When central metal ion is same (here Ni2+ ), the absorption of colour depends on the ligand. According to spectrochemical series, various ligands are as follows I−<Br−<Cl−<NO3−<F−<H2O<NH3<NO2−<CN−<CO Thus, H2O is the weakest ligand among these, therefore the absorbed energy will be lowest in [Ni(H2O)6]2+, so it will absorb highest wavelength (red light). E∝
1
λ
∴ The order of increasing wavelength is [Ni(NO2)6]4−<[Ni(NH3)6]2+<[Ni(H2O)6]2+