Iron(II) chloride

Iron(II) chloride, also known as ferrous chloride, is the chemical compound of formula FeCl2. It is a high melting point, paramagnetic solid, usually obtained as an off-white solid. FeCl2 crystallizes from water as the greenish tetrahydrate, which is how the salt is most commonly encountered in commerce and the laboratory.

Synthesis
A classical synthesis of metal halides involves the action of hydrogen chloride on the metal.


 * Fe + 2 HCl → FeCl2 + H2

For FeCl2, the synthesis is more conveniently conducted using a methanol solution of concentrated hydrochloric acid. Reaction of this with iron gives the methanol solvate [Fe(MeOH)6]Cl2, which upon heating in a vacuum at ca. 160 °C gives pure FeCl2. FeBr2 and FeI2 can be prepared analogously.

An alternative laboratory synthesis of FeCl2 entails the reaction of FeCl3 with chlorobenzene:
 * 2FeCl3 + C6H5Cl → 2FeCl2 + C6H4Cl2 + HCl

FeCl2 prepared in this way exhibits convenient solubility in tetrahydrofuran, a common solvent for chemical reactions.

For one of his two classic syntheses of ferrocene, Wilkinson generated FeCl2 by heating FeCl3 with iron powder.

Hydrated and aqueous ferrous chloride
FeCl2 forms complexes with many ligands. Its most common derivative is the hydrate, FeCl2(H2O)4.

Hydrated forms of ferrous chloride are generated by treatment of wastes from steel production with hydrochloric acid. Such solutions are designated spent acid, especially when the hydrochloric acid is not completely consumed. The spent acid requires treatment before its disposal.

Reactions
FeCl2 reacts with two molar equivalents of [(C2H5)4N]Cl to give the salt [(C2H5)4N]2[FeCl4]. Related compounds that can be prepared similarly include the [MnCl4]2-, [MnBr4]2-, [MnI4]2-, [FeBr4]2-, [CoCl4]2-, [CoBr4]2-, [NiCl4]2- and [CuCl4]2- salts.