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According to me, $\ce{MnO4^2-}$ is paramagnetic and $\ce{MnO4^-}$ is diamagnetic. On the basis of this, the corresponding potassium salts inherit these magnetic properties, i.e. $\ce{K2MnO4}$ is paramagnetic and $\ce{KMnO4}$ is diamagnetic.

However, according to my class teacher $\ce{KMnO4}$ is paramagnetic.

Is potassium manganate(VII) dia- or paramagnetic?

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Actually it is diamagnetic but due to a phenomenon called charge transfer spectrum (CTS), oxygen transfers one electron to manganese and $\ce{KMnO4}$ as a whole becomes paramagnetic. This is also the reason why it shows a characteristic colour while diamagnetic complexes are colourless due to absence of d-d transitions.

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    $\begingroup$ What would be the percentage of the excited state from charge transfer, in let's say a solution of KMnO4 ? Because a significant number of the molecules have to be in excited state before the bulk material gains paramagnetic properties. $\endgroup$ – S R Maiti Apr 26 at 20:37

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