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.[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]

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.[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]

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.[17]

The barium salt Ba
3(MnO
4)
2 has interesting magnetic properties.[18]

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]

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.[9]

See also
  • Dimanganite, a manganate(III) anion Mn
    2    O6−
    6
  • Manganate or manganate(VI), MnO2−
    4
  • Permanganate or manganate(VII), MnO−
    4

References
  1. Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 1221–22. ISBN 978-0-08-022057-4..
  2. D. Reinen, W. Rauw, U. Kesper, M. Atanasov, H. U Güdel, M. Hazenkamp, and U. Oetliker (1997): "Colour, luminescence and bonding properties of tetrahedrally coordinated chromium(IV), manganese(V) and iron(VI) in various oxide ceramics" Journal of Alloys and Compounds, volume 246, issue 1-2, pages 193-208. doi:10.1016/S0925-8388(96)02461-9
  3. zur Loye, K. D.; Chance, W. M.; Yeon, J.; zur Loye, H.-C. (2014). "Synthesis, Crystal Structure, and Magnetic Properties of the Oxometallates KBaMnO4 and KBaAsO4". Solid State Sciences. 37: 86–90. doi:10.1016/j.solidstatesciences.2014.08.013.CS1 maint: uses authors parameter (link)
  4. K. Dardenne, D. Vivien, and D. Huguenin (1999): "Color of Mn(V)-substituted apatites A10((B, Mn)O4)6F2, A = Ba, Sr, Ca; B= P, V". Journal of Solid State Chem.istry, volume 146, issue 2, pages 464-472. doi:10.1006/jssc.1999.8394
  5. Grisafe, D.A. and Hummel, F.A. (1970): "Pentavalent ion substitutions in the apatite structure, part A: Crystal chemistry". Journal of Solid State Chemistry, volume 2, issue 2, pages 160-166 doi:10.1016/0022-4596(70)90064-2
  6. P. Jiang, J. Li, A. Ozarowski, A. W. Sleight, and M. A, Subramanian (2013): "Intense turquoise and green colors in brownmillerite-type oxides based on Mn5+ in Ba
    2    In
    2-x    Mn
    x    O
    5+x    " Inorganic Chemistry, volume 52, issue 3, pages 1349-1357. doi:10.1021/ic3020332
  7. Herrman Lux (1946): "Über Salze des fünfwertigen Mangans." Zeitschrift für Naturforschung, volume 1, pages 281-283.
  8. Carrington, A.; Symons, M. C. R. (1956), "Structure and reactivity of the oxy-anions of transition metals. Part I. The manganese oxy-anions", J. Chem. Soc.: 3373–80, doi:10.1039/JR9560003373
  9. Lee, Donald G.; Chen, Tao (1993), "Reduction of manganate(VI) by mandelic acid and its significance for development of a general mechanism of oxidation of organic compounds by high-valent transition metal oxides", J. Am. Chem. Soc., 115 (24): 11231–36, doi:10.1021/ja00077a023.
  10. Cotton, F. Albert; Wilkinson, Geoffrey (1980), Advanced Inorganic Chemistry (4th ed.), New York: Wiley, p. 746, ISBN 0-471-02775-8.
  11. Weast, Robert C., ed. (1981). CRC Handbook of Chemistry and Physics (62nd ed.). Boca Raton, FL: CRC Press. p. D-134. ISBN 0-8493-0462-8..
  12. Manganese – compounds – standard reduction potentials, WebElements, retrieved 2010-06-26.
  13. Sekula-Brzezińska, K.; Wrona, P. K.; Galus, Z. (1979), "Rate of the MnO4−/MnO42− and MnO42−/MnO43− electrode reactions in alkaline solutions at solid electrodes", Electrochim. Acta, 24 (5): 555–63, doi:10.1016/0013-4686(79)85032-X.
  14. Rush, J. D.; Bielski, B. H. J. (1995), "Studies of Manganate(V), -(VI), and -(VII) Tetraoxyanions by Pulse Radiolysis. Optical Spectra of Protonated Forms", Inorg. Chem., 34 (23): 5832–38, doi:10.1021/ic00127a022.
  15. Carrington, A.; Symons, M. C. R. (1956), "Structure and reactivity of the oxy-anions of transition metals. Part I. The manganese oxy-anions", J. Chem. Soc.: 3373–80, doi:10.1039/JR9560003373.
  16. Lee, Donald G.; Chen, Tao (1989), "Oxidation of hydrocarbons. 18. Mechanism of the reaction between permanganate and carbon-carbon double bonds", J. Am. Chem. Soc., 111 (19): 7534–38, doi:10.1021/ja00201a039.
  17. L. D. Merkle, Y. Guyot, and B. H. T. Chai (1995): "Spectroscopic and laser investigations of Mn5+:Sr5(VO4)3F". Journal of Applied Physics, volume 77, issue 2, pages 474-480. doi:10.1063/1.359585
  18. M. B. Stone, M. D. Lumsden, Y. Qiu, E. C. Samulon, C. D. Batista, and I. R. Fisher (2008): "Dispersive magnetic excitations in the S=1 antiferromagnet Ba
    3    Mn
    2    O
    8    ". Physics Review B, volume 77, page 134406 doi:10.1103/PhysRevB.77.134406
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