Determining relative f and d orbital contributions to M-Cl covalency in MCl6(2-) (M = Ti, Zr, Hf, U) and UOCl5(-) using Cl K-edge X-ray absorption spectroscopy and time-dependent density functional theory

The Counterintuitive Relationship between Orbital Energy, Orbital Overlap, and Bond Covalency in CeF62- and CeCl62

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.

A tetrahedral neptunium(V) complex

Neptunium is an actinide element sourced from anthropogenic production, and, unlike naturally abundant uranium, its coordination chemistry is not well developed in all accessible oxidation states. High-valent neptunium generally requires stabilization from at least one metal-ligand multiple bond, and departing from this structural motif poses a considerable challenge. Here we report a tetrahedral molecular neptunium(V) complex ([Np5+(NPC)4][B(ArF5)4], 1-Np) (NPC = [NPtBu(pyrr)2]-; tBu = C(CH3)3; pyrr = pyrrolidinyl (N(C2H4)2); B(ArF5)4 = tetrakis(2,3,4,5,6-pentafluourophenyl)borate). Single-crystal X-ray diffraction, solution-state spectroscopy and density functional theory studies of 1-Np and the product of its proton-coupled electron transfer (PCET) reaction, 2-Np, demonstrate the unique bonding that stabilizes this reactive ion and establishes the thermochemical and kinetic parameters of PCET in a condensed-phase transuranic complex. The isolation of this four-coordinate, neptunium(V) complex reveals a fundamental reaction pathway in transuranic chemistry.

Analyzing the Intensities of K-Edge Transitions in X2 Molecules (X = F, Cl, Br) for Use in Ligand K-Edge X-ray Absorption Spectroscopy

Ligand K-edge X-ray absorption spectroscopy (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 due 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 a 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.

Understanding covalency in molecular f-block compounds from the synergy of spectroscopy and quantum chemistry

One of the most intensely studied areas of f-block chemistry is the nature of the bonds between the f-element and another species, and in particular the role played by covalency. Computational quantum chemical methods have been at the forefront of this research for decades and have a particularly valuable role, given the radioactivity of the actinide series. The very strong agreement that has recently emerged between theory and the results of a range of spectroscopic techniques not only facilitates deeper insight into the experimental data, but it also provides confidence in the conclusions from the computational studies. These synergies are shining new light on the nature of the f element-other element bond.

Determination of Uranium Central-Field Covalency with 3d4f Resonant Inelastic X-ray Scattering

Understanding the nature of metal-ligand bonding is a major challenge in actinide chemistry. We present a new experimental strategy for addressing this challenge using actinide 3d4f resonant inelastic X-ray scattering (RIXS). Through a systematic study of uranium(IV) halide complexes, [UX6]2-, where X = F, Cl, or Br, we identify RIXS spectral satellites with relative energies and intensities that relate to the extent of uranium-ligand bond covalency. By analyzing the spectra in combination with ligand field density functional theory we find that the sensitivity of the satellites to the nature of metal-ligand bonding is due to the reduction of 5f interelectron repulsion and 4f-5f spin-exchange, caused by metal-ligand orbital mixing and the degree of 5f radial expansion, known as central-field covalency. Thus, this study furthers electronic structure quantification that can be obtained from 3d4f RIXS, demonstrating it as a technique for estimating actinide-ligand covalency.

Quantifying Covalency and Environmental Effects in RASSCF-Simulated O K-Edge XANES of Uranyl

A RASSCF approach to simulate the O K-edge XANES spectra of uranyl is employed, utilizing three models that progressively improve the representation of the local crystal environment. Simulations successfully reproduce the observed three-peak profile of the experimental spectrum and confirm peak assignments made by Denning. The [UO2Cl4]2- model offers the best agreement with experiment, with peak positions (to within 1 eV) and relative peak separations accurately reproduced. Establishing a direct link between a specific electronic transition and peak intensity is complicated, as a large number of possible transitions can contribute to the overall peak profile. Furthermore, a relationship between oxygen character in the antibonding orbital and the strength of the transition breaks down when using a variety of orbital composition approaches at larger excitation energy. Covalency analysis of the U-O bond in both the ground- and excited-state reveals a dependence on the crystal environment. Orbital composition analysis reveals an underestimation of the uranium contribution to ground-state bonding orbitals when probing O K-edge core-excited states, regardless of the uranyl model employed. However, improving the environmental model provides core-excited state electronic structures that are better representative of that of the ground-state, validating their use in the determination of covalency and bonding.

