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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 dimanganite 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 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 oxoanions, 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.
^ 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{{citation}}: CS1 maint: multiple names: authors list (link)
^ 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.

[RedBook-1] 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..

[C&W-2] Cotton, F. Albert; Wilkinson, Geoffrey (1980), Advanced Inorganic Chemistry (4th ed.), New York: Wiley, p. 746, ISBN 0-471-02775-8.

[PulseRad-3] 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.

[H2O2-4] 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.

[Xstal-5] Palenik, Gus J. (1967), "Crystal structure of potassium manganate", Inorg. Chem., 6 (3): 507–11, doi:10.1021/ic50049a016.

[JCS-6] 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 (1–2): 285–88, Bibcode:1982JMoSt..79..285J, doi:10.1016/0022-2860(82)85067-9

[9] Nyholm, R. S.; Woolliams, P. R. (1968), "Manganates(VI)", Inorg. Synth., 11: 56–61

[10] 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.

[Epot-11] 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.

[12] 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.

[13] 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.

[14] 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{{citation}}: CS1 maint: multiple names: authors list (link)

[Brachtel-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.

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[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

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[14]

[15]

@@ Line 7: / Line 7: @@
 A manganate(IV) anion {{chem|Mn|O|4|4-}} has been prepared by [[radiolysis]] of dilute solutions of [[permanganate]].<ref name="PulseRad"/><ref name="H2O2">{{citation | title = Oxidation of hydrocarbons. 18. Mechanism of the reaction between permanganate and carbon-carbon double bonds | first1 = Donald G. | last1 = Lee | first2 = Tao | last2 = Chen | journal = J. Am. Chem. Soc. | year = 1989 | volume = 111 | issue = 19 | pages = 7534–38 | doi =  10.1021/ja00201a039}}.</ref> It is mononuclear in dilute solution, and shows a strong absorption in the ultraviolet and a weaker absorption at 650&nbsp;nm.<ref name="PulseRad"/>
-==Structure and properties==
+==Structure==
 [[File:Manganate.jpg|thumb|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|Mitscherlich]] in 1831.<ref name="Xstal">{{citation | title = Crystal structure of potassium manganate | first = Gus J. | last = Palenik | journal = Inorg. Chem. | year = 1967 | volume = 6 | issue = 3 | pages = 507–11 | doi = 10.1021/ic50049a016}}.</ref> The [[manganese]]–[[oxygen]] distance is 165.9&nbsp;pm, about 3&nbsp;pm longer than in [[permanganate]].<ref name="Xstal"/> As a d<sup>1</sup> ion, it is [[Paramagnetism|paramagnetic]], but any [[Jahn–Teller effect|Jahn–Teller distortion]] is too small to be detected by [[X-ray crystallography]].<ref name="Xstal"/> Manganates are dark green in colour, with a visible absorption maximum of ''λ''<sub>max</sub>&nbsp;= 606&nbsp;nm (''ε'' = {{nowrap|1710&nbsp;dm<sup>3</sup> mol<sup>−1</sup> cm<sup>−1</sup>}}).<ref name="JCS">{{citation | journal = J. Chem. Soc. | year = 1956 | pages = 3373–80 | doi =   10.1039/JR9560003373 | title = Structure and reactivity of the oxy-anions of transition metals. Part I. The manganese oxy-anions | first1 = A. | last1 = Carrington | first2 = M. C. R. | last2 = Symons}}</ref><ref name="Mandelate">{{citation | title = 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 | first1 = Donald G. | last1 = Lee | first2 = Tao | last2 = Chen | journal = J. Am. Chem. Soc. | year = 1993 | volume = 115 | issue = 24 | pages = 11231–36 | doi = 10.1021/ja00077a023}}.