Manganate

{{Short description|Chemical compound}} thumb|150px|Structure of manganate In inorganic nomenclature, a '''manganate''' is any negatively charged molecular entity with manganese as the central atom.<ref name="RedBook">{{RedBook2005|pages=74–75, 77–78, 313, 338}}.</ref> However, the name is usually used to refer to the '''tetraoxidomanganate(2−)''' anion, MnO{{su|b=4|p=2−}}, also known as '''manganate(VI)''' because it contains manganese in the +6 oxidation state.<ref name="RedBook"/> Manganates are the only known manganese(VI) compounds.<ref name="C&W">{{Cotton&Wilkinson4th|page=746}}.</ref>

Other manganates include hypomanganate or manganate(V), {{chem|MnO|4|3-}}, permanganate or manganate(VII), {{chem|MnO|4|-}}, and the dimanganate or dimanganate(III) {{chem|Mn|2|O|6|6-}}.

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== 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 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 d¹ ion it is paramagnetic and should exhibit Jahn–Teller distortion, but any deviation from the tetrahedral geometry 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''_max&nbsp;= 606&nbsp;nm (''ε'' = {{nowrap|1710&nbsp;dm³ mol⁻¹ cm⁻¹}}).<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 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== Sodium and potassium manganates 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⁻}} → {{nowrap|4 MnO{{su|b=4|p=2−}}}} + {{nowrap|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.<ref name="C&W"/> :{{nowrap|2 MnO₂}} + {{nowrap|4 OH⁻}} + O₂ → {{nowrap|2 MnO{{su|b=4|p=2−}}}} + {{nowrap|2 H₂O}}

==Disproportionation== Manganates are unstable towards disproportionation in all but the most alkaline of aqueous solutions.<ref name="C&W"/> The ultimate products are permanganate and manganese dioxide, but the 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 MnO₄⁻/MnO₄²⁻ and MnO₄²⁻/MnO₄³⁻ 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, 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.<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 | isbn = 9780471936237 | 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 hydrazones to diazo compounds.<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== Manganate is formally the conjugate base of hypothetical manganic acid {{chem|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:<ref name="PulseRad">{{citation | title = Studies of Manganate(V), -(VI), and -(VII) Tetraoxyanions by Pulse Radiolysis. Optical Spectra of Protonated Forms | first1 = J. D. | last1 = Rush | first2 = B. H. J. | last2 = Bielski | journal = Inorg. Chem. | year = 1995 | volume = 34 | issue = 23 | pages = 5832–38 | doi = 10.1021/ic00127a022}}</ref> :HMnO{{su|b=4|p=−}} {{eqm}} MnO{{su|b=4|p=2−}} + H⁺&nbsp;&nbsp;&nbsp;p''K''_a = 7.4 ± 0.1

===Manganites=== The name "manganite" is used for compounds formerly believed to contain the anion {{chem|MnO|3|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{{su|b=2|p=III,IV}}O₄) or sodium chloride (LiMn^IIIO₂, NaMn^IIIO₂) structures.

One exception is potassium dimanganate(III), K₆Mn₂O₆, which contains discrete Mn₂O{{su|b=6|p=6−}} anions.<ref name="Brachtel">{{citation | title = Das erste Oxomanganat(III) mit Inselstruktur: K₆[Mn₂O₆&#93; | journal = Naturwissenschaften | volume = 63 | issue = 7 | year = 1976 | doi = 10.1007/BF00597313 | page = 339 | first1 = G. | last1 = Brachtel | first2 = R. | last2 = Hoppe|bibcode = 1976NW.....63..339B | s2cid = 45344152 }}.</ref>

==References== {{reflist|30em}}

Category:Manganates Category:Transition metal oxyanions Category:Oxometallates