An aqueous solution of Nitrate Salt is mixed with an equal volume of freshly prepared $FeS{O}_4$  Solution. When concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$  It is added drop wise along the walls of the tube without disturbing. The ‘X’ appears at the interface (magnitude), with a cationic ligand. The oxidation state of iron in the ‘X’ compound is,------------