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.

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

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