The Vibrational Fine Structure of the 1T1g←1A1g Absorption Band of [Co(NH3′)6]3+

The crystal absorption spectra of [Co(NH3)6][Ir(CN)6] and [Co(ND3)6][Ir(CN)6] were studied at 4.2 K. The vibrational fine structure observed on the 1T1g←1A1g absorption band of the hexaamminecobalt(III) complexes was interpreted in terms of a vibronic intensity scheme in comparison with the vibrational frequencies of the complexes. The deuteration shifts helped to confirm the assignments of the structure. It was found that, as well as the skeletal vibrations of the CoN6 group, the NH3 (and ND3) rocking and the H–N–H symmetric deformation modes are effective in the vibronic intensity mechanism of the bands.