Disproportionation
When a particular oxidation state becomes less stable relative to other oxidation states, one lower, one higher, it is said to undergo disproportionation.
Example
Manganese ( VI) becomes unstable relative to manganese ( VII) and manganese ( IV) in acidic solution.
3+6MnO2−4+ 4 H+→ 2+7MnO−4++4MnO2+2H2O