On the origin of low-valent uranium oxidation state

The significant interest in actinide bonding has recently focused on novel compounds with exotic oxidation states. However, the difficulty in obtaining relevant high-quality experimental data, particularly for low-valent actinide compounds, prevents a deeper understanding of 5f systems. Here we show X-ray absorption near-edge structure (XANES) measurements in the high-energy resolution fluorescence detection (HERFD) mode at the uranium M4 edge for the UIII and UIV halides, namely UX3 and UX4 (X = F, Cl, Br, I). The spectral shapes of these two series exhibit clear differences, which we explain using electronic structure calculations of the 3d-4f resonant inelastic X-ray scattering (RIXS) process. To understand the changes observed, we implemented crystal field models with ab initio derived parameters and investigated the effect of reducing different contributions to the electron-electron interactions involved in the RIXS process. Our analysis shows that the electron-electron interactions weaken as the ligand changes from I to F, indicative of a decrease in ionicity both along and between the UX3 and UX4 halide series.

Rechargeable Seawater-Based Chloride-Ion Batteries Enabled by Covalent Surface Chemistry in MXenes

Rechargeable aqueous chloride-ion batteries (ACIBs) using Cl- ions as charge carriers represent a promising energy-storage technology, especially when natural seawater is introduced as the electrolyte, which can bring remarkable advantages in terms of cost-effectiveness, safety, and environmental sustainability. However, the implementation of this technology is hindered by the scarcity of electrodes capable of reversible chloride-anion storage. Here, we show that a Ti3C2Clx MXene with Cl surface terminations enables reversible Cl- ion storage in aqueous electrolytes. Further, we developed seawater-based ACIBs that show a high specific capacity and an exceptionally long lifespan (40000 cycles, more than 1 year) in natural seawater electrolyte. The pouch-type cells achieve a high energy density (50 Wh Lcell-1) and maintain stable performance across a broad temperature range (-20 to 50 °C). Our investigations reveal that the covalent interaction between Cl- ions and Cl-terminated MXene facilitates Cl- ion intercalation into the MXene interlayer, promoting rapid ion migration with a low energy barrier (0.10 eV). Moreover, this MXene variant also enables the reversible storage of Br- ions in an aqueous electrolyte with a long cycle life. This study may advance the design of anion storage electrodes and enable the development of sustainable aqueous batteries.

Alkyl Coordination in meso-(ONO)2- Supported Uranium(IV) Complexes

A series of U(IV) complexes bearing alkyl and chloride ligands in the trans configuration was synthesized and characterized. Starting with the diastereopure U(IV) trans-dichloride complex meso-( tBu2PONO)UCl2(dtbpy) (1, tBu2PONO = 2,6-bis((di-tert-butylphosphino)methanolato)pyridine), four distinct alkyl groups were employed to prepare ( tBu2PONO)U(R)Cl(dtbpy), where R = (trimethylsilyl)methyl (neosilyl), 2a, R = 2,2-dimethyl propyl (neopentyl), 2b, and R = 2-methyl-2-phenyl propyl (neophyl), 2c. Alkylation occurs with specificity but generates a predominant species and a minor species corresponding to anti/syn regioisomers relative to the tBu2P groups of the ligand. For synthesis using R = methyl, the dimethyl complex ( tBu2PONO)U(Me)2(dtbpy), 2d, was prepared; the addition of 1 equiv of MeLi produced a mixture of products. Complexes 2a-2d were characterized using single crystal X-ray diffraction (SC-XRD), UV-vis-nIR, and 1H and 31P NMR spectroscopies.

