$\mathrm{M}\mathrm{n}$ had outer electronic configuration as $4{\mathrm{s}}^2\mathrm{}3{\mathrm{d}}^5$ , first two electrons are removed from $4\mathrm{S}-$ orbital, but the removal of $3^{\mathrm{r}\mathrm{d}}$ electron is from half filled $3\mathrm{d}-$ orbital, which requires higher ionization energy then other metal.