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.

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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.^[1] FeBr₂ and FeI₂ can be prepared analogously.

An alternative laboratory synthesis of FeCl₂ entails the reaction of FeCl₃ with chlorobenzene:^[2][3]

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.^[4]

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.^[5]

References

1. ^ G. Winter, Inorganic Synthesis, 1973, volume 14, pp. 101-104.
2. ^ P. Kovacic and N. O. Brace, Inorganic Synthesis, 1960, volume 6, p. 172.
3. ^ P. Kovacic and N. O. Brace, J. Am. Chem. Soc., 1954, volume 76, p. 5491.
4. ^ G. Wilkinson, "Ferrocene", Organic Syntheses, Coll. Vol. 4, p.473 (1963); Vol. 36, p.31 (1956).
5. ^ N. S. Gill & F. B. Taylor, Inorganic Synthesis, 1967, volume 9, pp. 136-142.

See also

• Hydrochloric acid
• Iron(III) chloride
• Iron(II) sulfate