Reaction of the potassium pentamethylcyclopentadienyl iridate tris-hydride K[IrCp*H₃] with UCl₄ and ThCl₄(DME)₂ led to the complete replacement of the halide ligands to generate multimetallic complexes U{(μ-H)₃IrCp*}₄ (1) and Th{[(μ-H₂)(H)IrCp*]₂[(μ-H)₃IrCp*]₂} (2), respectively. These analogues feature a significant discrepancy in hydride bonding modes; 1 contains twelve bridging hydrides while 2 contains ten bridging hydrides and two terminal, Ir-bound hydrides. Use of a U(iii) starting material, UI₃(1,4-dioxane)_1.5, resulted in the octanuclear complex {U[(μ²-H₃)IrCp*]₂[(μ³-H₂)IrCp*]}₂ (3). Computational studies indicate significant bonding character between U/Th and Ir in 1 and 2, with f-orbital involvement in the singly-occupied molecular orbitals of the uranium species 1. In addition, these studies attribute the variation in hydride bonding between 1 and 2 to differences in dispersion effects.

EXAFS provides the capability to interrogate nanoparticle (NP) structure in atomistic detail without relying on long-range crystallinity. There is a limitation in that EXAFS provides averaged structural information, making it difficult to separate a small amount of heterogeneous structure from bulk. In this work, models were developed to extract surface-specific information from conventional EXAFS measurements collected on UO2 NPs of varying size. Specifically, the surface terminating species of UO2 NPs was determined from comparison of coordination numbers with geometric models while the origin of static disorder was interrogated from user-defined simulations. Results show that the degree of oxygenation on the NP surface does not significantly deviate from bulk surface and that static disorder is highly enhanced in NP surface layers but cannot be attributed to surface relaxation effects alone. The approach described herein has the potential to be adapted to a range of inorganic NP systems to interrogate surface structure.

A single-source-precursor approach was developed to synthesize uranium-based materials outside of the typically-studied oxides. This approach allows for shorter reaction times, milder reaction conditions, and control over the chemicals present in synthesis. To this end, the first homoleptic uranium thioamidate complex was synthesized as a precursor for US₂ materials. Pyrolysis of the thioamidate results in decomposition via an alkene elimination pathway and formation of γ-US₂, which has historically been hard to access without the need for a secondary sulfur source. Despite the oxophilicity of uranium, the method successfully forms US₂ without the inclusion of oxygen in the bulk final product. These findings are supported by simultaneous thermal analysis, elemental analysis, powder X-ray diffraction, and uranium L₃-edge X-ray absorption fine-structure spectroscopy. This work represents the first example of a single-source precursor approach to target and synthesize actinide materials other than the oxides.

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 theoretical 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 higher 4f orbital overlap with the F 2p orbitals compared to the Cl 3p. This result is counterintuitive since the Ce-F bonds display less 4f covalency despite their higher 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 higher 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.

5f covalency in [U(C₇H₇)₂]^- was probed with carbon K-edge X-ray absorption spectroscopy (XAS) and electronic structure theory. The results revealed U 5f orbital participation in δ-bonding in both the ground- and core-excited states; additional 5f ϕ-mixing is observed in the core-excited states. Comparisons with U(C₈H₈)₂ show greater δ-covalency for [U(C₇H₇)₂]^-.

Correction for 'Enhanced 5f-δ bonding in [U(C₇H₇)₂]^-: C K-edge XAS, magnetism, and ab initio calculations' by Yusen Qiao et al., Chem. Commun., 2021, 57, 9562-9565, DOI: 10.1039/D1CC03414F.

We report the structural properties of ultra-small ThO₂ and UO₂ nanoparticles (NPs), which were synthesized without strong binding surface ligands by employing a covalent organic framework (COF-5) as an inert template. The resultant NPs were used to observe how structural properties are affected by decreasing grain size within bulk actinide oxides, which has implications for understanding the behavior of nuclear fuel materials. Through a comprehensive characterization strategy, we gain insight regarding how structure at the NP surface differs from the interior. Characterization using electron microscopy and small-angle X-ray scattering indicates that growth of the ThO₂ and UO₂ NPs was confined by the pores of the COF template, resulting in sub-3 nm particles. X-ray absorption fine structure spectroscopy results indicate that the NPs are best described as ThO₂ and UO₂ materials with unpassivated surfaces. The surface layers of these particles compensate for high surface energy by exhibiting a broader distribution of Th-O and U-O bond distances despite retaining average bond lengths that are characteristic of bulk ThO₂ and UO₂. The combined synthesis and physical characterization efforts provide a detailed picture of actinide oxide structure at the nanoscale, which remains highly underexplored compared to transition metal counterparts.