The last two types give rise to many strongly coloured complexes suitable for trace analysis.
Bands due to d–d transitions are responsible for the colours of transition metal ions in aqueous
solutions. Absorption of radiation results in the movement of electrons between filled and half-filled or
empty metal d orbitals which differ in energy because of the electrostatic field created by coordination
of the ligands. Various colours are produced depending on the metal and the nature of the coordinating
ligand. The absorption band shifts towards the UV with increasing strength of ligand field, the order for
some of the more common ligands being I– < Br– < Cl– < S^2 – < OH– < H 2 O< NCS– < EDTA^4 – < NH 3 <
ethylenediamine < o-phenanthroline. This is known as a spectrochemical series. In most
cases, the bands are of low intensity because the transitions are spectroscopically forbidden by rules of
symmetry. The fact that they occur at all is probably due to vibrational distortions which relax the rules.
This does not apply to tetrahedral and square planar complexes which have no centre of symmetry and
which generally have quite intense absorption bands.
A large number of metal complexes involve organic ligands in which the absorption bands of the ligand
are modified to a varying degree by coordination to the metal. The effect on the spectrum of the ligand
depends on whether the metal-ligand bonds are predominantly covalent or ionic. In complexes where
bonding to the metal is essentially ionic small shifts in bands due to n → π and π → π transitions are
observed with little change in intensity, the spectrum of the metal complex being similar to that of the
protonated ligand. Examples of this type include metal complexes with hydroxynaphthylazo dyes such
as that formed between magnesium and eriochrome black T (Figure 9.9(a)). The absorption maximum
of the ligand
Figure 9.9