Absorption spectroscopy of octahedral nickel(II) complexes: A case study of interactions between multiple electronic excited states

Absorption spectra of octahedral nickel(II) complexes are used to illustrate that the limitation to only the initial and Born–Oppenheimer final states of an electronic transition is not adequate in order to rationalize the intensity and vibronic structure of the lowest-energy spin-forbidden transition of these compounds. Qualitative and quantitative models are applied and discussed for a series of absorption spectra measured in solution at room temperature and for the low-temperature single-crystal absorption spectra of nickel(II) ions doped into bromide host lattices. The spectra and models illustrate the influence of multiple allowed transitions on the lowest-energy spin-forbidden band.