Iron(III) chloride

Iron(III) chloride, known as ferric chloride, is an inorganic compound with the formula FeCl₃ and exists in several hydrated forms. These are among the most significant and widespread iron compounds. Both the anhydrous and hydrated versions are hygroscopic, and all contain iron in the +3 oxidation state. The anhydrous form acts as a Lewis acid, while all forms serve as mild oxidizing agents. Common uses include water purification and metal etching.

All ferric chloride types are paramagnetic due to five unpaired electrons in their 3d orbitals, regardless of whether the iron center is octahedral or tetrahedral. This high-spin d⁵ configuration makes d-d electronic transitions doubly forbidden—both spin-forbidden and Laporte-forbidden—resulting in pale-colored solutions. While aqueous ferric sulfate and ferric nitrate are nearly colorless, chloride solutions appear yellow because chloride ligands strongly affect the iron’s optical properties.

The anhydrous compound is a hygroscopic crystalline solid melting at 307.6 °C. Its color shifts with viewing angle: dark green in reflected light, purple-red in transmitted light. It adopts a structure with octahedral Fe(III) centers linked by two-coordinate chloride ligands. With a low melting point, it boils around 315 °C, and its vapor contains dimers similar to aluminum chloride. At higher temperatures, these dimers dissociate into monomers with D₃h symmetry, competing with reversible decomposition into iron(II) chloride and chlorine gas.

Upon exposure to water, ferric chloride forms hydrates, reflecting its Lewis acidity. All hydrates are deliquescent, absorbing moisture to become liquid. Hydration yields aquo complexes with the formula [FeCl₂(H₂O)₄]⁺, which can adopt cis or trans stereochemistry. Four hydrates have been characterized by X-ray crystallography: the dihydrate, disesquihydrate, trisesquihydrate, and hexahydrate. They differ in the cation’s stereochemistry, the anion identity, and the presence of water of crystallization. The first three contain the tetrahedral tetrachloroferrate anion.

Aqueous solutions of ferric chloride contain octahedral [FeCl₂(H₂O)₄]⁺ with unspecified stereochemistry. Speciation is challenging because components lack distinct spectroscopic signatures. Iron(III) complexes are kinetically labile, meaning ligands rapidly exchange, and solutions are strongly acidic. Iron aquo complexes undergo olation, forming polymeric oxo derivatives. Dilute solutions yield soluble nanoparticles with molecular weights around 10⁴ that “age” over days. These hydrolysis polymers are crucial for water treatment. In contrast, solutions in diethyl ether or tetrahydrofuran form well-behaved 1:2 adducts, FeCl₃(ether)₂, with pentacoordinate iron.

Anhydrous iron(III) chloride is produced annually in quantities of several hundred tons. The main method, direct chlorination, uses scrap iron at several hundred degrees Celsius to produce gaseous product. Excess chlorine ensures conversion of intermediate ferrous chloride to ferric state. Aqueous solutions are industrially made from iron oxides or by oxidizing iron metal with hydrochloric acid followed by chlorination. Hydrates do not readily yield anhydrous ferric chloride upon heating, as thermal dehydration produces hydrochloric acid and iron oxychloride. In the lab, hydrated iron(III) chloride can be converted to anhydrous form using thionyl chloride or trimethylsilyl chloride.

Despite being kinetically labile—with Cl⁻ and H₂O ligands exchanging rapidly—iron(III) chlorides are thermodynamically robust, as shown by the vigorous methods used in their synthesis.