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

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Abstract
The magnetic circular dichroism (M.C.D.) and absorption spectra of ReCl6 2- and ReBr6 2- have been measured at liquid helium temperatures in the crystalline hosts Cs2ZrCl6, Cs2NaYCl6, and Cs2ZrBr6 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 ReCl6 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 OsX6 2- and IrX6 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.