thumb|150px|Structure of manganate
In inorganic nomenclature, a manganate is any negatively charged molecular entity with manganese as the central atom. However, the name is usually used to refer to the tetraoxidomanganate(2−) anion, MnO, also known as manganate(VI) because it contains manganese in the +6 oxidation state.
Other manganates include hypomanganate or manganate(V), , permanganate or manganate(VII), , and the dimanganate or dimanganate(III) .
A manganate(IV) anion has been prepared by radiolysis of dilute solutions of permanganate. It is mononuclear in dilute solution, and shows a strong absorption in the ultraviolet and a weaker absorption at 650 nm. The manganese–oxygen distance is 165.9 pm, about 3 pm longer than in permanganate. The Raman spectrum has also been reported.
Preparation
Sodium and potassium manganates are usually prepared in the laboratory by stirring the equivalent permanganate in a concentrated solution (5–10 M) of the hydroxide for 24 hours
: + → + + O<sub>2</sub>
Potassium manganate is prepared industrially, as an intermediate to potassium permanganate, by dissolving manganese dioxide in molten potassium hydroxide with potassium nitrate or air as the oxidizing agent.
Uses
Manganates, particularly the insoluble barium manganate, BaMnO<sub>4</sub>, have been used as oxidizing agents in organic synthesis: they will oxidize primary alcohols to aldehydes and then to carboxylic acids, and secondary alcohols to ketones. Barium manganate has also been used to oxidize hydrazones to diazo compounds.
Related compounds
Manganate is formally the conjugate base of hypothetical manganic acid , which cannot be formed because of its rapid disproportionation. However, its second acid dissociation constant has been estimated by pulse radiolysis techniques:
:HMnO MnO + H<sup>+</sup> pK<sub>a</sub> = 7.4 ± 0.1
Manganites
The name "manganite" is used for compounds formerly believed to contain the anion , with manganese in the +3 oxidation state. However, most of these "manganites" do not contain discrete oxyanions, but are mixed oxides with perovskite (LaMn<sup>III</sup>O<sub>3</sub>, CaMn<sup>IV</sup>O<sub>3</sub>), spinel (LiMnO<sub>4</sub>) or sodium chloride (LiMn<sup>III</sup>O<sub>2</sub>, NaMn<sup>III</sup>O<sub>2</sub>) structures.
One exception is potassium dimanganate(III), K<sub>6</sub>Mn<sub>2</sub>O<sub>6</sub>, which contains discrete Mn<sub>2</sub>O anions.