Modern inorganic chemistry

THE TRANSITION ELEMENTS 385

it differs from the preceding transition metals in having a very stable

4- 2 oxidation state, corresponding to loss of only the 4s^2 electrons,

and indicative of the stability of the half-filled d levels.

Oxidation state + 7

Apart from two unstable oxide halides, MnO 3 F and MnO 3 Cl,

this state is exclusively represented by the oxide Mn 2 O 7 and the

anion

MANGANESE(VII) OXIDE, DIMANGANESE HEPTOXIDE, Mn 2 O 7

This oxide is obtained by adding potassium manganate(VII) to
concentrated sulphuric acid, when it appears as a dark coloured oil
which readily decomposes (explosively on heating) to manganese(IV)
oxide and oxygen:
2KMnO 4 + 2H 2 SO 4 -> Mn 2 O 7 + 2KHSO 4 + H 2 O
2Mn 2 O 7 -» 4MnO 2 + 3O 2

It is a powerful and violent oxidising agent. It dissolves in water,
and manganic(VII) acid (permanganic acid) HMnO 4 and its
dihydrate HMnO 4 .2H 2 O can be isolated as purple solids by low
temperature evaporation of the frozen solution. Manganic(YII)
acid is also a violent oxidising agent, especially with any organic
material; it decomposes quickly at 276 K.

THE MANGANATES(VII)

The purple manganate(VII) or permanganate anion, MnO^ is
tetrahedral; it owes its intense colour to charge transfer (since the
manganese has no d electrons). The potassium salt KMnO 4 is the
usual form, but many other cations from soluble manganate(VII)
salts (all purple); those with large unipositive cations (for example
Cs"^) are less soluble. Potassium manganate(VII) can be prepared
by (a) electrolytic oxidation of manganese metal (oxidation from
0 to +7) using a manganese anode in potassium carbonate solution,
(b) oxidation of manganate(II) (oxidation + 2 to + 7), using the
peroxodisulphate ion S 2 Og~* and a manganese(II) salt, and (c)

* This ion oxidises thus:

S 2 O^ ~ + 2e~ -+ 2SOi ~ : £^ = 2.0 V