Progress in Nonaqueous Molecular Uranium Chemistry: Where to Next?

There is long-standing interest in nonaqueous uranium chemistry because of fundamental questions about uranium's variable chemical bonding and the similarities of this pseudo-Group 6 element to its congener d-block elements molybdenum and tungsten. To provide historical context, with reference to a conference presentation slide presented around 1988 that advanced a defining collection of top targets, and the challenge, for synthetic actinide chemistry to realize in isolable complexes under normal experimental conditions, this Viewpoint surveys progress against those targets, including (i) CO and related π-acid ligand complexes, (ii) alkylidenes, carbynes, and carbidos, (iii) imidos and terminal nitrides, (iv) homoleptic polyalkyls, -alkoxides, and -aryloxides, (v) uranium-uranium bonds, and (vi) examples of topics that can be regarded as branching out in parallel from the leading targets. Having summarized advances from the past four decades, opportunities to build on that progress, and hence possible future directions for the field, are highlighted. The wealth and diversity of uranium chemistry that is described emphasizes the importance of ligand-metal complementarity in developing exciting new chemistry that builds our knowledge and understanding of elements in a relativistic regime.

Contemporary Assessment of Energy Degeneracy in Orbital Mixing with Tetravalent f-Block Compounds

The f block is a comparatively understudied group of elements that find applications in many areas. Continued development of technologies involving the lanthanides (Ln) and actinides (An) requires a better fundamental understanding of their chemistry. Specifically, characterizing the electronic structure of the f elements presents a significant challenge due to the spatially core-like but energetically valence-like nature of the f orbitals. This duality led f-block scientists to hypothesize for decades that f-block chemistry is dominated by ionic metal-ligand interactions with little covalency because canonical covalent interactions require both spatial orbital overlap and orbital energy degeneracy. Recent studies on An compounds have suggested that An ions can engage in appreciable orbital mixing between An 5f and ligand orbitals, which was attributed to "energy-degeneracy-driven covalency". This model of bonding has since been a topic of debate because different computational methods have yielded results that support and refute the energy-degeneracy-driven covalency model. In this Viewpoint, literatures concerning the metal- and ligand-edge X-ray absorption near-edge structure (XANES) of five tetravalent f-block systems─MO2 (M = Ln, An), LnF4, MCl62-, and [Ln(NP(pip)3)4]─are compiled and discussed to explore metal-ligand bonding in f-block compounds through experimental metrics. Based on spectral assignments from a variety of theoretical models, covalency is seen to decrease from CeO2 and PrO2 to TbO2 through weaker ligand-to-metal charge-transfer (LMCT) interactions, while these LMCT interactions are not observed in the trivalent Ln sesquixodes until Yb. In comparison, while XANES characterization of AnO2 compounds is scarce, computational modeling of available X-ray absorption spectra suggests that covalency among AnO2 reaches a maximum between Am and Cm. Moreover, a decrease in covalency is observed upon changing ligands while maintaining an isostructural coordination environment from CeO2 to CeF4. These results could allude to the importance of orbital energy degeneracy in f-block bonding, but there are a variety of data gaps and conflicting results from different modeling techniques that need to be addressed before broad conclusions can be drawn.

Perplexing EPR Signals from 5f36d1 U(II) Complexes

Metal complexes with unpaired electrons in orbitals of different angular momentum quantum numbers (e.g., f and d orbitals) are unusual and opportunities to study the interactions among these electrons are rare. X-band electron paramagnetic resonance (EPR) data were collected at <10 and 77 K on 10 U(II) complexes with 5f36d1 electron configurations and on some analogous Ce(II), Pr(II), and Nd(II) complexes with 4fn5d1 electron configurations. The U(II) compounds unexpectedly display similar two-line axial signals with g|| = 2.04 and g⊥ = 2.00 at 77 K. In contrast, U(II) complexes with 5f4 configurations are EPR-silent. Unlike U(II), the congenic 4f35d1 Nd(II) complex is EPR-silent. The Ce(II) complex with a 4f15d1 configuration is also EPR-silent, but a signal is observed for the Pr(II) complex, which has a 4f25d1 configuration. Whether or not an EPR signal is expected for these complexes depends on the coupling between f and d electrons. Since the coupling in U(II) systems is expected to be sufficiently strong to preclude an EPR signal from compounds with a 5f36d1 configuration, the results are viewed as unexplained phenomena. However, they do show that 5f36d1 U(II) samples can be differentiated from 5f4 U(II) complexes by EPR spectroscopy.

