Hypomanganate

In chemistry, hypomanganate, also called manganate(V) or tetraoxidomanganate(3−), is a trivalent anion (negative ion) composed of manganese and oxygen, with formula.

Hypomanganates are usually bright blue. Potassium hypomanganate is the best known salt, but sodium hypomanganate, barium hypomanganate , and the mixed potassium-barium salt  is also known. The anion can replace phosphate in synthetic variants of the minerals apatite  and brownmillerite.

History
The manganate(V) anion was first reported in 1946 by Hermann Lux, who synthesized the intensely blue sodium hypomanganate by reacting sodium oxide and manganese dioxide  in fused sodium nitrite  at 500 °C. He also crystalized the salt from strong (50%) sodium hydroxide solutions as the decahydrate ·10.

Structure and properties
Manganate(V) is a tetrahedral oxyanion structurally similar to sulfate, manganate, and permanganate. As expected for a tetrahedral complex with a d2 configuration, the anion has a triplet ground state.

The anion is a bright blue species with a visible absorption maximum at wavelength λmax = 670 nm (ε = 900 dm3 mol−1 cm−1).

Stability
Hypomanganate is unstable towards disproportionation to manganate(VI) and manganese dioxide: The estimated electrode potentials at pH 14 are:
 * MnO$2− 4$ + e− MnO$3− 4$   E = +0.27 V
 * MnO$3− 4$ + e− + 2 H2O MnO2 + 4 OH−   E = +0.96 V

However, the reaction is slow in very alkaline solutions (with OH− concentration above 5–10 mol/L).

The disproportionation is believed to pass through a protonated intermediate, with the acid dissociation constant for the reaction HMnO$2− 4$ MnO$3− 4$ + H+ being estimated as pKa = 13.7 ± 0.2. However, K3MnO4 has been cocrystallized with Ca2Cl(PO4), allowing the study of the UV–visible spectrum of the hypomanganate ion.

Preparation
Hypomanganates may be prepared by the careful reduction of manganates with sulfite, hydrogen peroxide or mandelate.

Hypomanganates can also be prepared by the solid state method under flow near 1000 °C. They can be prepared also via low temperature routes such as hydrothermal synthesis or flux growth. It is produced by dissolving manganese dioxide in molten sodium nitrite.

Uses
The strontium vanadate fluoride compound, with hypomanganate substituted for some vanadate units, has been investigated for potential use in near infrared lasers.

The barium salt has interesting magnetic properties.

Related compounds
In theory, hypomanganate would be the conjugate base of hypomanganic acid. This acid cannot be formed because of its rapid disproportionation, but its third acid dissociation constant has been estimated by pulse radiolysis techniques:
 * HMnO$2− 4$ MnO$3− 4$ + H+   pKa = 13.7 ± 0.2

Cyclic esters of hypomanganic acid are thought to be intermediates in the oxidation of alkenes by permanganate.