VARIABLE-ENERGY PHOTOELECTRON SPECTROSCOPIC COMPARISON OF THE BONDING IN FERRIC SULFIDE AND FERRIC-CHLORIDE - AN ALTERNATIVE DESCRIPTION OF THE NEAR-IR VISIBLE SPIN-FORBIDDEN TRANSITIONS IN HIGH-SPIN D5 COMPLEXES

Variable photon energy, valence-band, and core-level photoelectron spectroscopy (PES) have been used to determine the electronic structure and bonding in tetrahedral high-spin d5 FeS45−. The valence-band PES spectra over the range 25-100 eV show strong similarities with our previous results on tetrahedral FeCl4−. The three-peak pattern and their energy splittings and intensity ratios all parallel the data on FeCl4−. Also, as in ferric chloride, the major resonance enhancement appears in the deepest binding energy portion of the main band, indicating that dominant metal character is present in the bonding levels at deepest binding energy. No off-resonance PES intensity is observed in the satellite, indicating that little relaxation occurs upon ionization. These results demonstrate that the ground-state electronic structure of ferric sulfide parallels that of ferric chloride and is inverted from the normal description for transition-metal complexes, which places the dominant metal character in the antibonding levels at lowest binding energy. This inverted bonding scheme results from the large spin-polarization effects present in high-spin d5 complexes. Analysis of the Fe 2p core level PES spectra allows a comparison of the covalency of tetrahedral ferric chloride and sulfide. The difference is relatively small and is due to the lower ionization energy of the sulfide relative to the chloride ligands. Alternatively, there is a large difference observed between the bound-state optical absorption spectra of ferric chloride relative to the sulfide (and thiolate) complex, which is not satisfactorily accounted for by ligand field theory but is explained by spin-unrestricted Xα calculations. These studies indicate that the lowest energy spin-forbidden transitions in high-spin d5 complexes, which are normally described as d → d transitions in ligand field theory, have extensive ligand-to-metal charge-transfer character. © 1990, American Chemical Society. All rights reserved.