The electronic and vibrational spectra of t r a n s-[NiCl4(H2O)2]2− anions in crystalline Rb2NiCl4⋅2H2O and the effective symmetry of mixed-ligand nickel(II) complexes

Raman, far infrared, and electronic absorption spectra have been recorded for the [NiCl4(H2O)2]2− anion in dirubidium tetrachlorodiaquonickelate(II), Rb2NiCl4⋅2H2O. The vibrational spectra are analyzed in terms of an effective anion symmetry sufficiently low to remove all eigenstate degeneracies, while the d-electron ligand-field band shapes do not reflect the low crystallographic site symmetry of the Ni2+ ion. The effect of low symmetry and magnetic ordering on the polarized intensities of some zero-phonon electronic fine-structure lines is discussed, and the energy separations between these zero-phonon origins and vibronic side bands are correlated with the observed anion normal modes of vibration.