Variable energy photoelectron spectroscopy on model catalytic active sites: The electronic structure and bonding in tetrahedral iron chlorides and sulfides

Variable photon energy photoelectron spectroscopy (PES) using synchrotron radiation over the range 25-150 eV is used to elucidate the valence band electronic structure and bonding in high spin d{dollar}sp5{dollar} and d{dollar}sp6{dollar} iron chlorides and d{dollar}sp5{dollar} iron sulfide. Valence band PES spectra for tetrahedral d{dollar}sp5{dollar} CsFeCl{dollar}sb4{dollar} are presented. PES spectra taken at the Fe 3p absorption edge, which provide insight into the bonding description of the ionized complex, show dramatic resonance intensity enhancement of the main band peaks as well as the deep binding energy satellite. Quantitative analysis of the resonance intensity data gives an experimental estimate of the covalent mixing in the HOMO as 38% Fe 3d. Both the inverted bonding scheme and the small relaxation have been reproduced by spin-unrestricted SCF-X{dollar}alpha{dollar}-SW calculations on FeCl{dollar}sb4sp-{dollar}. The origin of this unusual bonding pattern and its implications with respect to Fe{dollar}sp{lcub}3+{rcub}{dollar} redox chemistry are discussed. The variable energy PES data for d{dollar}sp6{dollar} tetrahedral Cs{dollar}sb3{dollar}FeCl{dollar}sb5{dollar} and Rb{dollar}sb3{dollar}FeCl{dollar}sb5{dollar} are presented, and compared to the d{dollar}sp5{dollar} system. The remainder of the valence band shows an intensity profile consistent with extensive covalent mixing. The ground state bonding description and relaxation in the final state are compared to the results of SCF-X{dollar}alpha{dollar}-SW calculations and to the high spin ferric system. Resonance PES spectra show enhancement of the satellite, but the main band is also enhanced, indicating that the relaxation has not shifted the metal character entirely to the satellite. The relative contributions of coulomb repulsion and exchange to the relaxation are considered, and the implications of these results with respect to the oxidation of high spin ferrous complexes are discussed. Chapter 4 presents the variable energy PES spectra for tetrahedral d{dollar}sp5{dollar} KFeS{dollar}sb2{dollar}. These data are qualitatively similar to the ferric chloride, but show quantitative differences due to the difference in bonding of sulfur ligands and chloride ligands. Comparison is made to literature X{dollar}alpha{dollar} results on FeS{dollar}sb4sp{lcub}-5{rcub}{dollar} as well as Fe(SR){dollar}sb4sp-{dollar} complexes. These PES results are used to explore the relative contributions of ligand covalency and exchange effects to the ground state bonding description. The implications of these results with respect to the unusual optical absorption spectra of the ferric sulfide and thiolate complexes are discussed. (Abstract shortened with permission of author.)...