Modern inorganic chemistry

390 THE TRANSITION ELEMENTS

The oxide MnO is obtained by heating the carbonate MnCO 3.

Oxidation of manganese(II) in aqueous acid solution requires a

strong oxidising agent, for example

MnO; (aq) + 8H 3 O+ + 5e' -* Mn^2 +(aq) + 12H 2 O : £e = 1.52 V

MnO 2 (s) + 4H 3 O+ +2e~ ^Mn2+(aq)-h6H 2 O: £^+ 1.35 V

Thus, for example, peroxodisulphate(VI) will oxidise Mn(II) to

Mn(VII):

2Mn2+ + 5SO- + 8HO -» 2MnO + 4 16H+ 4-

However, the Mn(II) ion forms a variety of complexes in solution,

some of which may be more easily oxidised ; these complexes can

be either tetrahedral, for example [MnCl 4 ]^2 ", or octahedral, for

example [Mn(CN) 6 ]^4 '. Addition of ammonia to an aqueous solu-

tion of a manganese(II) salt precipitates Mn(OH) 2 ; reaction of

ammonia with anhydrous manganese(II) salts can yield the ion

[Mn(NH 3 ) 6 ]^2 +.

Low oxidation states

Manganese forms a decacarbonyl Mn 2 (CO) 10 in which each man-

ganese has the required share in 18 electrons to achieve the noble gas

configuration. Reduction of this covalent compound with sodium

amalgam gives the salt Na[Mn(CO) 5 ], sodium pentacarbonyl-

manganate ( - 1); in the ion Mn(CO)^ the noble gas structure is

again attained.

TESTS FOR MANGANESE

Fusion of a manganese compound with sodium carbonate and

potassium nitrate (on porcelain) gives a green manganate(YI) (p. 386).

TEST FOR MnO 4 ION

The purple colour of this ion alone is a sufficient test for its presence:
addition of sulphuric acid and hydrogen peroxide discharges the
colour.

TEST FOR Mn2+ IONS

If a manganese(II) salt is boiled with a strong oxidising agent such