${\mathrm{VO}}_4^{-},{\mathrm{CrO}}_4^{2-}\text{ and }{\mathrm{MnO}}_4^{-}$ are pale yellow, strong yellow and inense purple respectively in aqueous solution. The darkening of colour is due to

Let us understand the reason for the color by taking permanganate ion. The oxidation state of Mn in KMnO4 is +7, which means that Mn does not have any d electrons left in its orbital. Actually, Mn7+ complexes should not be colored because electronic transitions are not possible. However, KMnO4 has a deep purple color because of charge transfer from the ligand or we can say O2– to the metal center. This is called a ligand-to-metal charge transfer, which leads to absorption of energy and color.

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