</ref> The [[Raman spectroscopy|Raman spectrum]] has also been reported.<ref>{{citation | journal = J. Mol. Struct. | volume = 79 | issue = 1–2 | year = 1982 | pages = 285–88 | doi = 10.1016/0022-2860(82)85067-9 | title = Normal and resonance Raman spectra of some manganates | first1 = A. H. | last1 = Juberta | first2 = E. L. | last2 = Varettia|bibcode = 1982JMoSt..79..285J }}</ref>
+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|Mitscherlich]] in 1831.<ref name="Xstal">{{citation | title = Crystal structure of potassium manganate | first = Gus J. | last = Palenik | journal = Inorg. Chem. | year = 1967 | volume = 6 | issue = 3 | pages = 507–11 | doi = 10.1021/ic50049a016}}.</ref> The [[manganese]]–[[oxygen]] distance is 165.9&nbsp;pm, about 3&nbsp;pm longer than in [[permanganate]].<ref name="Xstal"/> As a d<sup>1</sup> ion, it is [[Paramagnetism|paramagnetic]], but any [[Jahn–Teller effect|Jahn–Teller distortion]] is too small to be detected by [[X-ray crystallography]].<ref name="Xstal"/> Manganates are dark green in colour, with a visible absorption maximum of ''λ''<sub>max</sub>&nbsp;= 606&nbsp;nm (''ε'' = {{nowrap|1710&nbsp;dm<sup>3</sup> mol<sup>−1</sup> cm<sup>−1</sup>}}).<ref name="JCS">{{citation | journal = J. Chem. Soc. | year = 1956 | pages = 3373–80 | doi =   10.1039/JR9560003373 | title = Structure and reactivity of the oxy-anions of transition metals. Part I. The manganese oxy-anions | first1 = A. | last1 = Carrington | first2 = M. C. R. | last2 = Symons}}</ref><ref>{{citation | title = 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 | first1 = Donald G. | last1 = Lee | first2 = Tao | last2 = Chen | journal = J. Am. Chem. Soc. | year = 1993 | volume = 115 | issue = 24 | pages = 11231–36 | doi = 10.1021/ja00077a023}}.</ref> The [[Raman spectroscopy|Raman spectrum]] has also been reported.<ref>{{citation | journal = J. Mol. Struct. | volume = 79 | issue = 1–2 | year = 1982 | pages = 285–88 | doi = 10.1016/0022-2860(82)85067-9 | title = Normal and resonance Raman spectra of some manganates | first1 = A. H. | last1 = Juberta | first2 = E. L. | last2 = Varettia|bibcode = 1982JMoSt..79..285J }}</ref>
-===Preparation===
+==Preparation==
 [[Sodium manganate|Sodium]] and [[potassium manganate]]s are usually prepared in the laboratory by stirring the equivalent [[permanganate]] in a concentrated solution (5–10&nbsp;M) of the [[hydroxide]] for 24&nbsp;hours<ref name="JCS"/> or with heating.<ref>{{citation | authorlink1 = Ronald S. Nyholm | last1 = Nyholm | first1 = R. S. | last2 = Woolliams | first2 = P. R. | title = Manganates(VI) | journal = Inorg. Synth. | volume = 11 | year = 1968 | pages = 56–61}}</ref>
 :{{nowrap|4 MnO{{su|b=4|p=−}}}} + {{nowrap|4 OH<sup>−</sup>}} → {{nowrap|4 MnO{{su|b=4|p=2−}}}} + {{nowrap|2 H<sub>2</sub>O}} + O<sub>2</sub>
@@ Line 18: / Line 18: @@
 :{{nowrap|2 MnO<sub>2</sub>}} + {{nowrap|4 OH<sup>−</sup>}} + O<sub>2</sub> → {{nowrap|2 MnO{{su|b=4|p=2−}}}} + {{nowrap|2 H<sub>2</sub>O}}
+==Disproportionation==
-===Uses===
