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21.8A: The Metal

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    34447
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    Oxides

    Table \(\PageIndex{1}\): Manganese oxides
    Formula Color 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 decolorise commercial glass. When added to molten glass a small amount of red-brown Mn(III) results that masks the blue-green color from iron impurities. That is, by adding a reagent with the complimentary color 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.

    \[3MnO_4^{2-} + 4H^+ → 2MnO_4^- + MnO_2+ 2H_2O\]

    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 \(MnO_2\) followed by the electrolytic oxidation of manganate(VI):

    \[2MnO_2+ 4KOH + O2 → 2K_2MnO_4+ 2H_2O\]

    \[2K_2MnO_4+ 2H_2O → 2KMnO_4+ 2KOH + H_2\]

    The most important manganate(VII) is KMnO4 and the very intense purple color is due to a charge transfer band and not a d-d transition. It is a well-known oxidizing agent; the usual conditions for its use are 0.02 M KMnO4 and 1.5 M H2SO4.

    Redox properties of \(KMnO_4\)
    • strong base

    \[MnO_4^- + e^- → MnO_4^{2-} \,\,\, \text{ (RAPID)} \]

    with \(E=0.56\,V\)

    \[MnO_4^{2-} + 2H_2O + e^- → MnO_2 + 4OH^-\,\,\, \text{ (SLOW)} \]

    with \(E=0.60\,V\)

    • moderate base

    \[MnO_4^- + 2H_2O + 3e^- → MnO_2 + 4OH^-\]

    with \(E=0.59\,V\)

    • dil. H2SO4

    \[MnO_4^- + 8H_2O + 5e^- → Mn^{2+} + 4H_2O\]

    with \(E=1.51\,V\)

    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.

    Table \(\PageIndex{2}\): Manganese(II) halides
    Formula Color 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

    \[MnCO_3 + HX → MnX_2 + CO_2 + H_2O\]

    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.

    Contributors and Attributions


    21.8A: The Metal is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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