Correct increasing order for the wavelength of absorption in the...

Correct increasing order for the wavelength of absorption in the visible region for the complexes of

\mathrm{Co}^{3+}

Options

A $\left[\mathrm{Co}(\mathrm{CN})_6\right]^{3-},\left[\mathrm{Co}$ \left(\mathrm{NH}_3\right)_6\right]^{3+} $, \left[\mathrm{Co}\left(\mathrm{NH}_3\right)_5\left(\mathrm{H}_2 \mathrm{O}\right)\right]^{3+},\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_5 \mathrm{Cl}\right]^{2+}$
B $\begin{aligned} & {\left[\mathrm{Co}(\mathrm{CN})_6\right]^{3-},\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_5\left(\mathrm{H}_2 \mathrm{O}\right)\right]^{3+}} \\ & {\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_5 \mathrm{Cl}\right]^{2+},\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}}\end{aligned}$
C $\begin{aligned} & {\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+},\left[\mathrm{Co}(\mathrm{CN})_6\right]^{3-},} \\ & {\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_5\left(\mathrm{H}_2 \mathrm{O}\right)\right]^{3+},\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_5 \mathrm{Cl}\right]^{2+}}\end{aligned}$
D $\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_5 \mathrm{Cl}\right]^{2+},\left[\mathrm{Co}$ \left(\mathrm{NH}_3\right)_5\left(\mathrm{H}_2 \mathrm{O}\right)\right]^{3+} $, \left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+},\left[\mathrm{Co}(\mathrm{CN})_6\right]^{3-}$

Correct answer

A. $\left[\mathrm{Co}(\mathrm{CN})_6\right]^{3-},\left[\mathrm{Co}$ \left(\mathrm{NH}_3\right)_6\right]^{3+} $, \left[\mathrm{Co}\left(\mathrm{NH}_3\right)_5\left(\mathrm{H}_2 \mathrm{O}\right)\right]^{3+},\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_5 \mathrm{Cl}\right]^{2+}$

Step-by-step solution

Strength of ligands attached with

\mathrm{Co}^{3+}

ion is in the order of

\mathrm{CN}^{-}>\mathrm{NH}_3>\mathrm{H}_2 \mathrm{O}>\mathrm{Cl}^{-}

. So, order of crystal field splitting energy (values of

\Delta_o

) will be in the same order.

\therefore \quad

Wavelength of absorbing light

\propto \frac{1}{\Delta_o}

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