HexaaquaironII

Hexaaquairon(II), commonly represented as hexaaquairon(II) or [Fe(H2O)6]2+, is a coordination complex in which a central iron(II) ion is octahedrally coordinated by six water molecules. In aqueous solution it exists as the hexaaquairon(II) cation and is a fundamental species in iron(II) chemistry.

Structure and properties: The complex adopts an octahedral geometry and is high-spin, reflecting the weak-field nature of water as a ligand. It is paramagnetic due to the presence of four unpaired electrons. In solution, the ion is typically pale green, and its color can vary with accompanying ligands and redox state.

Formation and reactivity: Hexaaquairon(II) forms readily from iron(II) salts such as FeSO4 when dissolved in water, and the aqua ligands are labile, permitting ligand exchange to form a variety of coordination complexes. The hexaaqua complex is subject to oxidation by dissolved oxygen, converting Fe(II) to Fe(III) and altering both color and reactivity. At higher pH, hydrolysis occurs, producing hydroxo-iron species and, eventually, Fe(OH)2 or related ferric hydroxide precipitates upon oxidation.

Significance: As a primary aquated form of iron(II) in water, hexaaquairon(II) serves as a key intermediate in inorganic and bioinorganic processes, including corrosion chemistry, mining and geochemical cycling, as well as the synthesis of more complex iron(II) coordination compounds.