In `MnO_(4)^(-)` ion oxidation state of `Mn is +7`.It cannot increase its oxidation state any more but can decrease the same Therefore, `MnO_(4)^(-)` does not undergo disproportination.But in `MnO_(4)^(2-)` the oxidation state of Mn is +6.It can undergo an increase as well as decrease in its oxidtion state and undergoes disproportionation .
`(overset(+6)MnO_(4))^(2-)+4H^(+)rarroverset(+7)(MnO_(4))^(-)+overset(+4)MnO_(2)+2H_(2)O`
