

• A. A>B>C>D
• B. A<B<C<D
• C. B<A<C<D
• D. C>A>B>D

Hint:

Wavelength absorbed is inversely proportional to crystal field splitting energy.

Answer 1

The Correct Option is B

Step 1: Arrange ligands according to spectrochemical series.
\[ CN^->NH_3>H_2O>F^- \]
Step 2: Relate ligand strength to crystal field splitting.
Stronger ligand $\Rightarrow$ larger $\Delta_o$ $\Rightarrow$ shorter wavelength absorbed.
Step 3: Order of wavelength absorbed.
\[ A<B<C<D \]