Low-temperature charge-transfer spectra of hexahalogenoiridates(IV) and -osmates(IV)

Abstract

The charge-transfer spectra of IrCl₆ ^2– and OsCl₆ ^2– doped into (MeNH₃)₂SnCl₆ and IrBr₆ ^2– and OsBr₆ ^2– doped into (EtNH₃)₂SnBr₆ have been measured at 4 °K. The extensive vibrational and spin-orbit fine structure is discussed and the origins of the transitions are identified. Expressions are derived for the splitting of the ground and excited terms by spin-orbit coupling at both the metal and the halogen. With these expressions the spectra have been fitted by a least-squares procedure to yield values of the effective ligand spin-orbit coupling constant, the t_1u(π)—t_1u(σ) mixing coefficient and the baricentres of the charge-transfer terms. Oscillator strengths for transitions to each of the spin-orbit states are also calculated. The remarkable correspondence between the osmium(IV) and iridium(IV) spectra is commented on. It is suggested that the U_u′(²T_2u) state of IrBr₆ ^2– suffers a strong Jahn–Teller distortion.