-Manganates, particularly the insoluble [[barium manganate]], BaMnO<sub>4</sub>, have been used as [[oxidizing agent]]s in [[organic synthesis]]: they will oxidize primary [[alcohol]]s to [[aldehyde]]s and then to [[carboxylic acid]]s, and secondary alcohols to [[ketone]]s.<ref>{{citation | first1 = G. | last1 = Procter | first2 = S. V. | last2 = Ley | first3 = G. H. | last3 = Castle | contribution = Barium Manganate | title = Encyclopedia of Reagents for Organic Synthesis | editor-first = L. | editor-last = Paquette | year = 2004 | publisher = Wiley | location = New York | doi = 10.1002/047084289X | hdl = 10261/236866 | hdl-access = free }}.</ref><ref>{{citation | title = Barium Manganate. A Versatile Oxidant in Organic Synthesis | first1 = Habib | last1 = Firouzabadi | first2 = Zohreh | last2 = Mostafavipoor | journal = Bull. Chem. Soc. Jpn. | volume = 56 | issue = 3 | year = 1983 | pages = 914–17 | doi = 10.1246/bcsj.56.914| doi-access = free }}.</ref> Barium manganate has also been used to oxidize [[hydrazone]]s to [[diazo compound]]s.<ref>{{citation | last1 = Guziec | first1 = Frank S., Jr. | last2 = Murphy | first2 = Christopher J. | last3 = Cullen | first3 = Edward R. | title = Thermal and photochemical studies of symmetrical and unsymmetrical dihydro-1,3,4-selenadiazoles | journal = J. Chem. Soc., Perkin Trans. 1 | year = 1985 | pages = 107–13 | doi = 10.1039/P19850000107}}</ref>
-===Disproportionation===
 Manganates are unstable towards [[disproportionation]] in all but the most alkaline of [[aqueous solution]]s.<ref name="C&W"/> The ultimate products are [[permanganate]] and [[manganese dioxide]], but the [[Chemical kinetics|kinetics]] are complex and the mechanism may involve protonated and/or manganese(V) species.<ref>{{citation | title = Kinetics of the disproportionation of manganate in acid solution | first1 = Joan H. | last1 = Sutter | first2 = Kevin | last2 = Colquitt | first3 = John R. | last3 = Sutter | journal = Inorg. Chem. | year = 1974 | volume = 13 | issue = 6 | pages = 1444–46 | doi = 10.1021/ic50136a037}}.</ref><ref name="Epot">{{citation | title = Rate of the MnO<sub>4</sub><sup>−</sup>/MnO<sub>4</sub><sup>2−</sup> and MnO<sub>4</sub><sup>2−</sup>/MnO<sub>4</sub><sup>3−</sup> electrode reactions in alkaline solutions at solid electrodes | first1 = K. | last1 = Sekula-Brzezińska | first2 = P. K. | last2 = Wrona | first3 = Z. | last3 = Galus | journal = Electrochim. Acta | year = 1979 | volume = 24 | issue = 5 | pages = 555–63 | doi = 10.1016/0013-4686(79)85032-X}}.</ref>
+==Uses==
+Manganates, particularly the insoluble [[barium manganate]], BaMnO<sub>4</sub>, have been used as [[oxidizing agent]]s in [[organic synthesis]]: they will oxidize primary [[alcohol]]s to [[aldehyde]]s and then to [[carboxylic acid]]s, and secondary alcohols to [[ketone]]s.<ref>{{citation | first1 = G. | last1 = Procter | first2 = S. V. | last2 = Ley | first3 = G. H. | last3 = Castle | contribution = Barium Manganate | title = Encyclopedia of Reagents for Organic Synthesis | editor-first = L. | editor-last = Paquette | year = 2004 | publisher = Wiley | location = New York | doi = 10.1002/047084289X | hdl = 10261/236866 | hdl-access = free}}.</ref><ref>{{citation | title = Barium Manganate. A Versatile Oxidant in Organic Synthesis | first1 = Habib | last1 = Firouzabadi | first2 = Zohreh | last2 = Mostafavipoor | journal = Bull. Chem. Soc. Jpn. | volume = 56 | issue = 3 | year = 1983 | pages = 914–17 | doi = 10.1246/bcsj.56.914| doi-access = free}}.</ref> Barium manganate has also been used to oxidize [[hydrazone]]s to [[diazo compound]]s.<ref>{{citation | last1 = Guziec | first1 = Frank S., Jr. | last2 = Murphy | first2 = Christopher J. | last3 = Cullen | first3 = Edward R. | title = Thermal and photochemical studies of symmetrical and unsymmetrical dihydro-1,3,4-selenadiazoles | journal = J. Chem. Soc., Perkin Trans. 1 | year = 1985 | pages = 107–13 | doi = 10.1039/P19850000107}}</ref>
 ==Related compounds==