Manganese(II) chloride

Manganese(II) chloride describes a series of compounds with the formula MnCl₂(H₂O)_x, where the value of x can be 0, 2, or 4. The tetrahydrate is a pink salt that occurs naturally as the rare mineral scacchite. Most commonly, the term "manganese(II) chloride" refers to the tetrahydrate MnCl₂·4H₂O, which consists of octahedral trans-Mn(H₂O)₄Cl₂ molecules. The dihydrate MnCl₂·2H₂O is also known. Many Mn(II) species are characteristically pink, the paleness of the color being characteristic of transition metal complexes with high spin d⁵ configurations.

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Preparation

Manganese(II) chloride can be prepared by treating manganese metal or manganese(II) carbonate with hydrogen chloride or hydrochloric acid, depending on whether one seeks the anhydrous or hydrated forms:

Mn + 2 HCl → MnCl₂ + H₂

MnCO₃ + 2 HCl → MnCl₂ + H₂O + CO₂

It also forms when manganese(IV) oxide is heated with concentrated hydrochloric acid; this reaction was once used for the manufacture of chlorine.

MnO₂ + 4 HCl → MnCl₂ + 2 H₂O + Cl₂

This reaction illustrates the oxidizing power of MnO₂ and the fact that oxides tend to exist in higher oxidation states than chlorides.

Chemical properties

MnCl₂ is a polymeric solid which adopts a layered structure. The hydrates dissolve in water to give mildly acidic solutions with a pH of around 4. Such aqueous solutions are used to prepare a variety of manganese compounds, e.g.,

MnCl₂(aq) + K₂CO₃(aq) → MnCO₃(s) + 2 KCl

In these equations, "MnCl₂(aq)" refers to [Mn(H₂O)₆]²⁺, which is the principal form of manganese in aqueous solutions of any manganese chloride.

It is a weak Lewis acid, reacting with chloride ions to produce a series of solids containing the following ions [MnCl₃]^-, [MnCl₄]^2- , and [MnCl₆]^4-. Both [MnCl₃]^- and [MnCl₄]^2- are polymeric.

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 manganese compounds. For example, manganocene is prepared by reaction of MnCl₂ with a solution of sodium cyclopentadienide in THF.

MnCl₂ + 2 NaC₅H₅ → Mn(C₅H₅)₂ + 2 NaCl

Paramagnetism

Manganese chloride is paramagnetic, this fact is utilized when MnCl₂ is used as a contrast agent in MRI scanning. A dramatic example is using MnCl₂ to highlight neural pathways.

Precautions

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

References

1. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
2. Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
3. A. F. Wells, 'Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984.
4. The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
5. Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.