Hypomanganate
# Hypomanganate

In chemistry, hypomanganate, also called manganate(V) or tetraoxidomanganate(3−), is a trivalent anion (negative ion) composed of manganese and oxygen, with formula $MnO^{3−}$ $_4.$

Hypomanganates are usually bright blue. Potassium hypomanganate K $_3MnO$ $_4$ is the best known salt, but sodium hypomanganate Na $_3MnO$ $_4,$ barium hypomanganate Ba $_3(MnO$ $_4)$ $_2,$ and the mixed potassium-barium salt KBaMnO $_4$ is also known. The anion can replace phosphate $PO^{3−}$ $_4$ 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 Na $_2O$ and manganese dioxide MnO $_2$ in fused sodium nitrite NaNO $_2$ at 500 °C. He also crystalized the salt from strong (50%) sodium hydroxide solutions as the decahydrate Na $_3MnO$ $_4·10H$ $_2O.$

## Structure and properties

Manganate(V) is a tetrahedral oxyanion structurally similar to sulfate, manganate, and permanganate. As expected for a tetrahedral complex with a $d^2$ configuration, the anion has a triplet ground state.

The anion is a bright blue species with a visible absorption maximum at wavelength $λ_{max}$ = 670 nm (ε = 900 $dm^3$ $mol^{−1}$ $cm^{−1}).$

## 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 $H_2O$ ⇌ $MnO_2$ + 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 $pK_a$ = 13.7 ± 0.2. However, $K_3MnO_4$ has been cocrystallized with $Ca_2Cl(PO_4),$ 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 $O_2$ 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 Sr $_5(VO$ $_4)$ $_3F$ compound, with hypomanganate substituted for some vanadate units, has been investigated for potential use in near infrared lasers.

The barium salt Ba $_3(MnO$ $_4)$ $_2$ has interesting magnetic properties.

## Related compounds

In theory, hypomanganate would be the conjugate base of hypomanganic acid H $_3MnO$ $_4.$ This acid cannot be formed because of its rapid disproportionation, but its third acid dissociation constant has been estimated by pulse radiolysis techniques:

    $HMnO^{2−}$ $_4$ ⇌ $MnO^{3−}$ $_4$ + $H^+$ $pK_a$ = 13.7 ± 0.2

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

