The compounds ${A}_{2}\text{Fe}{X}_{5}\ensuremath{\cdot}{\text{H}}_{2}\text{O}$ ($A=\text{alkali}$ or ${\text{NH}}_{4}$; $X=\text{Cl}$, Br) form a series of collinear antiferromagnets with transition temperatures in the range from 6 to 23 K. These ordering temperatures are much higher than in other hydrated salts of transition-metal ions with similar distances between magnetic ions. Spin-density distributions have been determined in ${\text{Rb}}_{2}{\text{FeBr}}_{5}\ensuremath{\cdot}{\text{H}}_{2}\text{O}$ and ${\text{K}}_{2}{\text{FeCl}}_{5}\ensuremath{\cdot}{\text{H}}_{2}\text{O}$ by means of the polarized-neutron-diffraction technique and ab initio calculations in order to elucidate the mechanism of such enhancement. The results show a large spin-density delocalization $(\ensuremath{\sim}20%)$ toward the ligand atoms, which explains the efficiency of the superexchange pathways in transmitting the magnetic interaction.