Manganese Chemistry

History

For the chapter on Manganese chemistry from the Elsevier text "Chemistry of the Elements" by Greenwood and Earnshaw see On-Line Metals Based Surveys

Occurrence
Manganese is the 12th most abundant element and 3rd most abundant transition metal (cf. Fe, Ti). A number of forms of manganese occur in nature (~ 300 minerals) giving an overall abundance of 0.106%. 12 of these minerals are economically viable including: pyrolusite (MnO2), manganite (Mn2O3.H2O), hausmannite (Mn3O4) rhodochrosite (MnCO3) and Mn-nodules.
The main deposits are found in South Africa and the Ukraine (> 80%) and other important manganese deposits are in China, Australia, Brazil, Gabon, India, and Mexico.

The International Centre for Environmental and Nuclear Sciences (ICENS) has an on-going programme of mapping the geochemical content of Jamaica. 'A Geochemical Atlas of Jamaica' was published in 1995 and is available from Amazon or ICENS.
The results found for Manganese are shown below (courtesy of Prof G.C. Lalor).
ICENS Mn data
Extraction
The metal is obtained by reduction with Al, or in a Blast furnace. The metal resembles iron in being moderately reactive and at high temperatures reacts vigorously with a range of non-metals. For example it burns in N2 at 1200 °C to form Mn3N2 and roasting in air gives Mn3O4

Uses
85-90% of the Manganese produced goes in to the fabrication of ferromanganese alloys. The 1 and 2 Euro coins contain manganese since there it is more abundant and cheaper than nickel.
Manganese dioxide has been used in the cathodes of dry cell batteries and is used in newer alkaline batteries as well.
Manganese salts have been used in glass making since the Eygyptian and Roman times and found in paints from as early as 17,000 years ago.
Its use in glass is either to add colour or to reduce the effect iron impurities have on the colour of glass, see below.

Manganese in Biology
Manganese is an essential trace element for all forms of life. It accumulates in mitochondria and is essential for their function. The manganese transport protein, transmanganin, is thought to contain Mn(III). Several metalloenzymes are known: arginase, pyruvate carboxylase and superoxide dismutase.

Humans excrete roughly 10 kg of urea per year, this results from the hydrolysis of arginine by the enzyme arginase found on the liver which is the final step of the urea cycle. This reaction allows for the disposal of nitrogenous waste from the breakdown of proteins.

In mammalian arginases I and II, binuclear manganese clusters are present at the active site. In the structure 1rla the Manganese nearest neighbours were identifed as: Asp124, Asp128, Asp232, Asp234, His101, His126.


Oxides

Manganese oxides
Formula Colour Oxidation State MP °C
Mn2O7 green oil Mn7+ 5.9
MnO2 black Mn4+ 535d
Mn2O3 black Mn3+ 1080d
Mn3O4 - Haussmanite black Mn2/3+ 1705
MnO grey-green Mn2+ 1650

Preparations:

Mn3O4 is prepared from the other oxides by heating in air at 1000 °C
MnO is prepared from the other oxides by heating with H2 at temperatures below 1200 °C above that Mn metal is produced.
MnO2 has been used for many years to decolourise commercial glass. When added to molten glass a small amount of red-brown Mn(III) results that masks the blue-green colour from iron impurities. That is, by adding a reagent with the complimentary colour of the impurity, the resultant effect is to balance out and give a clear glass.
MnO2 is used as an oxidant for the conversion of aniline to hydroquinone.
Mn2O7 is dangerously explosive above 3 °C. It is thought that some accidents have occurred when instead of adding conc HCl to solid KMnO4 to produce Cl2 the wrong bottle is selected and conc H2SO4 was used leading to the formation of a green oil that explodes.

High Oxidation State Oxide Salts

Fusion of MnO2 with an alkali metal hydroxide and an oxidizing agent such as KNO3 produces very dark-green manganate(VI) salts (manganates) which are stable in strongly alkaline solution but which disproportionate readily in neutral or acid solution.

3MnO42- + 4H+ → 2MnO4- + MnO2+ 2H2O

The deep-purple manganate(VII) salts (permanganates) may be prepared in aqueous solution by oxidation of manganese(II) salts with very strong oxidizing agents such as PbO2 or NaBiO3. They are manufactured commercially by alkaline oxidative fusion of MnO2 followed by the electrolytic oxidation of manganate(VI):

2MnO2+ 4KOH + O2 → 2K2MnO4+ 2H2O

2K2MnO4+ 2H2O → 2KMnO4+ 2KOH + H2

The most important manganate(VII) is KMnO4 and the very intense purple colour is due to a charge transfer band and not a d-d transition. It is a well-known oxidizing agent.

Redox properties of KMnO4.

  strong base
  MnO4- + e-      →  MnO42-      E=0.56V (RAPID)
  MnO42-  + 2H2O  + e-  →  MnO2  + 4OH-  E=0.60V (SLOW)

  moderate base
  MnO4- + 2H2O  + 3e- →  MnO2  + 4OH-    E=0.59V

  dil. H2SO4
  MnO4- + 8H2O  + 5e- →  Mn2+  + 4H2O    E=1.51V
The usual conditions for its use are 0.02M KMnO4 and 1.5M H2SO4.

In the industrial production of saccharin and benzoic acid, KMnO4 is the oxidant, medically, it has been used as a disinfectant. It is gaining in use for water purification, since it has an advantage over chlorine that it does not affect the taste, and has the bonus that the MnO2 produced acts as a coagulant for colloidal impurities.

Halides
Manganese(II) halides
Formula Colour MP °C BP °C m (BM) Structure
MnF2 pale-pink 920 - - rutile
MnCl2 pink 652 1190 5.73 CdCl2
MnBr2 rose 695 - 5.82 -
MnI2 pink 613 - 5.88 CdI2
Preparations:

Prepared from MnCO3 + HX → MnX2 + CO2 + H2O

Manganese complexes

Octahedral complexes of Mn(III) are expected to show Jahn-Teller distortions. It was of interest therefore to compare the structures of Cr(acac)3 with Mn(acac)3 since the Cr(III) ion is expected to give a regular octahedral shape. In fact the Mn-O bond distances were all found to be equivalent.

An unusual Mn complex is obtained by the reaction of Mn(OAc)2 with KMnO4 in HOAc. This gives [Mn3O(OAc)6 3H2O]OAc. It is used as an industrial oxidant for the conversion of toluene to phenol.

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The Department of Chemistry, University of the West Indies,
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Created October 2003. Links checked and/or last modified 21st November 2007.
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