Modern inorganic chemistry

THE TRANSITION ELEMENTS 413

tion gives a blue solid Cu(NO 3 ) 2 .N 2 O 4 ; this gives the blue

anhydrous nitrate Cu(NO 3 ) 2 on heating. This compound is covalent;

it is volatile and can readily be sublimed, to give a blue vapour

containing molecules with the geometrical structure

A ^

O—N Cu N—O

\/

x

°"

O

Addition of water gives the hydrated nitrate Cu(NO 3 ) 2. 3H 2 O, the
product obtained when copper (or the +2 oxide or carbonate) is
dissolved in nitric acid. Attempts to dehydrate the hydrated nitrate,
for example by gently heating in vacuo, yield a basic nitrate, not the
anhydrous salt.

Copper{H) sulphide, CuS, is obtained as a black precipitate when

hydrogen sulphide is passed into a solution of a copper(II) salt.

COMPLEXES OF COPPER(ll)

When a copper(II) salt dissolves in water, the complex aquo-ion
[Cu(H 2 O) 6 ]^2 is formed; this has a distorted octahedral (tetragonal)
structure, with four "near water molecules in a square plane around
the copper and two kfar' water molecules, one above and one below
this plane. Addition of excess ammonia replaces only the four planar
water molecules, to give the deep blue complex [Cu(NH3)4(H 2 O)2]2+
(often written as [Cu(NH 3 ) 4 ]^2 * for simplicity). To obtain
[Cu(NH 3 ) 6 ]2+, water must be absent, and an anhydrous copper(II)
salt must be treated with liquid ammonia.
Addition of halide ions to aqueous copper(II) solutions can give
a variety of halo-complexes; for example [CuCl 4 ]^2 " (yellow
square-planar, but in crystals with large cations becomes a flattened
tetrahedron); [CuCl 3 ]~ (red, units linked together in crystals to
give tetrahedral or distorted octahedral coordination around each
copper).
Addition of aqueous cyanide ion to a copper(II) solution gives a
brown precipitate of copper(II) cyanide, soluble in excess cyanide to
give the tetracyanocuprate(II) complex [Cu(CN) 4 ]^2 ~. However,
copper(II) cyanide rapidly decomposes at room temperature, to
give copper(I) cyanide and cyanogen(CN) 2 (cf. the similar de-
composition of copperfll) iodide, below); excess cyanide then gives
the tetracyanocuprate(I) [Cu(CN) 4 ]^3 ~.