The optical absorption and magnetic circular dichroism spectra of Re4+ in the cubic hosts, Cs2ZrCl6, Cs2NaYCl6, and Cs2ZrBr6

The magnetic circular dichroism (M.C.D.) and absorption spectra of ReCl 6 2- and ReBr 6 2- have been measured at liquid helium temperatures in the crystalline hosts Cs 2 ZrCl 6 , Cs 2 NaYCl 6 , and Cs 2 ZrBr 6 over the energy regions 13 000-39 000 cm -1 , 28 000-40 000 cm -1 and 19 000-31 000 cm -1 respectively. All of the observed bands are assigned to allowed or parity forbidden charge-transfer transitions, except those in ReCl 6 2- at energies between 13 000 and 16 000 cm -1 , for which the M.C.D. spectra support the d-d interpretation of earlier work. The experimental data have been compared with predicted spectra based on a model which includes spin-orbit coupling and electrostatic interactions within the metal and ligand configurations, but neglects metal-ligand electrostatic interactions except as perturbations. Comparisons have been made with the spectra of OsX 6 2- and IrX 6 2- whenever appropriate, and it is suggested that the spin-orbit splitting of the t 1u (π + σ) ligand orbital in the chloride complexes is ∼ 1500 cm -1 , appreciably greater than the emaximum of 3/2ζcl ∼ 880 cm -1 predicted by simple MO theory.