Complexes, (I) \(\left[\mathrm{Ni}\left(\mathrm{NH}_3ight)_6ight]^{2+}\),
(II) \(\left[\mathrm{Ni}\left(\mathrm{H}_2 \mathrm{O}ight)_6ight]^{2+}\) and (III) \(\left[\mathrm{Ni}\left(\mathrm{NO}_2ight)_6ight]^{4-}\) are made of \(\mathrm{Ni}^{2+}\) and ligands \(-\mathrm{NH}_3,-\mathrm{H}_2 \mathrm{O}\) and \(-\mathrm{NO}_2^{-}\)respectively. The field strength order of the ligands is
\(\mathrm{H}_2 \mathrm{O} < \mathrm{NH}_3 < \mathrm{NO}_2^{-}\)
So, the order CFSE values: \(\Delta_0^{\mathrm{II}} < \Delta_0^{\mathrm{I}} < \Delta_0^{\mathrm{III}}\)
We know, \(E=\frac{h c}{\lambda}\) or \(E \propto \frac{1}{\lambda}\)
So, the order of wavelength of absorption in visible region will be,
\(\lambda_{\text {III }} < \lambda_{\text {I }} < \lambda_{\text {II }}\)