Are 4f-Orbitals Engaged in Covalent Bonding Between Lanthanides and Triphenylphosphine Oxide? An Oxygen K-Edge X-ray Absorption Spectroscopy and Density Functional Theory Study

The bonding covalency between trivalent lanthanides (Ln = La, Pr, Nd, Eu, Gd) and triphenylphosphine oxide (TPPO) is studied by X-ray absorption spectra (XAS) and density functional theory (DFT) calculations on the LnCl3(TPPO)3 complexes. The O, P, and Cl K-edge XAS for the single crystals of LnCl3(TPPO)3 were collected, and the spectra were interpreted based on DFT calculations. The O and P K-edge XAS spectra showed no significant change across the Ln series in the LnCl3(TPPO)3 complexes, unlike the Cl K-edge XAS spectra. The experimental O K-edge XAS spectra suggest no mixing between the Ln 4f- and the O 2p-orbitals in the LnCl3(TPPO)3 complexes. DFT calculations indicate that the amount of the O 2p character per Ln-O bond is less than 0.1% in the Ln 4f-based orbitals in all of the LnCl3(TPPO)3 complexes. The experimental spectra and theoretical calculations demonstrate that Ln 4f-orbitals are not engaged in the covalent bonding of lanthanides with TPPO, which contrasts the involvement of U 5f-orbitals in covalent bonding in the UO2Cl2(TPPO)2 complex. Results in this work reinforce our previous speculation that bonding covalency is potentially responsible for the extractability of monodentate organophosphorus ligands toward metal ions.

31P Nuclear Magnetic Resonance Spectroscopy as a Probe of Thorium-Phosphorus Bond Covalency: Correlating Phosphorus Chemical Shift to Metal-Phosphorus Bond Order

We report the use of solution and solid-state 31P Nuclear Magnetic Resonance (NMR) spectroscopy combined with Density Functional Theory calculations to benchmark the covalency of actinide-phosphorus bonds, thus introducing 31P NMR spectroscopy to the investigation of molecular f-element chemical bond covalency. The 31P NMR data for [Th(PH2)(TrenTIPS)] (1, TrenTIPS = {N(CH2CH2NSiPri3)3}3-), [Th(PH)(TrenTIPS)][Na(12C4)2] (2, 12C4 = 12-crown-4 ether), [{Th(TrenTIPS)}2(μ-PH)] (3), and [{Th(TrenTIPS)}2(μ-P)][Na(12C4)2] (4) demonstrate a chemical shift anisotropy (CSA) ordering of (μ-P)3- > (═PH)2- > (μ-PH)2- > (-PH2)1- and for 4 the largest CSA for any bridging phosphido unit. The B3LYP functional with 50% Hartree-Fock mixing produced spin-orbit δiso values that closely match the experimental data, providing experimentally benchmarked quantification of the nature and extent of covalency in the Th-P linkages in 1-4 via Natural Bond Orbital and Natural Localized Molecular Orbital analyses. Shielding analysis revealed that the 31P δiso values are essentially only due to the nature of the Th-P bonds in 1-4, with largely invariant diamagnetic but variable paramagnetic and spin-orbit shieldings that reflect the Th-P bond multiplicities and s-orbital mediated transmission of spin-orbit effects from Th to P. This study has permitted correlation of Th-P δiso values to Mayer bond orders, revealing qualitative correlations generally, but which should be examined with respect to specific ancillary ligand families rather than generally to be quantitative, reflecting that 31P δiso values are a very sensitive reporter due to phosphorus being a soft donor that responds to the rest of the ligand field much more than stronger, harder donors like nitrogen.

