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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 dimanganate or dimanganate(III) Mn
₂O⁶⁻
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]

Structure

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 Eilhard Mitscherlich in 1831.^[5] The manganese–oxygen 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 λ_max = 606 nm (ε = 1710 dm³ mol⁻¹ cm⁻¹).^[6]^[7] The Raman spectrum has also been reported.^[8]

Preparation

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 OH⁻ → 4 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⁻ + O₂ → 2 MnO²⁻
₄ + 2 H₂O

Disproportionation

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.^[10]^[11]

Uses

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.^[12]^[13] Barium manganate has also been used to oxidize hydrazones to diazo compounds.^[14]

Related compounds

Manganate is formally the conjugate base of hypothetical manganic acid H
₂MnO
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⁺ pK_a = 7.4 ± 0.1

Manganites

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 oxyanions, but are mixed oxides with perovskite (LaMn^IIIO₃, CaMn^IVO₃), spinel (LiMn^III,IV
₂O₄) or sodium chloride (LiMn^IIIO₂, NaMn^IIIO₂) structures.

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

References

^ ^a ^b International Union of Pure and Applied Chemistry (2005). Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005). Cambridge (UK): RSC–IUPAC. ISBN 0-85404-438-8. pp. 74–75, 77–78, 313, 338. Electronic version..
^ ^a ^b ^c Cotton, F. Albert; Wilkinson, Geoffrey (1980), Advanced Inorganic Chemistry (4th ed.), New York: Wiley, p. 746, ISBN 0-471-02775-8.
^ ^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
^ 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.
^ ^a ^b ^c Palenik, Gus J. (1967), "Crystal structure of potassium manganate", Inorg. Chem., 6 (3): 507–11, doi:10.1021/ic50049a016.
^ ^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
^ 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.
^ Juberta, A. H.; Varettia, E. L. (1982), "Normal and resonance Raman spectra of some manganates", J. Mol. Struct., 79 (1–2): 285–88, Bibcode:1982JMoSt..79..285J, doi:10.1016/0022-2860(82)85067-9
^ Nyholm, R. S.; Woolliams, P. R. (1968), "Manganates(VI)", Inorg. Synth., 11: 56–61
^ 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.
^ 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.
^ Procter, G.; Ley, S. V.; Castle, G. H. (2004), "Barium Manganate", in Paquette, L. (ed.), Encyclopedia of Reagents for Organic Synthesis, New York: Wiley, doi:10.1002/047084289X, hdl:10261/236866, ISBN 9780471936237.
^ 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.
^ 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
^ 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, S2CID 45344152.