Modern inorganic chemistry

THE TRANSITION ELEMENTS 403

orbitals occurs, and all these electrons are paired in a more stable

energy level (p. 366). Such an arrangement is stable with respect to

oxidation or reduction. "Appropriate' ligands are those containing

a nitrogen donor atom, for example ammonia NH 3 , cyanide CN"

gnd nitro — NO^, and cobalt has a strong affinity for all these. Thus

if cobalt(II) chloride is oxidised by air in presence of ammonia, with

ammonium chloride added to provide the required anion, the

orange hexamminocobalt(III) chloride is precipitated :

4[Co(H 2 O) 6 ]Cl 2 + 4NH 4 C1 + 20NH 3 + O 2

-* 4[Co(NH 3 ) 6 ]Cl 3 + 26H 2 O

For this reaction, charcoal is a catalyst; if this is omitted and

hydrogen peroxide is used as the oxidant, a red aquopentammino-

cobalt(III) chloride, [Co(NH 3 ) 5 H 2 O]Cl 3 , is formed and treatment

of this with concentrated hydrochloric acid gives the red chloro-

pentammino-cobalt(III) chloride, [Co(NH 3 ) 5 Cl]Cl 2. In these latter

two compounds, one ammonia ligand is replaced by one water

molecule or one chloride ion ; it is a peculiarity of cobalt that these

replacements are so easy and the pure products so readily isolated.

In the examples quoted, the complex cobalt(III) state is easily

obtained by oxidation of cobalt(II) in presence of ammonia, since

[Co(NH 3 ) 6 ]3+(aq) + <T -> [Co(NH 3 ) 6 ]^2 + (aq):£^ = +0.1 V

Cobalt(II) is also easily oxidised in the presence of the nitrite ion

NO 2 as ligand. Thus, if excess sodium nitrite is added to a cobalt(II)

salt in presence of ethanoic acid (a strong acid would decompose

the nitrite, p. 244), the following reaction occurs :

Co2+(aq) + 7NO 2 - + 2H+ -> NO + H 2 O + [Co(NO 2 ) 6 ]^3 -

Here, effectively, the Co2+(aq) is being oxidised by the nitrite ion

and the latter (in excess) is simultaneously acting as a ligand to form

the hexamtrocobaltate(III) anion. In presence of cyanide ion CN~.

cobalt(II) salts actually reduce water to hydrogen since

[Co(CN) 6 ]^3 -(aq) + <T -> [Co(CN) 5 (H 2 O)]^3 "(aq) + CN~ :

E^ - -0.8V

Oxidation state + 2

SALTS

In some respects these salts resemble those of iron; the aquo-cation

[Co(H 2 O) 6 ]2+ (pink) occurs in solution and in some solid salts, for