Jahn-Teller Effect in the Eg Vibrational Mode of Hexafluoride Molecules. The Infrared Spectra of RuF6 and RhF6

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Abstract
The infrared spectra of RuF6 and RhF6 are reported and are interpreted in terms of a regular octahedral structure, point group Oh. The spectrum for RuF6 shows fewer bands than any of the known hexafluoride molecules, presumably owing to a Jahn‐Teller effect. The fundamental vibrational frequencies (cm−1) for RuF6 are v1=675, v3=735, v4=275, v5=262; for RhF6, v1=634, v2=595, v3=724, v4=283, v5=269, and v6=192; for RuF6 v2 is estimated to be 573. A broad band arising from an electronic transition in RuF6 is also observed with maxima at 1950 and 1400 cm−1. The symmetry requirements for a Jahn‐Teller effect in a regular octahedral hexafluoride molecule are summarized. The evidence for a Jahn‐Teller effect in the Eg mode for hexafluoride molecules is reviewed. An analogous effect is not observed for the F2g mode although it is symmetry allowed. Comparison of band widths for (v1+v3) and (v2+v3) permits the following estimates of the Jahn‐Teller splittings (cm−1) for the Eg mode: TcF6=80, ReF6=25, and OsF6=54. RhF6, like IrF6, does not show a Jahn‐Teller effect of comparable magnitude, presumably because of the ``spin character'' of its Γ8 electronic ground state. The corresponding effect for RuF6 is relatively larger and cannot be evaluated in this way because the band (v2+v3) is so broadened as to make it extremely difficult to characterize.

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