Question 2

The energy of the absorption band ($Δ_o$) depends on the strength of the ligands in the spectrochemical series. According to the spectrochemical series, the strength of the ligands is: $Cl^- < H_2O < NH_3 < CN^-$. The crystal field splitting energy ($Δ_o$) follows the same order as ligand strength. Since energy ($E$) is inversely proportional to wavelength ($λ$), $E = \frac{hc}{λ}$, the order of wavelength maxima ($λ_{max}$) will be the inverse of the ligand strength order: $CN^- < NH_3 < H_2O < Cl^-$. Therefore, the order of wavelength maxima is: $[Co(CN)_6]^{3-} < [Co(NH_3)_6]^{3+} < [Co(NH_3)_5(H_2O)]^{3+} < [Co(NH_3)_5(Cl)]^{2+}$.