Iron(III) iodide

Iron iodide is an inorganic compound with the chemical formula FeI₃. It is a thermodynamically unstable compound that is difficult to prepare. Nevertheless, iron iodide has been synthesised in small quantities in the absence of air and water.

Preparation

Iron and iodide tend to undergo a redox reaction in which Fe³⁺ is reduced to Fe²⁺ and I⁻ is oxidised to I₂. This reaction can be avoided and iron iodide can be synthesised by a photochemical reaction. Iron pentacarbonyl reacts with excess iodine in hexane under argon, releasing carbon monoxide and forming the complex diiodotetracarbonyliron, Fe₄I₂, as a light red solution.
This complex then undergoes oxidative photodecarbonylation at −20 °C in the presence of further iodine and actinic light. A black film of FeI₃ is deposited as further carbon monoxide is evolved.

Reactivity

Iron iodide is prone to light-induced decomposition to iron iodide and iodine.
Donor solvents such as tetrahydrofuran, acetonitrile, pyridine and water also promote this reaction: iron iodide is extremely hygroscopic. It is sparingly soluble in dichloromethane. It reacts with iodide to form the tetraiodoferrate ion.
Iron iodide undergoes ligand exchange or metathesis with certain alkyl chlorides to reversibly form iron chloride and the corresponding alkyl iodides.
Adducts of FeI₃ are well known. An orange complex can be prepared from FeI₂ and I₂ in the presence of thiourea. Iron powder reacts with iodine-containing proligands to also give adducts of ferric iodide.