Manganese(II) chloride

Preparation

Manganese chloride is produced by treating manganese(IV) oxide with concentrated hydrochloric acid. :MnO2 + 4 HCl → MnCl2 + 2 H2O + Cl2 In the 19th century this reaction was used for the manufacture of chlorine. By carefully neutralizing the resulting solution with MnCO3, one can selectively precipitate iron salts, which are common impurities in manganese dioxide. In the laboratory, manganese chloride can be prepared by treating manganese metal or manganese(II) carbonate with hydrochloric acid: :Mn + 2 HCl + 4 H2O → MnCl2(H2O)4 + H2 :MnCO3 + 2 HCl + 3 H2O → MnCl2(H2O)4 + CO2

Structures

Anhydrous MnCl2 adopts a layered cadmium chloride-like structure. The tetrahydrate consists of octahedral cis-Mn(H2O)4Cl2 molecules. The trans isomer, which is metastable, is also known. The dihydrate MnCl2(H2O)2 is a coordination polymer. Each Mn center is coordinated to four doubly bridging chloride ligands. The octahedron is completed by a pair of mutually trans aquo ligands.

Chemical properties

The hydrates dissolve in water to give mildly acidic solutions with a pH of around 4. These solutions consist of the metal aquo complex [Mn(H2O)6]2+.

It is a weak Lewis acid, reacting with chloride ions to produce a series of salts containing the following ions [MnCl3]−, [MnCl4]2−, and [MnCl6]4−.

Upon treatment with typical organic ligands, manganese(II) undergoes oxidation by air to give Mn(III) complexes. Examples include Mn([EDTA)]−, Mn([CN)6]3−, and Mn([acetylacetonate)3]. Triphenylphosphine forms a labile 2:1 adduct: :MnCl2 + 2 Ph3P → [MnCl2(Ph3P)2]

Anhydrous manganese(II) chloride serves as a starting point for the synthesis of a variety of organomanganese compounds. For example, manganocene is prepared by reaction of MnCl2 with a solution of sodium cyclopentadienide in tetrahydrofuran (THF). :MnCl2 + 2 NaC5H5 → Mn(C5H5)2 + 2 NaCl Similar reactions are used in the preparation of the antiknock compound methylcyclopentadienyl manganese tricarbonyl.

Manganese chloride is a precursor to organomanganese reagents in organic chemistry.

Manganese chloride is mainly used in the production of dry cell batteries.

Magnetism

Manganese(II) salts are paramagnetic. As such the presence of such salts profoundly affect NMR spectra.

Natural occurrence

Scacchite is the natural, anhydrous form of manganese(II) chloride.

Precautions

Manganism, or manganese poisoning, can be caused by long-term exposure to manganese dust or fumes.

References

References

1. {{IDLH. 7439965. Manganese compounds (as Mn)
2. N. N. Greenwood, A. Earnshaw, ''Chemistry of the Elements'', 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
3. Reidies, Arno H.. (2002). "Ullmann's Encyclopedia of Industrial Chemistry". Wiley-VCH.
4. (1964). "Crystal structure of manganese dichloride tetrahydrate". Inorganic Chemistry.
5. A. F. Wells, ''Structural Inorganic Chemistry'', 5th ed., Oxford University Press, Oxford, UK, 1984.
6. (1965). "Crystal structures of manganese(II) and iron(II) chloride dihydrate". Journal of Chemical Physics.
7. Gérard Cahiez, François Chau, Bernard Blanchot. (1999). "Regioselective Monoalkylation of Ketones Via Their Manganese Enolates: 2-Benzyl-6-Methylcyclohexanone from 2-Methylcyclohexanone". Organic Syntheses.
8. S. Marquais, M. Alami, and G. Cahiez. (1995). "Manganese-Copper-Catalyzed Conjugate Addition of Organomagnesium Reagents to a,b-Ethylenic Ketones: 2-(1,1-Dimethylpentyl)-5-methyl-cyclohexanone from Pulegone". Organic Syntheses.
9. "Scacchite".