MANGANESE CHLORIDE: PROPERTIES, STRUCTURE, USES, RISKS - CHEMISTRY - 2025

The manganese chloride is an inorganic salt having the chemical formula MnCl ₂. It is made up of Mn ²⁺ and Cl ^- ions in a 1: 2 ratio; for each Mn ²⁺ cation there are twice as many Cl ^- anions.

This salt can form several hydrates: MnCl ₂ · 2H ₂ O, (dihydrate), MnCl ₂ · 4H ₂ O (tetrahydrate), and MnCl ₂ · 6H ₂ O (hexahydrate). The most common form of the salt is tetrahydrate.

Pink crystals of manganese chloride. Source: Ondřej Mangl

The physical properties of manganese chloride such as density, melting point and solubility in water, are influenced by its degree of hydration. For example, the melting point of the anhydrous form is much higher than that of the tetrahydrate form.

The color of manganese chloride is pale pink (top image). Paleness is characteristic of transition metal salts. Manganese chloride is a weak Lewis acid.

The mineral known as scacquita is the naturally anhydrous form of manganese (II) chloride; like kempita.

Manganese (II) chloride is used as an alloying agent; catalyst in chlorination reactions, etc.

Physical properties

Physical appearance

- Anhydrous form: pink cubic crystals.

- Tetrahydrate form: slightly deliquescent reddish monoclinic crystals.

Molar masses

- Anhydrous: 125.838 g / mol.

- Dihydrate: 161.874 g / mol.

- Tetrahydrate: 197.91 g / mol.

Melting points

- Anhydrous: 654 ºC.

- Dihydrate: 135 ºC.

- Tetrahydrate: 58 ºC.

Boiling point

Anhydrous form: 1,190 ºC.

Densities

- Anhydrous: 2,977 g / cm ³.

- Dihydrate: 2.27 g / cm ³.

- Tetrahydrate: 2.01 g / cm ³.

Water solubility

Anhydrous form: 63.4 g / 100 ml at 0 ° C; 73.9 g / 100 ml at 20 ° C; 88.5 g / 100 ml at 40 ° C; and 123.8 g / 100 ml at 100 ° C.

Solubility in organic solvents

Soluble in pyridine and ethanol, insoluble in ether.

Decomposition

Unless proper precautions are taken, dehydration of the hydrated forms to the anhydrous form can lead to hydrolytic dehydration, with the production of hydrogen chloride and manganese oxychloride.

pH

A 0.2 M solution of manganese chloride tetrahydrate in aqueous solution has a pH of 5.5.

Stability

It is stable, but sensitive to moisture and incompatible with strong acids, reactive metals, and hydrogen peroxide.

Structure of manganese chloride

Coordination complex for MnCl2 tetrahydrate. Source: Smokefoot

Starting with the tetrahydrated salt, with striking pink crystals, it must be made up of coordination complexes (top image). In them, the metal center of Mn ²⁺ is surrounded by an octahedron defined by four H ₂ O molecules and two Cl ^- anions.

Note that the Cl ^- ligands are in cis positions; they are all equivalent at the rectangular base of the octahedron, and it doesn't matter if Cl is "moved" ^- to any of the other three positions. Another possible isomer for this coordinate molecule is one in which both Cl ^- are in trans positions; that is, at different extremes (one above and the other below).

The four water molecules with their hydrogen bonds allow two or more octahedra to be joined by dipole-dipole forces. These bridges are highly directional, and adding the electrostatic interactions between Mn ²⁺ and Cl ^-, they establish an ordered structure characteristic of a crystal.

The pink color of MnCl ₂ · 4H ₂ O is due to the electronic transitions of Mn ²⁺ and its d ⁵ configuration. Likewise, the disturbances caused by the proximity of the water molecules and the chlorides modify the amount of energy necessary to be absorbed by these d ⁵ electrons in order to transit higher energy levels.

Dihydrate

Polymeric structure for MnCl2 · 2H2O. Source: Smokefoot

The salt has been dehydrated and its formula now becomes MnCl ₂ · 2H ₂ O. What happens to the octahedron above? Nothing, except that the two H ₂ O molecules they left behind are replaced by two Cl ^-.

