Flach, Max: Halogenated and microhydrated transition metal ions: insights from x-ray absorption spectroscopy. , Dissertation, Albert-Ludwigs-Universität Freiburg, 2025, 2025
https://freidok.uni-freiburg.de/data/266816
10.6094/UNIFR/266816
Open Access Version
Abstract:
Transition metals are of great interest to fields like catalysis, electrochemistry, and biochemistry as their electronic structure enables a rich chemistry. Therefore, the refining of spectroscopic methods sensitive to the electronic structure of transition metals is essential to advances in aforementioned fields. In this study gas phase x-ray absorption spectroscopy data of diatomic iron and nickel halide cations in the oxidation state of +2 are presented. The sensitivity of the used method enables insights into the changes of the electronic structure of transition metal halogen bonds upon changing of the halogen ligand. Combining the experimental data with charge transfer multiplet and multiplet calculations enables a detailed investigation of gradual and abrupt changes in the valence electronic configuration of the diatomic transition metal halide cations. A shift of the median of the iron L₃-edge excitation energy towards lower energy with increasing covalency of the iron-halogen bond within the oxidation state of +2 is observed. The observed shift amounts to 420±60meV from [FeF]⁺ the most ionic species of the series to the more covalently bonded [FeI]⁺. In addition, a shift of the nickel L₃-edge excitation energy median of 2.3±0.2eV is discerned between monatomic Ni⁺ in the ground state and a low-lying excited state differing by exact one electron in 3d occupation. Furthermore, an abrupt change in the nickel L₃-edge spectrum between [NiF]⁺ and [NiCl]⁺ is observed. This abrupt change suggests that the ground state wave function of [NiF]⁺ is best described by a 3d⁸ valence configuration. In contrast, the ground state wave functions of [NiX]⁺ (X = Cl, Br, I) are best described by two electronic configurations with a dominating 3d⁹Ḻ configuration that includes a ligand hole and a 3d⁸ configuration with no ligand hole. Therefore, [NiF]⁺ electronic structure can be described by a classic ligand field while the electronic structures of [NiX]⁺ (X = Cl, Br, I) are best described by an inverted ligand field. Moreover, iron (II) with its first complete hydration shell was prepared in the gas phase. The x-ray absorption spectra of the gas phase [Fe(H₂O)₆]²⁺ at the iron L-edge and oxygen K-edge is compared to the x-ray absorption spectra of iron in aqueous solutions as well as molecular water. In contrast, to liquid phase experiments with a dominating signal of bulk solvent molecules at the oxygen K-edge in the gas phase the signal originates solely from the water ligands in the first solvation shell of the metal. This enables detailed insight to the electronic structure of the water metal bond. Hence, observation and assignment of spectral features at the oxygen K-edge of [Fe(H₂O)₆]²⁺ originating from the hybridization of the irons 3d valence orbitals with the water ligands valence orbitals is achieved. Supporting DFT calculations enabled the assignment of specific spectral features to transitions resulting from photon excitations.