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Iron(III) iodide

Iron(III) iodide is an inorganic compound with the chemical formula FeI₃. It is a thermodynamically unstable compound that is difficult to prepare. Nevertheless, iron(III) iodide has been synthesised in small quantities in the absence of air and water.^[1]

Iron(III) and iodide tend to undergo a redox reaction^[1] in which Fe³⁺ is reduced to Fe²⁺ and I⁻ is oxidised to I₂. This reaction can be avoided and iron(III) 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(II), Fe(CO)₄I₂, as a light red solution.^[1][2]

Fe(CO)₅ + I₂ → Fe(CO)₄I₂ + CO

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.^[1][2]

Fe(CO)₄I₂ + ½I₂ + hν → FeI₃ + 4CO

Iron(III) iodide is prone to light-induced decompositiontoiron(II) iodide and iodine.^[2][3][4]

FeI₃ + hν → FeI₂ + ½I₂

Donor solvents such as tetrahydrofuran, acetonitrile, pyridine and water also promote this reaction: iron(III) iodide is extremely hygroscopic. It is sparingly soluble in dichloromethane. It reacts with iodide to form the tetraiodoferrate(III) ion.^[2]

FeI₃ + I⁻ → FeI₄⁻

Iron(III) iodide undergoes ligand exchangeormetathesis with certain alkyl chloridestoreversibly form iron(III) chloride and the corresponding alkyl iodides.^[2]

FeI₃ + 3 RCl ⇌ FeCl₃ + 3 RI

Adducts of FeI₃ are well known. An orange complex can be prepared from FeI₂ and I₂ in the presence of thiourea.^[5][6] Iron powder reacts with iodine-containing proligands to also give adducts of ferric iodide.^[7]

• Iron(II) iodide, FeI₂