Darstellung und spektroskopische Charakterisierung von Tetrahalogenooxalato-Osmaten(IY) / Preparation and Spectroseopical Characterisation of Tetrahalooxalato-osmates(IV)

Reaction of [OsX6]2- (X = CI, Br, I) with oxalic acid in water gives the very stable and deeply coloured new complexes [OsX40x]2- which can be precipitated as alkali or tetraalkylammonium salts. The salts with long-chain alkylammonium ions are soluble in all common no polar solvents. The IR and Ra spectra and the depolarization ratios are consistent with the local C2v-symmetry. The coupling between the vibrations of coordinated oxalate and with the skeletal [OsX4O2]-modes is apparently small. The optical spectra have been studied at low temperature (10 K). Analogous to [OsXe]2- the very intense absorptions in the UV and VIS are assigned to the electric dipole allowed electron transfer from "(π+σ)t1u, πt2U(γ7), πt2u(γ8) and (σ+π)t1u" to "dt2g". By the descent in symmetry each of these transitions is further split in three components (A1, B1, B2) corresponding to the three different groups of ligands, X2tr-, X2c. and ox. The distinction between the X2tr. →Os and X2c- → Os is caused by a static trans-effect and depends on the difference of the optical electronegativities induced by ox. This splitting is not observed for [OsCl4ox]2- but is about 900 cm-1 for [OsBr4ox]2- and 1200 cm-1 for [OsI4ox]2-. The well defined bands in the NIR and VIS are assigned to the intraconfigurational d-dtransitions between the spin-orbit components in the octahedral doublegroup Oh* splitted by the lower C2v-symmetry. Some of the zero-phonon transitions within the split of the 3Tlg, Oh-groundstate exhibit extremely high intensities (e~2500 cm2/mmol). The assignment of these transitions is improved by the electronic Ra spectra. The finestructure of some of the d-d-transitions originates from coupling with A1 vibrational modes especially with νs(C-O).