MCD spectra of OsI₆^2- and the trans-mixed hexahalo complexes of Ir⁴⁺ and Os⁴⁺

Abstract

The absorption and magnetic circular dichroism (MCD) spectra of the n-tetrabutylammonium salts of OsI₆ ^2- and trans-IrCl₄Br₂ ^2-, OsCl₄Br₂ ^2- and OsBr₄Cl₂ ^2- and of the trans-tetraethylammonium salts of IrBr₄Cl₂ ^2-, OsCl₄I₂ ^2-, and OsI₄Cl₂ ^2- were measured in KBr pellets at liquid helium temperature. The bands in the absorption spectra, which are very similar for analogous Ir⁴⁺ and Os⁴⁺ mixed halides, are assigned with the help of MCD data to ligand-to-metal charge-transfer transitions using the j-j model previously found successful in interpreting the OsCl₆ ^2- and OsBr₆ ^2- spectra. The signs and, where possible, the magnitudes of the tetragonal distortion parameters are estimated for these mixed halides by fitting the spectra, and the signs are compared with those predicted by simple molecular orbital arguments. The principal bands of OsI₆ ^2- are assigned to charge-transfer transitions which correspond to the analogous ones in OsBr₆ ^2- and OsCl₆ ^2-. The importance of second-order ligand spin-orbit coupling is demonstrated.