BITSAT 2018 Slot 2 Solved Paper

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+