Probing Substituent Effect on Nickel-Sulfur Bond Covalency in Ni(II)-Dithiophosphinate Complexes by Sulfur K-Edge XAS and DFT Calculations

Complexes of Ni(II) with a series of aryl or alkyl substituent dithiophosphinic acids were characterized by crystallographic structure, sulfur K-edge X-ray absorption spectroscopy (XAS), and density functional theory (DFT). In these complexes, Ni(II) coordinates with four sulfur atoms from two dithiophosphinate anions form a well-defined square-planar structure. Despite the minor differences in the geometry parameters among the complexes, the electronic structure is affected significantly by the substituent group attached to dithiophosphinic acid. In particular, the addition of ortho-CF3 group to the aryl ring constrains the orientation of the aryl ring and enhances the conjugation between the aryl ring and the coordinating core. Sulfur K-edge XAS spectra help further reveal the electronic structure of the complexes. Both the pre-edge feature and rising-edge feature provide abundant information on the molecular orbitals and show a distinctive effect of the substituent groups on the electronic structure of the complexes, which is supposedly relevant to the ligand's performance in Ln(III)/An(III) separation.

Bounding [AnO2]2+ (An = U, Np) covalency by simulated O K-edge and An M-edge X-ray absorption near-edge spectroscopy

Restricted active space simulations are shown to accurately reproduce and characterise both O K-edge and U M4,5-edge spectra of uranyl in excellent agreement with experimental peak positions and are extended to the Np analogue. Analysis of bonding orbital composition in the ground and O K-edge core-excited states demonstrates that metal contribution is underestimated in the latter. In contrast, An M4/5-edge core-excited states produce bonding orbital compositions significantly more representative of those in the ground state. Quantum Theory of Atoms in Molecules analysis is employed to explain the discrepancy between K- and M-edge data and demonstrates that the location of the core-hole impacts the pattern of electron localisation in core-excited states. An apparent contradiction to this behaviour in neptunyl is rationalised in terms interelectronic repulsion between the unpaired 5f electron and the excited core-electron.

Tetrathiafulvalene-2,3,6,7-tetrathiolate linker redox-state elucidation via S K-edge X-ray absorption spectroscopy

Sulfur K-edge XAS data provide a unique tool to examine oxidation states and covalency in electronically complex S-based ligands. We present sulfur K-edge X-ray absorption spectroscopy on a discrete redox-series of Ni-based tetrathiafulvalene tetrathiolate (TTFtt) complexes as well as on a 1D coordination polymer (CP), NiTTFtt. Experiment and theory suggest that Ni-S covalency decreases with oxidation which has implications for charge transport pathways.

Core Excitations of Uranyl in Cs2UO2Cl4 from Relativistic Embedded Damped Response Time-Dependent Density Functional Theory Calculations

X-ray spectroscopies, by their high selectivity and sensitivity to the chemical environment around the atoms probed, provide significant insights into the electronic structures of molecules and materials. Interpreting experimental results requires reliable theoretical models, accounting for environmental, relativistic, electron correlation, and orbital relaxation effects in a balanced manner. In this work, we present a protocol for the simulation of core excited spectra with damped response time-dependent density functional theory based on the Dirac-Coulomb Hamiltonian (4c-DR-TD-DFT), in which environmental effects are accounted for through the frozen density embedding (FDE) method. We showcase this approach for the uranium M4- and L3-edges and oxygen K-edge of the uranyl tetrachloride (UO2Cl42-) unit as found in a host Cs2UO2Cl4 crystal. We have found that the 4c-DR-TD-DFT simulations yield excitation spectra that very closely match the experiment for the uranium M4-edge and the oxygen K-edge, with good agreement for the broad experimental spectra for the L3-edge. By decomposing the complex polarizability in terms of its components, we have been able to correlate our results with angle-resolved spectra. We have observed that for all edges, but in particular the uranium M4-edge, an embedded model in which the chloride ligands are replaced by an embedding potential reproduces rather well the spectral profile obtained for UO2Cl42-. Our results underscore the importance of the equatorial ligands to simulating core spectra at both uranium and oxygen edges.

