dianion at 650 nm is shifted to 530 nm by interaction with Mg2+. Similarly, a series of coloured
complexes are formed between Cu, Co, Ni, Fe(III), Ag, Bi, Hg(I), Pd and Pt and dithiooxamide or
rubeanic acid (Figure 9.9(b)). The stronger the complex formed, the more the absorption band is shifted
towards the UV.
Where the metal-ligand bond is strongly covalent and possibly includes back bonding from the metal
into vacant ligand orbitals, the spectrum of the ligand may be significantly changed. Ionic complexes
formed between Ca, Sr and Ba and the dyes metalphthalein, calcein and thymolphthalexone are strongly
coloured with similar absorption spectra to the dissociated form of the ligands whereas complexes with
metals forming more covalent bonds are shifted into the near UV because of modification of the
conjugated chromophores. Dithizone (diphenylthiocarbazone) (Figure 9.9(c)) forms highly coloured
complexes with metals which can back-donate into ligand orbitals. The range of absorption maxima
extends from 620 nm for the reagent mono-anion to 490 nm for the Hg(II) complex and 460 nm for the
Ag(I) complex both of which are essentially covalent.
The origins of charge-transfer bands are quite different to those for the complexes already discussed.
Associated mainly with covalent, and especially transition metal complexes they are due to transitions
between σ or π ligand orbitals and empty or antibonding metal orbitals. Transitions from metal σ
orbitals to vacant ligand orbitals are also sometimes involved. These transitions are allowed by
symmetry rules and consequently give much more intense bands than most d-d transitions. The intense
red colour of the Fe(III)-thiocyanate complex, the orange colour of the titanium(IV)-peroxy complex
and the purple permanganate ion are all a result of the transfer of electrons from metal to ligand orbitals.
Such transitions are often associated with metals in high oxidation states and have been described as
photochemical oxidation-reduction reactions. Organic ligands forming charge-transfer complexes
include sulphosalicylic acid, o-phenanthroline and other aromatic amines which have been used as
reagents in the determination of Fe, Cu(I), Zn and Cd.
Qualitative Analysis – The Identification of Structural Features
Visible and UV spectrometry are of secondary importance to other spectral methods for the
identification and structural analysis of unknown compounds. This is a direct consequence of the broad
bands and rather simple spectra which make differentiation between structurally related compounds
difficult. As an adjunct to infrared, magnetic resonance and mass spectrometry, however, they can play
a useful role. They can be particularly helpful in confirming the presence of acidic or basic groups in a
molecule from the changes in band position and intensity associated with changes in pH (p. 369).