Resonance-Raman spectra of nitro cobalt(III) complexes: K3[Co(NO2)6], Na3[Co(NO2)6], trans-[NH4][Co(NO2)4(NH3)2], and trans-[Co(NO2)2(NH3)4]NO3

Abstract

The resonance-Raman spectra of K₃[Co(NO₂)₆], Na₃[Co(NO₂)₆], trans-[NH₄][Co(NO₂)₄(NH₃)₂], and trans-[Co(NO₂)₂(NH₃)₄]NO₃ have been measured at ca. 80 K with a view towards understanding the nature of the visible absorption band of nitro cobalt(III) complexes. Both intra-ligand vibrations (NO₂ symmetric stretching and NO₂ bending modes) and skeletal vibrations (Co–NO₂ and Co–NH₃ stretching modes) were found to be resonance-enhanced within the visible absorption band. The resonance enhancement occurred for the non-totally symmetric modes, as well as for the totally symmetric modes. This observation indicates B-term scattering which is concerned with two electronic excited states. The behaviour of the excitation profiles is discussed on the basis of the visible absorption bands partly having charge-transfer character.