Iron(III) chloride

Lore & Background

Iron(III) chloride exists as an anhydrous crystalline solid and a series of hydrates. The anhydrous form has a BiI3 structure with octahedral Fe(III) centers, and its vapor consists of the dimer Fe2Cl6, which dissociates into monomeric FeCl3 at higher temperatures. Hydrates form upon exposure to water, exhibiting deliquescence, and two hydrates have been characterized by X-ray crystallography: FeCl3·6H2O and FeCl3·2.5H2O. Aqueous solutions consist of the octahedral [FeCl2(H2O)4]+ cation, and detailed speciation is challenging due to kinetic lability and olation.

Reader's Guide

Iron(III) chloride is significant for its dual role as a Lewis acid and mild oxidizing agent, enabling its use in water treatment and metal etching. Its paramagnetic nature, due to five unpaired electrons in a high spin d5 configuration, results in pale-colored solutions. The compound is produced industrially via direct chlorination of scrap iron or from iron oxides with hydrochloric acid. Its hydrates do not readily yield the anhydrous form upon heating, requiring chemical dehydration with thionyl chloride or trimethylsilyl chloride. The compound's reactivity includes comproportionation with iron to form ferrous chloride, and its oxophilicity leads to complexes with alkoxides and carboxylates. Despite its kinetic lability, iron(III) chloride is thermodynamically robust, as reflected in its vigorous synthesis methods.

Did You Know?

• Anhydrous iron(III) chloride appears dark green by reflected light but purple-red by transmitted light.
• The vapor of anhydrous iron(III) chloride consists of the dimer Fe2Cl6, similar to aluminum chloride.
• Aqueous solutions of ferric chloride are strongly acidic and prone to olation, forming polymeric oxo derivatives.
• Hydrated iron(III) chloride can be converted to the anhydrous form by treatment with thionyl chloride or trimethylsilyl chloride.