Coupled excited states in nickel(II) complexes probed by polarized absorption spectroscopy

The absorption spectra of Ni(H2O)6(2+), trans-NiCl2(H2O)(4), and NiCl64- show an unusual band shape for the (3)A(2g) --> T-3(1g), E-1(g) (O-h labels) electronic transitions in the near-infrared to visible wavelength range. A barely resolved broad band and an intense vibronic progression with a spacing larger than the totally symmetric ground-state metal-ligand vibrational frequency are superposed. Low-temperature polarized spectra are analyzed with a quantitative model and both the large interval and the band intensities are shown to arise from efficient intersystem crossings between the two excited states. Alternative assignments proposed in the literature are examined.