Die Schwingungsfeinstruktur der elektronischen Absorptionsspektren von [0s02(ox)2]2- und [0s02(mal)2]2- Vibrational Fine Structure on the Electronic Absorption Spectra of [0s02(ox)2]2- and [0s02(mal)2]2-

The complexes (TBA)2[Os02(ox)2] and (TBA)2[0s02(mal)2] have been prepared and characterized. The IR and Raman frequencies are assigned according to point group D2h. The electronic absorption spectra measured on thin films of the solid complex salts on silica discs at 10 K show well resolved vibrational fine structure. Two very weak bands (e: 4-58) in the visible region are assigned to d -d transitions from the singlet ground state ag2 (dxy) to a triplet and singlet term of the equivalent ji* orbitals b2g/b3g (dxz, dyz). The two more intense band systems (e: 500-1500) in the UV region are attributed to CT transitions from b2u/b3u n orbitals of tbe oxo ligands to the lowest unoccupied orbitals b2g/b3g of the central ion. These electronic transitions couple with vs(0s — 0) as indicated by the main progression of about 700 cm 1 . In comparison to the vibrational ground state this lowered frequency is caused by population of antibonding n levels. The loosening of the 0s = 0 bonds in the excited state is demonstrated by high anharmonic constants (—2.5 to —10.5 cm 1 ) and the estimated bond lengthening of about 12 pm. Moreover the vibrational fine structure of the malonato complex shows many combination modes of i's(0s = 0 ) with other symmetric fundamentals. Due to lower site symmetry in the solid state the CT transitions of the malonato compound are split into 2 components with a spacing of about 200 cm-1.