Arrange the following coordination complexes in the increasing...

The wavelength of light absorbed by a coordination complex is inversely proportional to the crystal field splitting energy ($\Delta_o$) of the complex, since $E = \dfrac{hc}{\lambda} = \Delta_o$.

The magnitude of $\Delta_o$ depends on the field strength of the ligands. According to the spectrochemical series, the increasing order of ligand field strength is:
$\mathrm{Cl}^- \lt \mathrm{H}_2\mathrm{O} \lt \mathrm{NH}_3 \lt \mathrm{CN}^-$

Therefore, the order of crystal field splitting energy ($\Delta_o$) for the given complexes is:
\left[\mathrm{Cr}(\mathrm{CN})_6\right]^{3-} \gt \left[\mathrm{Cr}(\mathrm{NH}_3)_6\right]^{3+} \gt \left[\mathrm{Cr}(\mathrm{H}_2\mathrm{O})_6\right]^{3+} \gt \left[\mathrm{CrCl}_6\right]^{3-}$<br>$\mathrm{B} \gt \mathrm{A} \gt \mathrm{C} \gt \mathrm{D}$<br><br>Since the wavelength of absorbed light is inversely proportional to$\Delta_o$, the increasing order of wavelength of light absorbed is:<br>$\mathrm{B} \lt \mathrm{A} \lt \mathrm{C} \lt \mathrm{D}$<br><br>Answer:$\mathrm{B} \lt \mathrm{A} \lt \mathrm{C} \lt \mathrm{D}