Chapter 1. Ligand K-edge XAS is regularly used to determine the ligand contribution to metal–ligand bonds. For quantitative studies, the pre-edge transition intensities must be referenced to an intensity standard, and pre-edge intensities obtained from different ligand atoms cannot be compared without standardization owing to different cross-sections at each absorption edge. In this work, the intensities of the 1s → σ* transitions in F2, Cl2, and Br2 are analyzed for their use as references for ligand K-edge XAS. We show that the intensities of these transitions are equal to the intensities of the 1s → np transitions in the unbound halogens. This finding is supported by comparison between the normalized experimental intensities for the molecules and the calculated oscillator strengths for the atoms. These results highlight the potential for these molecules to be used as intensity standards in F, Cl, and Br K-edge XAS experiments.

Chapter 2. Understanding the nature of covalent bonding in metal-based systems remains challenging. In particular, the relative roles of orbital energy degeneracy and overlap in enabling bond covalency remain poorly understood. Here, F K-edge X-ray absorption spectroscopy is used to experimentally determine bond covalency and orbital overlap in MF62− (M = Ti, Zr, Hf). Moreover, analysis on the previously studied analogous MCl62− complexes enables meaningful comparisons between nd–2p and nd–3p (n = 3, 4, 5) bonding. For each complex, transitions into π bonding molecular orbitals are resolved. From the intensities of these features, we find significantly enhanced π overlap in MF62− relative to MCl62−. For all MF62−, this greater π overlap enables greater π bond covalency vs. the corresponding MCl62− complexes, despite a greater energy difference between metal and ligand orbitals. For TiX62− (X = F, Cl), for which transitions into σ bonding molecular orbitals are resolved, similarly greater σ overlap is found for TiF62− vs. TiCl62−. However, in this case, the enhanced σ overlap is not sufficient to enable greater σ bond covalency in TiF62−.

Chapter 3. The 4f orbitals of Ce(IV) have shown appreciably enhanced covalent mixing with ligand orbitals relative to those of Ce(III). Here, X-ray spectroscopy, magnetic susceptibility measurements, and computational methods are used to investigate 4f covalency in CeF62− and CeCl62−. These techniques show covalent mixing between Ce 4f and F 2p orbitals to be about 25% less than mixing between Ce 4f and Cl 3p orbitals, placing CeF62− among the most ionic Ce(IV) compounds to-date. However, ligand field analysis using the experimental data shows significantly increased 4f orbital overlap with the F 2p orbital compared to the Cl 3p. This result is counterintuitive since the Ce–F bonds display less 4f covalency despite their enhanced orbital overlap, and greater overlap is traditionally associated with enhanced bond covalency. The weaker covalency is attributed to the large energy gap between Ce 4f and F 2p orbitals strongly counteracting the enhanced orbital overlap. These results highlight that only a concerted consideration of both atomic orbital overlap and energy matching in f-element systems leads to an accurate picture of their bonding.

Chapter 4. The An 5f orbitals have been shown to participate in covalent orbital mixing more readily than the radially contracted Ln 4f orbitals. However, precisely evaluating this mixing remains experimentally challenging. Here, F K-edge X-ray absorption spectroscopy is used to quantify the amount of orbital mixing between the An 5f and F 2p orbitals in the series AnF62− (An = U, Np, Pu). While 5f orbital mixing in AnCl62− was found previously to increase from UCl62− to NpCl62− to PuCl62−, it is found to be the same for each AnF62−. Notably, orbital mixing is greater for the U and Np fluorides vs chlorides and is the same for the Pu complexes. Ligand field analysis reveals that the An–F bonds are characterized by greater 5f orbital overlap than the An–Cl bonds. These results suggest that Pu marks a transitional element in the An(IV) series between overlap-driven and energy-driven covalency.