FROM ISOLATED COMPLEX UNITS TO ONE-DIMENSIONAL MOLECULAR CHAINS - OPTICAL INVESTIGATIONS OF TERNARY ALKALI-METAL THIOFERRATES(III)

Low-temperature absorption spectra in the near-infrared and visible region were recorded from KBr disks of iron-sulfur complexes which contain (pseudo-) tetrahedral [Fe(III)S4] units in their crystal structures, i.e., Na5FeS4, with isolated tetrathioferrate(III) chromophores, dimer Na3FeS3 and the chain compounds AFeS2 (A = K, Rb, Cs). Also, polarized measurements were carried out on CSGa1-xFexS2 (0.00 less-than-or-equal-to x less-than-or-equal-to 0.25) mixed crystals. The spectra are dominated by intense bands in the visible region which are assigned to LMCT transitions of the [Fe(III)S4] chromophore. The lowest CT transition measured from the mixed crystals is strongly z-polarized (E over arrow pointing right parallel-to chain axis) and exhibits band-edge behavior even at low iron concentrations. With increasing iron content the band-edge is shifted to lower energies, indicating formation of higher oligomers. Several weak transitions measured at higher sensitivity between 8000 ard 17 000 cm-1 are assigned to the d-d transitions of a high-spin d5 chromophore using polarization selection rules. On this basis also shoulders and inflections measured from the pure ferric compounds can be interpreted by comparison. A fit of band peaks by angular-overlap calculations leads to ligand field parameters smaller than those of the [FeCl4]- analogue. From CT transitions an optical electronegativity of 2.5 for the sulfur ligand is derived. The strong antiferromagnetic coupling between the iron units in polycenter complexes affects primarily band intensities and the energies of CT transitions. A low-spin ground state, which was assumed on the basis of magnetic measurements on some of these systems, could not be confirmed from present optical investigations.