At first you may give the wrong impression that there are four Cl ^- for every Mn ²⁺; however, the half of the octahedron (axially) is actually the repeating unit of the crystal.

Thus, it is true that there is an Mn ²⁺ coordinated to two Cl ^- and two water molecules in trans positions. But for this unit to interact with another it needs two Cl bridges, which in turn allows the coordination octahedron for manganese to be completed.

In addition to Cl bridges, water molecules also collaborate with their hydrogen bonds so that this MnCl ₂ · 2H ₂ O chain does not disassemble.

Anhydrous

Finally, the magnesium chloride has finished losing all the water contained in its crystals; you now have the anhydrous salt, MnCl ₂. Without the water molecules, the crystals noticeably lose the intensity of their pink coloration. The octahedron, as for hydrates, remains unchanged by the very nature of manganese.

Without water molecules, the Mn ²⁺ ends up surrounded by an octahedron composed only of Cl ^-. This coordination bond is both covalent and ionic; for this reason the structure of MnCl _{2 is} often referred to as a polymer crystal. In it there are alternating layers of Mn and Cl.

Nomenclature

Manganese has many possible oxidation states. Because of this, the traditional nomenclature for MnCl ₂ is not clear.

On the other hand, manganese chloride corresponds to its best known name, to which it would be necessary to add the '(II)' to make it agree with the stock nomenclature: manganese (II) chloride. And likewise, there is the systematic nomenclature: manganese dichloride.

Applications

Laboratory

Manganese chloride serves as a catalyst for the chlorination of organic compounds.

Industry

Manganese chloride is used as a raw material for the production of antiknockers for gasoline; welding material for non-ferrous metals; intermediary in the manufacture of pigments; and linseed oil drier.

It is used in the textile industry for printing and dyeing; in the production of various manganese salts, including methylcyclopentadienylmanganese tricarbonyl used as a brick colorant; and in the production of dry electric cells.

Manganese chloride is used as an alloying agent and is added to molten magnesium to produce manganese-magnesium alloys; as an intermediate in the preparation of drying agents for paint and varnish; and as a component of disinfectants.

It is also used in the purification of magnesium.

Fertilizer and animal feed

Manganese chloride is used as a source of manganese, an element that, although not a primary nutritional element for plants like nitrogen, phosphorus and potassium, is used in numerous biochemical reactions typical of these living beings.

It is also added to the feed of breeding animals to supply manganese, an essential trace element for the growth of animals.

Manganese chloride is a dietary component that supplies manganese, an element that is involved in many processes necessary for life, including: synthesis of fatty acids and sex hormones; assimilation of vitamin E; cartilage production; etc.

Risks

May cause redness, irritation and dermatitis on contact with the skin. Manganese chloride causes red, painful, and watery eyes.

When inhaled, salt causes a cough, sore throat and shortness of breath. On the other hand, ingestion could cause vomiting, nausea and diarrhea.

Chronic excessive inhalation of this salt can lead to lung inflammation and subsequent reactive airway disease.

Its excessive ingestion can cause mental disorders, dehydration, hypotension, liver and kidney failure, failure of the multiorgan system and death.

Neurotoxicity is the initial manifestation of the undesirable action of manganese, with headaches, dizziness, memory loss, hyperreflexia, and mild tremor.

Severe toxicity is manifested by symptoms and signs similar to those seen in Parkinson's disease.

References

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5. Toxicology Data Network. (sf). Manganese chloride. Toxnet. Recovered from: toxnet.nlm.nih.gov
6. Gérard Cahiez. (2001). Manganese (II) Chloride. doi.org/10.1002/047084289X.rm020
7. National Center for Biotechnology Information. (2019). Manganese dichloride. PubChem Database. CID = 24480. Recovered from: pubchem.ncbi.nlm.nih.gov
8. WebConsultas Healthcare, SA (2019). Minerals: manganese. Recovered from: webconsultas.com