Question

why the third ionisation energy of manganese is unexpectedly high?

Answer 1

The electronic configuration of manganese is -1s2, 2s2, 2p6, 3s2, 3p6, 3d5 4s2

When electrons are removed from the manganese atom, the first two electrons are removed with ease since it left the manganese ion with half filled d orbitals i.e. with five unpaired electrons. The half filled orbital imparts stability to the manganese ion due to greater exchange energy. Thus it does not prefer to lose one electron which disturbs its stability and hence not favorable from energy point of view.