Why is Mn3 + a good oxidizer

VII. Subgroup / 7. Group / manganese group

Manganese is the second most common transition metal (after iron) with a frequency of 0.1% in the earth's crust.
The most important manganese minerals are the oxides hausmannite (Mn3O4, Fig. 9.7.1.), Braunite (Mn2O3) and pyrolusite (MnO2, 'Braunstein', Fig. 9.7.2.).
9.7.1. Hausmannite, Mn3O4 (Octahedron, spinel structure) 9.7.2. Pyrolusite, MnO2 (Gremmelsbach, Triberg, Black Forest)
Manganese Vesuvian (see Fig. 7.X.) is a mixed ortho / disilicate, the Mn2+ and Mn3+ on the cation sites (basic formula: approx10(Mg, Fe)2Al4[Si2O7]2[SiO4] 5 (OH)4). 'Manganese nodules' (see Fig. 9.7.3.) Can be found at great depths (5000 m) in the sea and contain up to 30% manganese in the form of various Mn (II / III / IV) oxides / hydroxides or hydrous other minerals (e.g. buserite or birnessite). These tubers grow very slowly, but are currently also being discussed as raw materials.
9.7.3. Manganese tuber (USA, 1,000 miles southeast of Hawaii, from a depth of 5,000 m)
Further manganese minerals are manganite (MnO (OH)) and rhodochrosite ('Manganspat', MnCO3). The production of elementary metallic manganese can e.g. by the aluminothermic reaction of hausmannite according to
3 mn3O4 + 6 Al ⟶ 9 Mn + 4 Al2O3
be performed. Ferromanganese, an Mn-rich (approx. 70%) iron alloy, can also be produced technically by reduction with carbon directly in the blast furnace.
Elemental manganese is one of the few metallic elements that shows its own unusual and very complex crystal chemistry. Two of the three modifications have rather complicated cubic structures, which are described in detail here.
Fig. 9.7.4. elemental manganese
Manganese itself is relatively ignoble (Mn2+/ Mn0: -1.18 V) and is soluble in non-oxidizing acids:
Mn + 2 HCl ⟶ MnCl2 + H2
Elemental manganese burns to Mn in air3O4. Manganese melts at 1247 OC and is a very important alloying partner in steels.
Four oxidation states of manganese can be seen in the gradual reduction of permanganate (manganate (VII) with perborate:

Reduction of permanganate
With perborate (NaBO2 . H2O2 . 3 H.2O) permanganate can be gradually reduced:
MnVIIO4- (purple) ⟶ MnVIO42- (green) ⟶ MnV.O43- (blue) ⟶ MnIVO2 (brown)

The thermodynamic stability of the various oxidation states, which, like the species itself, depends heavily on the pH value of the solution, is shown in the Frost diagram in Fig. 9.7.5. (left). In the Pourbaix diagram (see Fig. 9.7.5. Right, for c= 1 mol / l) the species that are stable at the respective potentials and pH values ​​are also shown. (! Attention: these diagrams depend on the concentrations of the solutions and are based on thermodynamic data!)

9.7.5. 'Frost' diagram of Mn at pH 0 (red) and pH 14 (blue) (left) ‣SVG and 'Pourbaix' diagram (right) ‣SVG

It is clear from the two diagrams that Mn is present in low oxidation states under basic conditions in the form of the sparingly soluble oxides / hydroxides. Only at extreme pH values ​​does Mn (II) dissolve in the form of a hydroxido complex (green field in the Pourbaix diagram). Compounds with manganese in higher oxidation states have a strong oxidizing effect in acid. Under basic conditions (see experiment above) all oxidation states from Mn (VII) to Mn (IV) are available, while the light blue manganate (V) is unknown in acidic solutions. In the acidic Mileu, Mn (II) in particular (in the form of the hexaqua complex) is very stable, while under basic conditions Mn (IV) in the form of manganese dioxide is the most stable species, and thus mostly the end product of redox reactions (see analytics, e.g. at manganometry).

