Hypomanganate

In chemistry, hypomanganate, also called manganate(V) or tetraoxidomanganate(3−), is a trivalent anion (negative ion) composed of manganese and oxygen, with formula MnO3− 4.

Hypomanganates are usually bright blue. Potassium hypomanganate K3MnO4 is the best known salt, but sodium hypomanganate Na3MnO4, barium hypomanganate Ba3(MnO4)2, and the mixed potassium-barium salt KBaMnO4 is also known. The anion can replace phosphate PO3− 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 Na2O and manganese dioxide MnO2 in fused sodium nitrite NaNO2 at 500 °C. He also crystalized the salt from strong (50%) sodium hydroxide solutions as the decahydrate Na3MnO4·10H2O.

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= 670nm (ε = 900dm3 mol−1 cm−1).

Stability

Hypomanganate is unstable towards disproportionation to manganate(VI) and manganese dioxide: The estimated electrode potentials at pH14 are:

MnO2− 4 + e− ⇌ MnO3− 4E = +0.27 V

MnO3− 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 HMnO2− 4⇌ MnO3− 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 O2 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 Sr5(VO4)3F compound, with hypomanganate substituted for some vanadate units, has been investigated for potential use in near infrared lasers.

The barium salt Ba3(MnO4)2 has interesting magnetic properties.

Related compounds

In theory, hypomanganate would be the conjugate base of hypomanganic acid H3MnO4. This acid cannot be formed because of its rapid disproportionation, but its third acid dissociation constant has been estimated by pulse radiolysis techniques:

HMnO2− 4 ⇌ MnO3− 4 + H+pKa = 13.7 ± 0.2

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