High resolution absorption and magnetic circular dichroism spectra of Cs2ZrCl6 : Os4

The high resolution magnetic circular dichroism (M.C.D.) spectrum of Cs 2 ZrCl 6 : Os 4+ has been studied at liquid helium temperature over the region 17 000-34 000 cm -1 in conjunction with the corresponding absorption spectrum previously reported by Dorain, Patterson and Jordan. The absorption spectrum is reported for the first time in the 5 000 cm -1 and 11 000 cm -1 regions. All of the important features of the absorption and M.C.D. spectra between 23 000 cm -1 and 34 000 cm -1 can be explained on the basis of a ligand-to-metal charge-transfer interpretation formulated in the j-j coupling limit. The contention of Dorain, Patterson and Jordan that this region can be interpreted as arising from d → d transitions is contradicted by the M.C.D. data, and it is argued that such an interpretation is highly implausible on intensity grounds. The major M.C.D. features in the regions of strong absorption are accounted for convincingly on the basis of allowed charge-transfer transitions. Reasonable, though not unique, interpretations of the M.C.D. are suggested for the regions of intermediate intensity on the basis of forbidden (vibrationally-induced) charge-transfer transitions. The ordering of ligand-to-metal excitations from low to high energy is : t 1g (π) → t 2g (d), t 1u (π + σ) → t 2g (d), t 2g (π) → t 2g (d), t 2u (π) → t 2g (d), e g (σ) → t 2g (d). The regions of much weaker absorption at low energy < 18 000 cm -1 are attributable to d → d transitions.