The + II oxidation state is relatively stable with manganese, since all five are here d-States are simply occupied (d5-Electronic configuration). Transitions between the d-Levels are thus spin-forbidden, so that d-dTransitions do not play a role for the colors of the salts and complexes and many Mn (II) compounds only show a very pale color (mostly slightly pink) at most.
  • The hydroxide precipitates out of Mn (II) -containing solutions when ammonia is added (see analysis!):
    MnSO4 + 6 NH3 ⟶ Mn (OH)2 + Well2SO4
    which is insoluble in alkalis.
  • The anhydride for this is the monoxide MnO, which has a NaCl structure.
  • Manganese (II) forms with NH3 a hexammine complex:
    Mn2+ + 6 NH3 ⟶ [Mn (NH3)6]2+
  • With H2O2 oxidation to Mn (IV) produces brown-black precipitates of the oxide-hydroxide:
    MnII(OH)2 + H2O2 ⟶ MnIVO (OH)2
  • Other poorly soluble compounds of Mn (II) are the carbonate and the pale pink (see analysis) colored sulfide:
    Mn2+ + CO32- ⟶ MnCO3
    Mn2+ + S2- ⟶ MnS
    In terms of color, this is NEVER the sulphide in life!
Mn (III) is only known in solid compounds, occurs in solution - regardless of the pH value, see Frost diagram in Fig. 9.7.5. left - disproportionation to MnII and MnIV.
  • Well-known oxygen compounds and minerals with manganese in the + III oxidation state are Braunite, Mn2O3, and the mixed valence hausmannite (Mn3O4, see Fig. 9.7.1).
    • The hausmannite crystallizes in the spinel structure and contains a MnII- and two mnIIIIons per formula unit. It is a normal spinel, the octahedron is MnIII Jahn-Teller are distorted.
    • α-Mn2O3 crystallizes in the C-type structure (also Bixbyit or defect anti-CaF2-Type). Here the Mn2+-Cations form a cubic closest packing of spheres and the oxide ions occupy 3/4 of the tetrahedral holes (cubic, space group Ia3, with magnetic order pseudo-cubic Pbca).
    • The black γ form of Mn2O3 is isotypic to γ-Al2O3 and γ-Fe2O3 (Defect spinel) and, like this, is created when the hydroxides are carefully dehydrated.
  • Other compounds are MnO (OH) (manganite) and complexes such as [MnF6]3-.
  • The pigment 'Manganese Violet' is an ammonium Mn (III) pyrophosphate NH4Mn [P2O7] (see Fig. 9.7.6.). It is used in cosmetics and painting.
    9.7.6. Mn violet: NH4Mn [P2O7]
Mn (IV) comes as MnO2 in the pyrolusite (see Fig. 9.7.2.). "Brown stones" are mixtures of various precipitates, including those containing water and / or cations. Strictly speaking, it is a non-stoichiometric compound that is better than MnO2-x (Rutile structure, up x= 0.05) has to be formulated.
MnO2 is also a very strong oxidizing agent (see analytics, see Frost diagram in Fig. 9.7.5) and can, for example, release chlorine from HCl:
MnO2 + 4 HCl ⟶ MnCl2 + Cl2
MnO2 is amphoteric. With acids (like metals) it forms hydrated cations Mn4+, Manganates MnO are formed with alkalis44- or MnO32-. MnV. exists as MnO43- only in the alkaline (see video and frost diagram). At other pH values, disproportionation occurs. The ion is light blue. MnVI is stable in an alkaline medium in the form of the manganate (VI) ion MnO42-, on the other hand, under acidic conditions MnVII in the form of the well-known permanganate ion MnO4- in front. Manganates (VI) arise (see analytics) in oxidizing melt reactions (oxidation melt):
MnSO4 + 2 K2CO3 + 2 KNO3 ⟶ K2MnO4 + 2 KNO2
The manganate (VI) ion is green in solution. In the case of acidification, it is dispropotionated in MnVII and MnII via (see Frost diagram):
5 mnVIO42- + 8 H.+ ⟶ 4 mnVIIO4- + Mn2+ + 4 H.2O
The ion Mn is built into the barite structureVIO42- durable and the result is a pigment called 'Mn blue' (see Fig. 9.7.7.).
9.7.7. Mn blue: BaSO4: MnVI
Peroxosulphuric acid (with Ag+ as a catalyst), nitric acid or PbO2 can Mn2+ oxidize to permanganate in acidic solution, e.g.
2 mn2+ + 5 H.2S.2O8 + 8 H.2O ⟶ 2 MnO4- (purple) + 10 H.2SO4 + 6 H.+
Also Braunstein, MnO2 can be used with strong oxidizing agents such as Cl2, O3 or can be oxidized electrolytically to form permanganate.
The permanganate ion is itself a strong oxidizing agent, which under alkaline conditions turns into manganese dioxide MnO2, in acidic conditions to the Mn2+ is reduced. For example, sulphurous acid can be oxidized to sulfuric acid and iodide to iodine. Synproportionation reactions of Mn (VII) and Mn (II) lead to Mn (IV):
2 mnVIIO4- + 3 mn2+ + 4 OH- + 3 H.2O ⟶ 5 mnIVO (OH)2 (Brown black)
The anhydride of the associated acid HMnO4 is Mn2O7, is also strongly oxidizing and a volatile oil that breaks down when it explodes.
2 mn2O7 ⟶ 4 MnO2 + 3 O2
You can see this reaction when you try 'Sturm im Wasserglas' (also 'Mn lightning'):
2 KMnO4 + H2SO4 ⟶ K2SO4 + Mn2O7 + H2O

Mn flashes
Small crystals of potassium permanganate are concentrated on the interface between. Sulfuric acid (below) and absolute ethanol (above). The resulting green oil is Mn2O7, the anhydride of permanganic acid, which reacts with explosive decomposition.


Radio element (artificial element, produced during nuclear fission). Chemically very similar to rhenium.
Rhenium comes together with molybdenum in molybdenite MoS2 in front. It is represented by the reduction of the oxides or sulfides with hydrogen or electrolytically. It crystallizes in a hexagonal close packing of spheres and is used in alloys for thermocouples and filaments (because of its high melting point). The +7 oxidation state is fairly stable. re2O7 can be distilled from an oxidizing medium, i.e. it is largely covalent. The oxide ReO3, the namesake of a very simple and basic structure type, is red. Oxides, sulfides and halides are also well characterized by oxidation states IV, V and VI, but less stable than the above-mentioned oxides. ReCl3 contains clusters with a Re3-Triangle, [Re2Cl8]2- the famous Re-Re quadruple bond.
Typical reactions with perrhenates, [ReO4]- are:
2 ReO4- + 7 H.2S + 2 H.+ ⟶ Re2S.7 (black) + 8 H.2O
ReO4- + [(C.6H5)4As]+ ⟶ [(C6H5)4As] ReO4
Perrhenates show the characteristic reactions of all [XO4]--Ions.