Chapter 5: Metal Complexes

The circular dichroism (CD) spectroscopy of inorganic complexes is described. Two groups of complexes are examined, those with CD intensity associated with the π-system transitions of the ligand and those in which the d → d transitions of the central metal are the key factor. The CD intensity in the former case tends to be relatively strong and is usually generated by exciton coupling between neighbouring chromophores, while in contrast that of the latter group tends to be relatively weak, since it is associated primarily with the electric dipole forbidden but magnetic dipole allowed d → d transitions of the central metal. The exciton coupling of intense CD spectra of octahedral compounds containing bidentate o-phenanthroline (phen) and 2,2’-bipyridyl (bipy) provide examples of the analysis of CD intensity associated with ligand π-systems. The approach that was used to identify the d → d bands in the electronic absorption and CD spectra of ferrocytochrome c provides an example of how CD intensity associated with the central metal can be identified using spectral deconvolution. Differences in the anisotropy factors of the component spectral bands, defined as four times the rotational strength divided by the dipole strength, makes it possible to distinguish between the d → d bands, and the charge transfer and π → π* bands associated with the porphyrinoid ligand. The anisotropy factors are typically an order of magnitude higher in the former case.