From the following pairs of ions which one is not an iso-electronic pair?

$Fe^{2+}$: $_{26}Fe = 3d^64s^2$, $Fe^{2+} = 3d^6$ → 24 electrons $Mn^{2+}$: $_{25}Mn = 3d^54s^2$, $Mn^{2+} = 3d^5$ → 23 electrons So $Fe^{2+}$ and $Mn^{2+}$ are not isoelectronic. ($Na^+$/$Mg^{2+}$ and $O^{2-}$/$F^-$ have 10 electrons each, and $Mn^{2+}$/$Fe^{3+}$ have 23 each.)