Electronic spectra of transition metal complexes

Abstract

In this chapter our concern will be with the electronic spectra of transition metal complexes, particularly those of the first transition series. Although nowadays there are fewer studies of these spectra, per se, than previously, their study is essential if the electronic excited states of complexes are to be understood. The energy required for the promotion of an electron from one orbital to another or, more precisely, the excitation of a molecule from its electronic ground state to an electronic excited state, corresponds to absorption of light in the near-infrared, visible or ultraviolet regions of the spectrum. For transition metal complexes the absorption bands in the first two of these regions are relatively weak and are associated with transitions largely localized on the metal atom. The ultraviolet bands are intense. They are associated with the transfer of an electron from one atom to another and so are called charge-transfer bands. These bands are responsible for the colour changes associated with indicators for inorganic cations, such as the thiocyanate test for Fe^III or the indicators used in EDTA (compleximetric) titrations, but in these cases the charge-transfer bands fall in the visible region of the spectrum. The intense bands will be dealt with in the latter part of this chapter; for the moment our concern will be with the weaker bands. Whilst these are most simply explained by crystal field theory, a detailed comparison of the data with the theoretical predictions will show that ligand field theory provides a more appropriate explanation.