Low symmetry distortions of hexahalo anions of Re(IV) and Os(IV) investigated by polarized absorption spectra and ligand field calculations

Polarized absorption spectra of intraconfigurational transitions of Re⁴⁺ and Os⁴⁺ in trigonal environments (D _3d site symmetry of host lattices) were measured at low temperature (110 K to 1·9 K). The vibronic side bands and electronic origins arising from electric and magnetic dipole transitions are assigned to appropriate term symbols. The vibrational fine structure is explained by normal vibrations with quanta closely related to those of the ground state. Some of the complex octahedra of doped species do not adopt the type of distortion (stretched or squeezed) to which they are predisposed by the host crystals. In the case where the space provided is not adequate for the doped compound, additional local distortions are present which are different for Re and Os. The degree of splitting due to the trigonal symmetry on octahedral Re⁴⁺ energy levels has been found always larger for the ground state, even though it has only spin degeneracy, than for the excited states. Ligand field theory is able to explain this finding by choosing parameter sets out of a relatively limited range which can be readily scanned.