Similarly, anhydrous CuSO4
is white, but CuSO4 5H2O is
blue in colour The influence
of the ligand on the colour
of a complex may be illustrated by considering the [Ni(H2O)6]
2+ complex,
which forms when nickel(II) chloride is dissolved in water If the
didentate ligand, ethane-1,2-diamine(en) is progressively added in the
molar ratios en:Ni, 1:1, 2:1, 3:1, the following series of reactions and
their associated colour changes occur:
[Ni(H2O)6]
2+ (aq)
+ en (aq)
=
[Ni(H2O)4(en)]
2+(aq)
+ 2H2O
green
     pale blue
[Ni(H2O)4 (en)]
2+(aq) + en (aq)
=
[Ni(H2O)2(en)2]
2+(aq) + 2H2O
          blue/purple
[Ni(H2O)2(en)2]
2+(aq) + en (aq)
=
[Ni(en)3]
2+(aq)
+ 2H2O
           violet
This sequence is shown in Fig