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1. The Electronic Spectra of Coordination Compounds
2. Introduction Many transition metal complexes are colored. This is due to transitions of electrons between the molecular orbitals that are formed largely by the d orbitals on the metal.
Many transitions are in the visible range, with the color of the complex taking on the complementary color of the frequency or frequencies absorbed.
4. General Features Absorption bands in electronic spectra are usually broad, and occur much more rapidly than molecular vibrations. As a result, the spectra represent a snapshot of molecules in various vibrational and rotational states.
5. General Features Extinction coefficients will range from <1 up to 50,000 M-1cm-1 depending upon the type of electronic transition and whether it is permitted based on selection rules.
6. Selection Rules Electronic transitions obey the following selection rules:
1. ?S = 0. Electrons cannot change spin.
2. The Laporte rule states that there must be a change in parity. Only g?u or u?g transitions are allowed. Thus, all d?d transitions are forbidden by this selection rule.