Iron(III) iodide
# Iron(III) iodide

Iron(III) iodide is an inorganic compound with the chemical formula $FeI_3.$ 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.

## Preparation

Iron(III) and iodide tend to undergo a redox reaction in which $Fe^{3+}$ is reduced to $Fe^{2+}$ and $I^−$ is oxidised to $I_2.$ 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), $Fe(CO)_4I_2,$ as a light red solution.

    $Fe(CO)_5$ + $I_2$ → $Fe(CO)_4I_2$ + CO

This complex then undergoes oxidative photodecarbonylation at −20 °C in the presence of further iodine and actinic light. A black film of $FeI_3$ is deposited as further carbon monoxide is evolved.

    $Fe(CO)_4I_2$ + $½I_2$ + hν → $FeI_3$ + 4CO

## Reactivity

Iron(III) iodide is prone to light-induced decomposition to iron(II) iodide and iodine.

    $FeI_3$ + hν → $FeI_2$ + $½I_2$

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.

    $FeI_3$ + $I^−$ → $FeI_4^−$

Iron(III) iodide undergoes ligand exchange or metathesis with certain alkyl chlorides to reversibly form iron(III) chloride and the corresponding alkyl iodides.

    $FeI_3$ + 3 RCl ⇌ $FeCl_3$ + 3 RI

Adducts of $FeI_3$ are well known. An orange complex can be prepared from $FeI_2$ and $I_2$ in the presence of thiourea. Iron powder reacts with iodine-containing proligands to also give adducts of ferric iodide.

