Consider the following complexes

I) ${\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{+3}$

II) ${\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{+3}$

III) ${\left[\mathrm{Co}{\left(\mathrm{CN}\right)}_6\right]}^{-3}$

the increasing order of wavelength of light absorbed is

$$\begin{gathered} \mathrm{Energy}(\mathrm{or} \mathrm{frequency}) \mathrm{of} \mathrm{light} \mathrm{absorbed} \propto \mathrm{field} \mathrm{strength} \mathrm{of} \mathrm{ligand}; \\ \mathrm{Wavelength} \mathrm{of} \mathrm{light} \mathrm{absorbed} \propto \frac{1}{\mathrm{field} \mathrm{strength} \mathrm{of} \mathrm{ligand}}; \\ \mathrm{Field} \mathrm{strength} \mathrm{of} {\mathrm{CN}}^{-}>{\mathrm{NH}}_3>{\mathrm{H}}_2\mathrm{O} \\ \therefore \mathrm{Order} \mathrm{of} \mathrm{wavelength} \mathrm{absorbed} : \mathrm{III}<\mathrm{I}<\mathrm{II}\hspace{0.17em}; \end{gathered}$$