O is blue in colour. The influence of the ligand on the colour of a complex may be illustrated by considering the [Ni(H O) ] + complex, which forms when nickel(II) chloride is dissolved in water. If the didentate ligand, ethane- , -diamine(en) is progressively added in the molar ratios en:Ni, : , : , : , the following series of reactions and their associated colour changes occur: [Ni(H O) ] + (aq) + en (aq) = [Ni(H O) (en)] + (aq) + 2H O green pale blue [Ni(H O) (en)] + (aq) + en (aq) = [Ni(H O) (en) ] + (aq) + 2H O blue/purple [Ni(H O) (en) ] + (aq) + en (aq) = [Ni(en) ] + (aq) + 2H O violet This sequence is shown in Fig. .
. Fig. . Aqueous solutions of complexes of nickel(II) with an increasing number of ethane- , -diamine ligands.
[Ni(H O) ] (aq) [Ni(H O) ] (aq) en [Ni(H O) ] (aq) en [Ni(en) ] (aq) Colour of Some Gem Stones The colours produced by electronic transitions within the d orbitals of a transition metal ion occur frequently in everyday life. Ruby [Fig. . (a)] is aluminium oxide (Al O ) containing about .
- % Cr + ions ( d ), which are randomly distributed in positions normally occupied by Al + . We may view these chromium(III) species as octahedral chromium(III) complexes incorporated into the alumina lattice; d–d transitions at these centres give rise to the colour. Fig. .
: Transition of an electron in The crystal field model is successful in explaining the formation, structures, colour and magnetic properties of coordination compounds to a large extent. However, from the assumptions that the ligands are point charges, it follows that anionic ligands should exert the greatest splitting effect. The anionic ligands actually are found at the low end of the spectrochemical series. Further, it does not take into account the covalent character of bonding between the ligand and the central atom.