Modern inorganic chemistry

THE TRANSITION ELEMENTS 405

easily oxidised to the + 3 state. A large number of other cobalt(II)

complexes, cationic. neutral and anionia are known.

Lower oxidation states

Cobalt has an odd number of electrons, and does not form a simple

carbonyl in oxidation state 0. However, carbonyls of formulae

Co 2 (CO) 8 , Co 4 (CO) 12 and Co 6 (CO) 16 are known; reduction of

these by an alkali metal dissolved in liquid ammonia (p. 1 26) gives

the ion [Co(CO) 4 ] ~. Both Co 2 (CO) 8 and [Co(CO) 4 ] ~ are important

as catalysts for organic syntheses. In the so-called *oxo' reaction,

where an alkene reacts with carbon monoxide and hydrogen, under

pressure, to give an aldehyde, dicobalt octacarbonyl is used as

catalyst :

V c^ 4- ro -L H

C

°

2(CO)8

C=C + C0 + H "

400K

alkene 100atm H C~ H

aldehyde O

TESTS FOR COBALT

For a cobalt(H) salt the precipitation of the blue-^pink cobalt(II)
hydroxide by alkali, or precipitation of black cobalt(II) sulphide by
hydrogen sulphide provide useful tests; the hydroxide is soluble in
excess alkali and is oxidised by air to the brown 'CoO(OH)'.
Addition of excess potassium nitrite acidified with ethanoic acid
gives a precipitate of the potassium hexanitro-cobaltate(III),
K 3 [Co(NO 2 ) 6 ] (p. 403).
Decomposition of most cobalt(III) complexes by boiling with
alkali gives a brown precipitate of the hydrated oxide Co 2 O 3 .aq
(p. 402). This will quantitatively oxidise iodide to iodine.

NICKEL

THE ELEMENT

Nickel occurs more abundantly than cobalt but only a few deposits
are economically useful for extraction. The metal is obtained by