Potassium hypomanganate

Potassium hypomanganate, K₃MnO₄, also known as potassium manganate(V), is a bright blue salt and a rare example of a manganese(V) compound. It is formed:

by the reduction of potassium permanganate with excess potassium sulfite;

by the reduction of potassium manganate with hydrogen peroxide in 10 M potassium hydroxide solution;

by the reduction of potassium manganate with mandelate in 3–10 M potassium hydroxide solution;

by disproportionation when manganese dioxide is dissolved in a concentrated solution of potassium hydroxide;

The hypomanganate anion is unstable with respect to disproportionation in all but the most alkaline of solutions: estimated electrode potentials at pH 14 are

MnO2−
4 + e⁻ ⇌ MnO3−
4   E = +0.27 V MnO3−
4 + e⁻ + 2 H₂O ⇌ MnO₂ + 4 OH⁻   E = +0.96 V

The disproportionation is believed to pass through a protonated intermediate, with the acid dissociation constant for the reaction HMnO2−
4 ⇌ MnO3−
4 + H⁺ being estimated as pK_a = 13.7 ± 0.2. However, K₃MnO₄ has been cocrystallized with Ca₂Cl(PO₄), allowing the study of the UV–visible spectrum of the hypomanganate ion.