MnO4^2- undergoes disproportionate reaction in acidic medium but MnO4^- does not. Why?

Disproportionation is a type of redox reaction in which a species is simultaneously reduced and oxidised forming two different products.
In MnO42-, the oxidation state of manganese is +6. It can disproportionate to form MnO2 and MnO4-.
3MnO42- + 4H+ → MnO2 + 2MnO4- + 2H2O

​However, the oxidation state of Mn in MnO4- is +7 which is the maximum possible oxidation state of Mn (atomic number 25, 1s2 2s2 2p6 3s2 3p6 3d5 4s2) and hence it cannot undergo disproportionation reaction. 
 

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