In chemistry, hypomanganate, also called manganate(V) or tetraoxidomanganate(3−), is a trivalent anion (negative ion) composed of manganese and oxygen, with formula MnO3−4.
[3] The anion is a bright blue species[1] with a visible absorption maximum at wavelength λmax = 670 nm (ε = 900 dm3 mol−1 cm−1).
[8][9] Hypomanganate is unstable towards disproportionation to manganate(VI) and manganese dioxide:[10][1] The estimated electrode potentials at pH 14 are:[11][12][13] However, the reaction is slow in very alkaline solutions (with OH− concentration above 5–10 mol/L).
[1][7] The disproportionation is believed to pass through a protonated intermediate,[13] with the acid dissociation constant for the reaction HMnO2−4 ⇌ MnO3−4 + H+ being estimated as pKa = 13.7 ± 0.2.
[10][15] Hypomanganates may be prepared by the careful reduction of manganates with sulfite,[1] hydrogen peroxide[16] or mandelate.