Modern inorganic chemistry

THE TRANSITION ELEMENTS 401

COBALT

THE ELEMENT

Cobalt compounds have been in use for centuries, notably as
pigments ('cobalt blue') in glass and porcelain (a double silicate of
cobalt and potassium); the metal itself has been produced on an
industrial scale only during the twentieth century. Cobalt is rela-
tively uncommon but widely distributed; it occurs biologically in
vitamin B 12 (a complex of cobalt(III) in which the cobalt is bonded
octahedrally to nitrogen atoms and the carbon atom of a CN"
group). In its ores, it is usually in combination with sulphur or
arsenic, and other metals, notably copper and silver, are often
present. Extraction is carried out by a process essentially similar to
that used for iron, but is complicated because of the need to remove
arsenic and other metals.
Cobalt is a bluish silvery metal, exhibits ferromagnetism, and can
exist in more than one crystal form; it is used in alloys for special
purposes. Chemically it is somewhat similar to iron; when heated
in air it gives the oxides Co 3 O 4 and CoO, but it is less readily
attacked by dilute acids. With halogens, the cobalt(II) halides are
formed, except that with fluorine the (III) fluoride, CoF 3 , is obtained.
Like iron and the next transition element, nickel, cobalt is not
generally found in any oxidation state above + 3, and this and + 2
are the usual states. The simple compounds of cobalt(III) are
strongly oxidising:

[Co(H 2 O) 6 ]3+ + <?--> [Co(H 2 O) 6 ]^2 + :E^ = +1.81 V

and hence the simple cobalt(III) cation cannot exist in aqueous
solution (which it would oxidise to oxygen). However, the chemistry
of cobalt is notable for the ease with which complexes are formed,
and for the big effect which complex formation has on the relative
stabilities of the + 2 and + 3 states. Historically, this was observed
as early as 1798; Tassaert observed that an ammoniacal solution of
a cobalt(II) salt changed colour on exposure to air, and some years
later it was shown that, if cobalt(II) chloride was oxidised in presence
of ammonia, the yellow product had the formula CoCl 3. 6NH 3 , a
formula which posed a valency problem to the chemists of that
time. Alfred Werner, in the period 1890-1913 (he was awarded the
Nobel Prize for chemistry in 1913), was primarly concerned with
elucidating the nature of fcCoC! 3. 6NH 3 ' and similar compounds;
his investigations (carried out in the absence of the structural
methods available to us today) showed conclusively that the
compound was a complex [Co(NH 3 ) 6 ]Cl 3 , hexamminocobalt(III)