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$\ce{Mn}$ in $\ce{KMnO4}$ is in $+7$ oxidation state, has no d electrons so d-d transition is not possible.

Then, how is $\ce{KMnO4}$ coloured?

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    $\begingroup$ It is not ionic bond; there is still $\mathrm d$ electrons. $\endgroup$ – DHMO Oct 12 '16 at 14:34
  • $\begingroup$ In $\ce{MnO4-}$, there are no electrons in the $e$ or $t_2$ orbitals which arise from the metal 3d orbitals and therefore what are conventionally known as d-d transitions are not possible. Whether it is ionic or covalent has no impact. $\endgroup$ – orthocresol Oct 12 '16 at 18:45