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 NiCl6 4- 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 wavel ength range. A barely resolved broad band and an intense vibronic prog ression with a spacing larger than the totally symmetric ground-state metal-ligand vibrational frequency are superposed. Low-temperature pol arized spectra are analyzed with a quantitative model and both the lar ge interval and the band intensities are shown to arise from efficient intersystem crossings between the two excited states. Alternative ass ignments proposed in the literature are examined.