Modern inorganic chemistry

408 THE TRANSITION ELEMENTS

there is some double bonding between the nickel and carbon atoms;

i.e.

CO

I

Ni

/u-V

CO \\ CO

CO

If the -f 2 complex K 2 [Ni(CN) 4 ] (see above) is dissolved in liquand

ammonia, addition of potassium produces the yellow K 4 [Ni(CN) 4 ] ;

the [Ni(CN) 4 ]^4 ~ ion has nickel in oxidation state 0, is isoelectronic

with Ni(CO) 4. and is believed to be tetrahedral.

TESTS FOR NICKEL

The reactions of aqueous solutions of nickel(II) salts with hydroxide

ions, with excess ammonia, with sulphide ion and with dimethyl-

glyoxime (see above) all provide useful tests for nickel(II) ions.

COPPER

THE ELEMENT

Copper has been used, especially in alloys with tin (bronze), since
about 3000 B.C., and the Romans used it extensively. Small amounts
of the free metal are found naturally, but its compounds (mostly
sulphides) are abundant; the most important ore is chalcopyrite or
copper pyrites CuFeS. Other natural forms include the basic
carbonates CuCO 3. Cu(OH) 2 (malachite) and 2CuCO 3 .Cu(OH) 2
(azurite). The process of extraction consists essentially of (a) separa-
tion of the ore from rock, by flotation (selective wetting), (b) con-
version of the sulphide ore to the crude metal, by blowing air through
the molten ore. (c) purification of the crude metal usually by
electrolysis; the crude copper is the anode in an electrolyte of
acidified aqueous copper(II) sulphate, and the pure metal deposits
on 'starting' sheets of copper as cathode. The metal is extensively
used for electrical purposes, for water tanks and pipes, and for
roofing. Alloys include the bronzes containing tin, and sometimes
phosphorus (for hardness—phosphor-bronze); brass, containing
zinc and cupro-nickel (for coinage). Compounds of copper are used
as fungicides, and as catalysts. Copper is found in plants and
animals; some lower animals (for example snails and crabs)