Polarized resonance Raman spectra of hexahalide complexes of transition metals: The effect of electronic degeneracy on the polarization of vibrational Raman scattering

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Abstract
Polarized vibrational resonance Raman spectra have been measured for eight hexahalide complexes of transition metals, low‐spin d4 OsCl62−, OsBr62−, and low‐spin d5 IrCl62−, IrBr62−, and low‐spin d6 PdCl62−, PtCl62−, PtBr62−, and PtI62−. Polarization anomaly due to asymmetric Raman tensor was found for the d5 complexes, whereas such was not observed in the cases of the d4 and d6 complexes. The result is analyzed on the basis of a symmetry consideration in vibronic space. Electronic and/or vibrational degeneracy is shown to induce the antisymmetric part of vibrational Raman tensors. It is concluded that the former is primarily responsible for the polarization anomaly found for the d5 complexes. Under resonance conditions, the hot transitions originating from the excited levels of degenerate vibrations can give different Raman tensor patterns from those for the corresponding 1←0 transitions. It is stressed that the selection and polarization rules of vibrational Raman scattering should be discussed in relation to the transition and not to the vibrational mode involved in it.

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