The d-d transitions observed in the absorption spectra of octahedral nickel(II) complexes provide detailed experimental information that can be used to test theoretical models. We present and analyze spectra of complexes whose ligand field strength leads to a forbidden spin-flip band close in energy to two spin-allowed transitions. Recently developed theoretical models are applied to rationalize the intensity of the spin-forbidden transition. The well-established limitation to only a single allowed transition from which intensity borrowing occurs is not adequate for many of these complexes. The spectra and models illustrate the simultaneous influence of multiple allowed transitions, leading to constructive or destructive interference and causing surprisingly low intensities of the spin-forbidden transition in some complexes. These observations can be generalized to qualitatively explain why only few forbidden absorption bands are observed in solution absorption spectra and why transitions between excited states often show unusual intensities and bandshapes.

E. González, A. Rodrigue-Witchel, C. Reber, Coord. Chem. Rev., special issue on Inorganic Electronic Spectroscopy, 2006, accepted for publication.