THE VIBRONIC STRUCTURE OF MIXED-LIGAND OS(IV) COMPLEXES .3. LOW-TEMPERATURE ABSORPTION-SPECTRA IN HIGH-RESOLUTION

Low temperature (2 K) absorption spectra in the region 4800-20 000 cm( -1) of hexahalogeno Os(IV) complexes have been recorded, for the near infrared by the use of a vacuum interferometer. The resolution of band s assigned to d-d transitions is remarkably improved to that obtained earlier from these and similar complexes which supply narrow line spec tra. The compounds investigated are Cs- and tetrabutylammonium (TBA) s alts of OsCl62-, Cs2OsBr6 and the mixed halogen complexes Cs-2[OsCl5Br ], Cs-2[OsCl4Br2] (cis and trans), Cs-2[OsBr4Cl2] (cis) and Cs-2[OsBr5 Cl]. Peak assignments to zero-phonon transitions (electronic origins) and vibrational side bands, the latter in general having higher intens ity, show that selection rules valid for octahedral coordination large ly hold as well for complexes of definitely lower symmetry. This is al so true for vibronic transitions promoted by vibrational modes which a re resultants from the corresponding parent compounds, i.e. from nu(3) (t(1u)), nu 4(t(1u)) and nu(6)(t(2u)) of octahedral symmetry. Apparent ly localization of d-electrons on the metal overrules the requirements imposed by molecular symmetry. The compounds are synthesized followin g a prescription in the literature, with an improvement of the separat ion procedure for avoiding contaminations with compounds of other halo gen ligand composition. (C) 1997 Elsevier Science B.V.