As metal ion is fixed, the increasing field strengths, i.e., the CFSE values of the ligands from the spectro-chemical series are in the order: \(\mathrm{H}_2 \mathrm{O} < \mathrm{NH}_3 < \mathrm{NO}_2^{-}\).
Thus, the energies absorbed for excitation will be in the order:
\(\left[\mathrm{Ni}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{2+} < \left[\mathrm{Ni}\left(\mathrm{NH}_3\right)_6\right]^{2+} < \left[\mathrm{Ni}\left(\mathrm{NO}_2\right)_6\right]^{4-}\)
As \(E=\frac{\text { he }}{\lambda}\) the wavelengths absorbed will be in the opposite order.