Hypomanganate
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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.[1][2] 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.[3] The anion can replace phosphate PO3−
4 in synthetic variants of the minerals apatite[4][5] and brownmillerite.[6]
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.[7][3] 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 d2 configuration, the anion has a triplet ground state.[3]
The anion is a bright blue species[1] with a visible absorption maximum at wavelength λmax = 670 nm (ε = 900 dm3 mol−1 cm−1).[8][9]
Stability
Hypomanganate is unstable towards disproportionation to manganate(VI) and manganese dioxide:[10][1] The estimated electrode potentials at pH 14 are:[11][12][13]
- MnO2−
4 + e− ⇌ MnO3−
4 E = +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).[1][7]
The disproportionation is believed to pass through a protonated intermediate,[13] with the acid dissociation constant for the reaction HMnO2−
4 ⇌ MnO3−
4 + H+ being estimated as pKa = 13.7 ± 0.2.[14] However, K3MnO4 has been cocrystallized with Ca2Cl(PO4), allowing the study of the UV–visible spectrum of the hypomanganate ion.[10][15]
Preparation
Hypomanganates may be prepared by the careful reduction of manganates with sulfite,[1] hydrogen peroxide[16] or mandelate.[9]
Hypomanganates can also be prepared by the solid state method under O2 flow near 1000 °C.[3][4][5][6] They can be prepared also via low temperature routes such as hydrothermal synthesis or flux growth.[3] It is produced by dissolving manganese dioxide in molten sodium nitrite.[17]
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.[18]
The barium salt Ba
3(MnO
4)
2 has interesting magnetic properties.[19]