Allowed and Forbidden d-d Bands in Octahedral Coordination Compounds: Intensity Borrowing and Interference Dips in Absorption Spectra

Abstract

Absorption spectra with weak spin-forbidden and more intense spin-allowed bands are observed for many different octahedral transition metal complexes. The simplest electron configurations showing these two different types of transitions are d² and d⁸ with singlet and triplet electronic states. Spectra of vanadium(III) and nickel(II) complexes show effects of interactions between singlet and triplet excited states, leading to intensity borrowing and interference dips. These effects are quantitatively analyzed with modern theoretical models based on coupled potential energy surfaces. Both effects can be rationalized in terms of quantum interference processes.