Why is the third ionization enthalpy of manganese (At. no.=25) unexpectedly high ?

Explanation

The electronic configuration of Mn(25) is `[Ar] 3d^5 4s^2` but that of `Mn^(2+)` is `[Ar]3d^5` . After losing two electrons, `Mn^(2+)` has half filled d-orbitals, which is more stable. That is why `Mn^(2+) ` has exceptionally high ionisation enthalpy.