The correct order of the complexes $\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_5\left(\mathrm{H}_2…

The correct order of the complexes $\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_5\left(\mathrm{H}_2 \mathrm{O}\right)\right]^{3+}$ (A),
$\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}$ (B),
$\left[\mathrm{Co}(\mathrm{CN})_6\right]^{3-}(\mathrm{C})$ and $\left[\mathrm{CoCl}\left(\mathrm{NH}_3\right)_5\right]^{2+}(\mathrm{D})$ in terms wavelength of light absorbed is :
  1. D>A>B>C
  2. C>B>D>A
  3. D>C>B>A
  4. C>B>A>D

Solution

We know $\mathrm{E}=\mathrm{h} v=\frac{\mathrm{hC}}{\lambda}$
$\mathrm{E} \propto \frac{1}{\lambda}$
Here all Co in +3 oxidation state.
So, as the ligand field strength $\uparrow, \operatorname{CFSE} \uparrow$
Order of field strength of ligand :
$\mathrm{CN} \gt \mathrm{NH}_3 \gt \mathrm{H}_2 \mathrm{O} \gt \mathrm{Cl}^{-}$
CFSE order : $\mathrm{C} \gt \mathrm{B} \gt \mathrm{A} \gt \mathrm{D}$
Wavelength order: $\mathrm{D} \gt \mathrm{A} \gt \mathrm{B} \gt \mathrm{C}$

Asked in: JEE Main 2025 (03 Apr Shift 1)

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