ULTRAVIOLET AND VISIBLE RESONANCE RAMAN-SPECTROSCOPY OF TRIS-(2,2'-BIPYRIDYL)IRON(II) - INTENSITY CONSIDERATIONS AND BAND ASSIGNMENTS

Resonance Raman spectra of Fe(bpy)3(2+) (bpy = 2,2'-bipyridine) have been recorded under excitation into two different metal-to-ligand d --> pi-* charge transfer absorption bands (lambda-ex = 354.7 or 532.0 nm) of the complex. The relative intensities of the active intraligand vibrations exhibit a marked dependence on excitation wavelength. Normal modes assigned to distortions along the long axis of the bpy molecule are found to undergo greater relative enhancement under UV excitation, while those modes associated with large displacements within the diimine core are preferentially enhanced under visible excitation. These observations are consistent with the coefficients and the phase relations of the 2p-pi atomic orbitals of the lowest lying pi-* orbitals of the bpy moiety as calculated by extended Huckel theory. Thus, the results lend strong support to previous assignments of the resonance Raman spectra of M(bpy)3(2+) systems.