402 THE TRANSITION ELEMENTS
chloride*, and Werner pioneered the study of coordination com-
pounds. We shall consider a few of the reactions investigated by
Werner later in this chapter.
COMPOUNDS OF COBALT
Oxidation state -f 3
As already noted, the simple salts in this oxidation state are powerful
oxidising agents and oxidise water. Since, also, Co(III) would
oxidise any halide except fluoride to halogen, the only simple halide
salt is CoF 3 , Cobalt(IlI) fluoride, obtained by reaction of fluorine
with cobalt(II) fluoride; it is a useful fluorinating agent.
Cobalt(III) oxide is obtained as a brown precipitate Co 2 O 3 .aq
when cobalt(II) hydroxide is oxidised in alkaline conditions (or
when a cobalt(III) is decomposed by aqueous alkali). On heating it
gives the black mixed oxide Co 3 O 4.
Hydrated cobalt(III) sulphate, Co 2 (SO 4 ) 3 .18H 2 O is obtained when
cobalt(II) sulphate is oxidised electrolytically in moderately con-
centrated sulphuric acid solution: it is stable when dry but liberates
oxygen from water. Some alums, for example KCo(SO 4 ) 2 .12H,O
can be obtained by crystallisation from sulphuric acid solutions. In
these and the sulphate, the cation [Co(H 2 O) 6 ]^34 ^ may exist; it is
both acidic and strongly oxidising.
Cobalt(III) nitrate Co(NO 3 ) 3 has been prepared by the reaction of
dinitrogen pentoxide with cobalt(III) fluoride.
COMPLEXES
Cobalt(III) contains six 3d electrons; in the presence of six appro-
priate ligands, arranged octahedrally, a large splitting of the d
The structure is octahedral, i.e.
3CL"
