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]

Structure of manganate

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]