Manganate

The manganate ion is MnO₄^2- and a manganate is any compound that contains the MnO₄^2- group. These salts, e.g. potassium manganate, are the conjugate base of the hypothetical species manganic acid, H₂MnO₄. Manganate salts are intermediates in the industrial synthesis of potassium permanganate. Occasionally permanganate and manganate salts are confused, but they behave quite differently.

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Structure

MnO₄^2- is tetrahedral, being structurally related to other tetrahedral oxides such permanganate and osmium tetroxide. Salts of MnO₄^2-, SO₄^2-, and CrO₄^2- are usually isostructural or nearly so, which means that they form solid solutions and have similar solubilities. Unlike sulfate and chromate, however, MnO₄^2- is paramagnetic.

Synthesis

See potassium manganate

Manganate ion is stable in water only under strongly basic conditions. Consequently, salts are found with electropositive metals, such as Na⁺, K⁺, and Ba²⁺ that do not form insoluble hydroxides. Manganates can be prepared by oxidation of MnO₂ or by reduction of permanganate.

Note that O₂ is insufficiently oxidizing to further convert Mn(VI) to Mn(VII). Thus, the Mn(VI)/Mn(VII) oxidation is effected using stronger oxidants including O₃ or Cl₂.

Reactions

MnO₄^2- is the conjugate base of the hypothetical manganic acid, H₂MnO₄, which is too unstable to be isolated. The half-deprotonated acid has, however, been examined. Thus, the pK_a of HMnO₄^- has been determined as 7.1. MnO₄^2- is about 100 times more basic than the acetate ion.^[1] At lower pH's, the manganate ion will disproportionate to permanganate ion and manganese dioxide:

Permanganate, manganate, hypomanganate, and manganite

The above reactions illustrate the close chemical relationship between the two principal soluble manganese oxides, permanganate and manganate. Two other, rarer members of the series are known. Hypomanganate, MnO₄^3-, is a blue-colored ion of Mn(V) that forms when MnO₄^2- is treated with manganite MnO₄^4- at pH>12. It has also been generated by treatment of KMnO₄ in 10M KOH with H₂O₂. The pK_a of HMnO₄^2- is 13.7,^[2] which indicates that solutions of hypomanganate contain significant amounts of the protonated form even at high pH's. Oxidations of alkenes by permanganate are proposed to proceed via the intermediacy of "esters" of hypomanganate, that is, species of the type MnO₂(OR)₂^- where R is an alkyl group.^[3]

The brown-colored manganite ion, a Mn(IV) species, in turn is produced by dissolving its parent acid anhydride MnO₂ in strong alkali. Alternatively, MnO₄^4- can be generated by reduction of MnO₄^3-.

Uses

BaMnO₄ and K₂MnO₄ are used to oxidize primary and secondary alcohols to aldehydes. Primary alcohols are oxidised to aldehydes, then carboxylic acids. Secondary alcohols are oxidised to ketones. Tertiary alcohols cannot be oxidised in this way. Its use is similar to that for the Jones reagent.

Literature cited

1. ^ Nyholm R.S. Woolliams P.R. (1986). ""Manganates(VI)"". Inorganic Syntheses XI: 56–61.
2. ^ Lee D.G. Chen T. (1993). "Reduction of Manganate(VI) by Mandelic Acid and Its Significance to Development of a General Mechanism for Oxidation of Organic Compounds by High-Valent Transition Metal Oxides". Journal of the American Chemical Society 115: 11231–11236.
3. ^ Rush J.D. Bielski B.H. (1995). "Studies of Manganate(V), -(VI), and -(VII) Tetraoxyanions by Pulse Radiolysis. Optical Spectra of Protonated Forms". Inorg. Chem. 43: 5832–8.

Other references

• G. Procter, S. V. Ley, G. H. Castle “Barium Manganate” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
• Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.

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