Modern inorganic chemistry

THE TRANSITION ELEMENTS 387

jn concentrated alkali, manganese(VI) is more stable than mangan

ese(VII) and the following reaction occurs:

4MnOj + 4OH~ -> 4MnO4~ + 2H 2 O -f O 2

(cf. the reverse reaction with chlorine, above).

Oxidation state + 6

This is only found in the green manganate( VI) ion, already described.

It is only stable in alkaline conditions; in neutral or acid solution

it disproportionates:

3MnO|~ + 2H 2 O -* MnO 2 + 2MnO4 + 4OH~

+ 6 +4 +7

Oxidation state + 5

This state exists as a manganate(V), the blue MnO^~ ; Na 3 MnO 4.

10H 2 O is isomorphous with Na 3 VO 4 (p. 374).

Oxidation state + 4

MANGANESE(IV) OXIDE, MnO 2

Manganese(IV) oxide is the only familiar example of this oxidation
state. It occurs naturally as pyrolusite, but can be prepared in an
anhydrous form by strong heating of manganese(II) nitrate:

Mnn(NO 3 ) 2 -> MnO 2 4- 2NO 2 t
It can also be precipitated in a hydrated form by the oxidation of
a manganese(II) salt, by, for example, a peroxodisulphate:

Mn2+ + S 2 Ol" + 2H 2 O -> 2SO|~ + MnO 2 l + 4H +
Manganese(IV) oxide is a dark-brown solid, insoluble in water
and dilute acids. Its catalytic decomposition of potassium chlor-
ate^) and hydrogen peroxide has already been mentioned. It
dissolves slowly in alkalis to form manganates(IV), but the constitu-
tion of these is uncertain. It dissolves in ice-cold concentrated hydro-
chloric acid forming the complex octahedral hexachloromangan-
ate(IV) ion:

MnO 2 + 6HC1 -> [MnlvCl 6 ]^2 ~ + 2H+ + 2H 2 O