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.
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]
Hypomanganate is unstable towards disproportionation to manganate(VI) and manganese dioxide: [10] [1] The estimated electrode potentials at pH 14 are: [11] [12] [13]
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]
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]
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]
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:
[14]
Cyclic esters of hypomanganic acid are thought to be intermediates in the oxidation of alkenes by permanganate. [9]
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.
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]
Hypomanganate is unstable towards disproportionation to manganate(VI) and manganese dioxide: [10] [1] The estimated electrode potentials at pH 14 are: [11] [12] [13]
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]
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]
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]
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:
[14]
Cyclic esters of hypomanganic acid are thought to be intermediates in the oxidation of alkenes by permanganate. [9]