THE TRANSITION ELEMENTS 365
light by the Cu^2 + cation is not in the visible part of the spectrum,
and the substance appears white. If the solid is now dissolved in
water, the Cu^2 + ion becomes surrounded by water molecules, and
complex species such as Cu(H 2 O)6 + are formed—these absorb light
in the visible part of the spectrum and appear pale blue. If this solu-
tion of copper(II) sulphate is allowed to crystallise, water molecules
remain coordinated round the Cu2+ ion in the solid copper(H)
sulphate pentahydrate (CuSO 4 .5H 2 O) and the solid is pale blue.
When an excess of ammonia is added to the original solution, some
of the water ligands around the copper(II) ion are replaced by
ammonia:
[Cu(H 2 O) 6 ]2+ + 4NH 3 -> [Cu(NH 3 ) 4 (H 2 O) 2 ]^2 + Hh 4H 2 O
pale blue deep blue
A different d orbital splitting results and the absorption now results
in a deep blue colour*
If excess chloride ion is added to a blue solution containing
[Cu(H 2 0) 6 ]^2 +
then [Cu(H 2 O) 6 ]2+ +4Cr-*[CuCl 4 ]^2 - +6H 2 O
distorted distorted
octahedral, tetrahedral,
pale blue yellow
and here the new Splitting results in a yellow-green colour.
The d orbital splitting depends on the oxidation state of a given
ion; hence two complex ions with the same shape, ligands and
coordination number can differ in colour, for example
[Co(NH 3 ) 6 ]2+ ^^ [Co(NH 3 ) 6 ]^3 +
+ 2 +3
octahedral. octahedral,
pink yellow
Magnetic properties
The splitting of the d orbital energy levels when ligands are bonded
to a central transition atom or ion has already been mentioned
(p. 60). Consider the two ions [Co(NH 3 ) 6 ]3+ and [Co(NH 3 ) 6 ]^2 +
we have just discussed. The splitting of the d orbital energy levels
for these two ions is shown in Figure 13.2.
- The change in colour when one ligand is replaced by another can be used to
determine the coordination number; thus if the colour change is measured in a
colorimeter as the new ligand is added, the intensity of new colour reaches a maximum
when the metal/ligand ratio is that in the new complex.
