Modern inorganic chemistry

THE TRANSITION ELEMENTS 407

COMPLEXES

Nickel(II) forms a great variety of complexes, in which there may be

either six ligands (octahedral or distorted octahedral), five ligands

(square pyramidal or trigonal biprism) or four (tetrahedral or

square planar), and which may be cationic, neutral or anionic. The

simple hydrated cation [Ni(H 2 O) 6 ]2+ is octahedral; addition of

concentrated aqueous ammonia in excess to an aqueous solution of

a nickel(II) salt gives the purple octahedral complex [Ni(NH 3 ) 6 ]^2 *

by replacement of the water ligands ; this forms sparingly soluble

salts with some anions, for example Br~. The scarlet-coloured

complex formed when dimethylglyoxime* is added to a nickel(II)

solution is a neutral planar complex :

CH,—C=NOH

;2 +

dimethylglyoxime

Ni

OH.... O

CH— C=N N=C— CH

Ni

CH 3 —C=N N=C—CH 3

i I

O.... HO

scarlet

( are hydrogen bonds)

If nickel(II) cyanide, Ni(CN) 2 , is dissolved in excess potassium
cyanide, the orange-red complex salt K 2 Ni(CN) 4. H 2 O can be
crystallised out; this contains the stable square-planar [Ni(CN) 4 ]z ~
anion.

Low oxidation states

Nickel tetracarbonyl Ni(CO) 4 was the first metal carbonyl to be
discovered, by Mond in 1890; it is obtained by passage of carbon
monoxide over nickel metal heated to 320 K. It is a volatile, toxic
liquid (b.p. 315 K), and has a tetrahedral structure. It has consider-
able stability, but inflames in air; it is believed that in the structure

More systematically named butanedione dioxime.