In inorganic nomenclature, a manganate is any negatively charged molecular entity with manganese as the central atom.[.] However, the name is usually used to refer to the tetraoxidomanganate(2−) anion, MnO, also known as manganate(VI) because it contains manganese in the +6 oxidation state. Manganates are the only known manganese(VI) compounds.[.]
Other manganates include hypomanganate or manganate(V), , permanganate or manganate(VII), , and the dimanganate or dimanganate(III) .
A manganate(IV) anion has been prepared by radiolysis of dilute solutions of permanganate.[.] It is mononuclear in dilute solution, and shows a strong absorption in the ultraviolet and a weaker absorption at 650 nm.
Structure
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.
[.] The
manganese–
oxygen distance is 165.9 pm, about 3 pm longer than in
permanganate.
As a d
1 ion, it is
Paramagnetism, but any Jahn–Teller distortion is too small to be detected by X-ray crystallography.
Manganates are dark green in colour, with a visible absorption maximum of
λmax = 606 nm (
ε = ).
[.] The Raman spectrum has also been reported.
Preparation
Sodium manganate 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
or with heating.
- + → + + 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.
- + + O2 → +
Disproportionation
Manganates are unstable towards disproportionation in all but the most alkaline of
.
The ultimate products are
permanganate and manganese dioxide, but the kinetics are complex and the mechanism may involve protonated and/or manganese(V) species.
[.][.]
Uses
Manganates, particularly the insoluble
barium manganate, BaMnO
4, have been used as
in organic synthesis: they will oxidize primary alcohols to
and then to
, and secondary alcohols to
.
[.][.] Barium manganate has also been used to oxidize
to
.
Related compounds
Manganate is formally the
conjugate base of hypothetical
manganic acid , which cannot be formed because of its rapid disproportionation. However, its second acid dissociation constant has been estimated by
pulse radiolysis techniques:
- HMnO MnO + H+ p Ka = 7.4 ± 0.1
Manganites
The name "manganite" is used for compounds formerly believed to contain the anion , with manganese in the +3 oxidation state. However, most of these "manganites" do not contain discrete
, but are
with perovskite (LaMn
IIIO
3, CaMn
IVO
3),
spinel (LiMnO
4) or
Halite structure (LiMn
IIIO
2, NaMn
IIIO
2) structures.
One exception is potassium dimanganate(III), K6Mn2O6, which contains discrete Mn2O anions.[.]