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Structure of manganate

In inorganic nomenclature, a manganate is any negatively charged molecular entity with manganese as the central atom.[1] However, the name is usually used to refer to the tetraoxidomanganate(2−) anion, MnO2−
4
, also known as manganate(VI) because it contains manganese in the +6 oxidation state.[1] Manganates are the only known manganese(VI) compounds.[2]

Other manganates include hypomanganate or manganate(V), MnO3−
4
, permanganate or manganate(VII), MnO
4
, and the dimanganite or dimanganate(III) Mn
2
O6−
6
.

A manganate(IV) anion MnO4−
4
has been prepared by radiolysis of dilute solutions of permanganate.[3][4] It is mononuclear in dilute solution, and shows a strong absorption in the ultraviolet and a weaker absorption at 650 nm.[3]

Contents

Structure and propertiesEdit

 
Solution containing the manganate(VI) ion

The manganate(VI) ion is tetrahedral, similar to sulfate or chromate: indeed, manganates are often isostructural with sulfates and chromates, a fact first noted by Mitscherlich in 1831.[5] The manganeseoxygen distance is 165.9 pm, about 3 pm longer than in permanganate.[5] As a d1 ion, it is paramagnetic, but any Jahn–Teller distortion is too small to be detected by X-ray crystallography.[5] Manganates are dark green in colour, with a visible absorption maximum of λmax = 606 nm (ε = 1710 dm3 mol−1 cm−1).[6][7] The Raman spectrum has also been reported.[8]

PreparationEdit

Sodium and potassium manganates are usually prepared in the laboratory by stirring the equivalent permanganate in a concentrated solution (5–10 M) of the hydroxide for 24 hours[6] or with heating.[9]

4 MnO
4
+ 4 OH4 MnO2−
4
+ 2 H2O + O2

Potassium manganate is prepared industrially, as an intermediate to potassium permanganate, by dissolving manganese dioxide in molten potassium hydroxide with potassium nitrate or air as the oxidizing agent.[2]

2 MnO2 + 4 OH + O22 MnO2−
4
+ 2 H2O

UsesEdit

Manganates, particularly the insoluble barium manganate, BaMnO4, have been used as oxidizing agents in organic synthesis: they will oxidize primary alcohols to aldehydes and then to carboxylic acids, and secondary alcohols to ketones.[10][11] Barium manganate has also been used to oxidize hydrazones to diazo compounds.[12]

DisproportionationEdit

Manganates are unstable towards disproportionation in all but the most alkaline of aqueous solutions.[2] The ultimate products are permanganate and manganese dioxide, but the kinetics are complex and the mechanism may involve protonated and/or manganese(V) species.[13][14]

Related compoundsEdit

Manganate is formally the conjugate base of hypothetical manganic acid H
2
MnO
4
, which cannot be formed because of its rapid disproportionation. However, its second acid dissociation constant has been estimated by pulse radiolysis techniques:[3]

HMnO
4
⇌ MnO2−
4
+ H+   pKa = 7.4 ± 0.1

ManganitesEdit

The name "manganite" is used for compounds formerly believed to contain the anion MnO3−
3
, with manganese in the +3 oxidation state. However, most of these "manganites" do not contain discrete oxoanions, but are mixed oxides with perovskite (LaMnIIIO3, CaMnIVO3), spinel (LiMnIII,IV
2
O4) or sodium chloride (LiMnIIIO2, NaMnIIIO2) structures.

One exception is potassium dimanganate(III), K6Mn2O6, which contains discrete Mn2O6−
6
anions.[15]

ReferencesEdit

  1. ^ a b International Union of Pure and Applied Chemistry (2005). Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005). Cambridge (UK): RSCIUPAC. ISBN 0-85404-438-8. pp. 74–75, 77–78, 313, 338. Electronic version..
  2. ^ a b c Cotton, F. Albert; Wilkinson, Geoffrey (1980), Advanced Inorganic Chemistry (4th ed.), New York: Wiley, p. 746, ISBN 0-471-02775-8.
  3. ^ a b c 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.
  4. ^ 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.
  5. ^ a b c Palenik, Gus J. (1967), "Crystal structure of potassium manganate", Inorg. Chem., 6 (3): 507–11, doi:10.1021/ic50049a016.
  6. ^ a b 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
  7. ^ Lee, Donald G.; Chen, Tao (1993), "Reduction of manganate(VI) by mandelic acid and its significance for development of a general mechanism of dationoxin of organic compounds by high-valent transition metal oxides", J. Am. Chem. Soc., 115 (24): 11231–36, doi:10.1021/ja00077a023.
  8. ^ Juberta, A. H.; Varettia, E. L. (1982), "Normal and resonance Raman spectra of some manganates", J. Mol. Struct., 79: 285–88, Bibcode:1982JMoSt..79..285J, doi:10.1016/0022-2860(82)85067-9
  9. ^ Nyholm, R. S.; Woolliams, P. R. (1986), "Manganates(VI)", Inorg. Synth., 11: 56–61
  10. ^ Procter, G.; Ley, S. V.; Castle, G. H. (2004), "Barium Manganate", in Paquette, L., Encyclopedia of Reagents for Organic Synthesis, New York: Wiley, doi:10.1002/047084289.
  11. ^ Firouzabadi, Habib; Mostafavipoor, Zohreh (1983), "Barium Manganate. A Versatile Oxidant in Organic Synthesis", Bull. Chem. Soc. Jpn., 56 (3): 914–17, doi:10.1246/bcsj.56.914.
  12. ^ Guziec, Frank S., Jr.; Murphy, Christopher J.; Cullen, Edward R. (1985), "Thermal and photochemical studies of symmetrical and unsymmetrical dihydro-1,3,4-selenadiazoles", J. Chem. Soc., Perkin Trans. 1: 107–13, doi:10.1039/P19850000107
  13. ^ Sutter, Joan H.; Colquitt, Kevin; Sutter, John R. (1974), "Kinetics of the disproportionation of manganate in acid solution", Inorg. Chem., 13 (6): 1444–46, doi:10.1021/ic50136a037.
  14. ^ 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.
  15. ^ Brachtel, G.; Hoppe, R. (1976), "Das erste Oxomanganat(III) mit Inselstruktur: K6[Mn2O6]", Naturwissenschaften, 63 (7): 339, Bibcode:1976NW.....63..339B, doi:10.1007/BF00597313.