Why is the third ionisation energy of manganese exceptionally high ?

Explanation

Electronic configuration of `Mn : [Ar]3d^(5)4s^(2)`
Electronic configuration of `Mn^(2+):[Ar]3d^(5)`
Since the `Mn^(2+)` ion has half - filled `(3d^(5))` d - subshell, it is highly stable. Hence, a large amount of energy is required to remove an electron from stable `Mn^(2+)` ion to form less stable `Mn^(3+)` ion.