Manganate

The manganate ion is MnO42− and a manganate is any compound that contains the MnO42− 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.

Structure
MnO42− is tetrahedral, being structurally related to other tetrahedral oxides such permanganate and osmium tetroxide. Salts of MnO42−, SO42−, and CrO42− are usually isostructural or nearly so, which means that they form solid solutions and have similar solubilities. Unlike sulfate and chromate, however, MnO42− 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
MnO42− 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 pKa of HMnO4− has been determined as 7.1. MnO42− is about 100 times more basic than the acetate ion. 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, MnO43−, is a blue-colored ion of Mn(V) that forms when MnO42− is treated with manganite MnO44− at pH>12. It has also been generated by treatment of KMnO4 in 10M KOH with H2O2. The pKa of HMnO42− is 13.7, 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.

The brown-colored manganite ion, a Mn(IV) species, in turn is produced by dissolving its parent acid anhydride MnO2 in strong alkali. Alternatively, MnO44− can be generated by reduction of MnO43−.

Uses
BaMnO4 and K2MnO4 are used to oxidize primary and secondary alcohols. 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.

Other references
See category for a list.
 * 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.