Iron(II) iodide

Iron(II) iodide is an inorganic compound with the chemical formula FeI2. It is used as a catalyst in organic reactions.

Preparation
Iron(II) iodide can be synthesised by the reaction of iron with iodine at 500 °C:


 * Fe + I2 -> FeI2

This is in contrast to the other iron(II) halides, which are best prepared by reaction of heated iron with the appropriate hydrohalic acid.


 * Fe + 2 HX -> FeX2 + H2

Alternatively, the synthesis can be carried out by treating freshly reduced iron with concentrated hydriodic acid under a nitrogen atmosphere in methanol. The initially obtained hexamethanol solvate is then thermally decomposed to anhydrous iodide:
 * Fe + 2 HI + 6 MeOH-> FeI2*6MeOH + H2
 * FeI2*6 MeOH -> FeI2 + 6 MeOH

Extremely finely divided iron(II) iodide is obtained by thermal decomposition of tetracarbonyldiiodidoiron(II) (Fe(CO)4I2).

In contrast to the ferrous fluoride, chloride and bromide, which form known hydrates, the diiodide is speculated to form a stable tetrahydrate but it not been characterized directly.

Chemical properties
Iron(II) iodide is a hygroscopic red-violet to black solid that is soluble in water, ethanol and diethyl ether. Rapid oxidation occurs in solution and in moist air. It turns whitish when exposed to air. The solution in water is colorless.

Dissolving iron metal in hydroiodic acid is another route to aqueous solutions of iron(II) iodide. Crystalline hydrates precipitate from these solutions.

Physical properties
Iron(II) iodide adopts the same crystal structure as cadmium iodide (CdI2). It crystallizes in the trigonal crystal system of the cadmium hydroxide type with the space group P$\overline{3}$m1 (space group no. 164) and lattice constants a = 404 pm, c = 675 pm.

Applications
Iron(II) iodide is used as an ingredient in homeopathic medicines. In the field of application it is known as Ferrum jodatum. However, the attributed effect, particularly against glandular diseases, has not been scientifically confirmed.

It is also used for the production of alkali iodides.