Synthesis and comparison of iso-structural f-block metal complexes (Ce, U, Np, Pu) featuring η6-arene interactions

Reaction of the terphenyl bis(anilide) ligand [{K(DME)₂}₂L^Ar] (L^Ar = {C₆H₄[(2,6-ⁱPr²C₆H₃)NC₆H₄]₂}^2-) with trivalent chloride "MCl₃" salts (M = Ce, U, Np) yields two distinct products; neutral L^ArM(Cl)(THF) (1^M) (M = Np, Ce), and the "-ate" complexes [K(DME)₂][(L^Ar)Np(Cl)₂] (2^Np) or ([L^ArM(Cl)₂(μ-K(X)₂)])_∞ (2^Ce, 2^U) (M = Ce, U) (X = DME or Et₂O) (2^M). Alternatively, analogous reactions with the iodide [MI₃(THF)₄] salts provide access to the neutral compounds L^ArM(I)(THF) (3^M) (M = Ce, U, Np, Pu). All complexes exhibit close arene contacts suggestive of η⁶-interactions with the central arene ring of the terphenyl backbone, with 3^M comprising the first structurally characterized Pu η⁶-arene moiety. Notably, the metal-arene bond metrics diverge from the predicted trends of metal-carbon interactions based on ionic radii, with the uranium complexes exhibiting the shortest M-C_centroid distance in all cases. Overall, the data presents a systematic study of f-element M-η⁶-arene complexes across the early actinides U, Np, Pu, and comparison to cerium congeners.

P K-Edge XANES Calculations of Mineral Standards: Exploring the Potential of Theoretical Methods in the Analysis of Phosphorus Speciation

Phosphorus K-edge X-ray absorption near-edge structure (XANES) spectroscopy is a technique routinely employed in the qualitative and quantitative analysis of phosphorus speciation in many scientific fields. The data analysis is, however, often performed in a qualitative manner, relying on linear combination fitting protocols or simple comparisons between the experimental data and the spectra of standards, and little quantitative structural and electronic information is thus retrieved. Herein, we report a thorough theoretical investigation of P K-edge XANES spectra of NaH₂PO₄·H₂O, AlPO₄, α-Ti(HPO₄)₂·H₂O, and FePO₄·2H₂O showing excellent agreement with the experimental data. We find that different coordination shells of phosphorus, up to a distance of 5-6 Å from the photoabsorber, contribute to distinct features in the XANES spectra. This high structural sensitivity enables P K-edge XANES spectroscopy to even distinguish between nearly isostructural crystal phases of the same compound. Additionally, we provide a rationalization of the pre-edge transitions observed in the spectra of α-Ti(HPO₄)₂·H₂O and FePO₄·2H₂O through density of states calculations. These pre-edge transitions are found to be enabled by the covalent mixing of phosphorus s and p orbitals and titanium or iron d orbitals, which happens even though neither metal ion is directly bound to phosphorus in the two systems.

Evaluating f-Orbital Participation in the UV═E Multiple Bonds of [U(E)(NR2)3] (E = O, NSiMe3, NAd; R = SiMe3)

The reaction of 1 equiv of 1-azidoadamantane with [U^III(NR₂)₃] (R = SiMe₃) in Et₂O results in the formation of [U^V(NR₂)₃(NAd)] (1, Ad = 1-adamantyl) in good yields. The electronic structure of 1, as well as those of the related U(V) complexes, [U^V(NR₂)₃(NSiMe₃)] (2) and [U^V(NR₂)₃(O)] (3), were analyzed with EPR spectroscopy, SQUID magnetometry, NIR-visible spectroscopy, and crystal field modeling. This analysis revealed that, within this series of complexes, the steric bulk of the E^2- (E═O, NR) ligand is the most important factor in determining the electronic structure. In particular, the increasing steric bulk of this ligand, on moving from O^2- to [NAd]^2-, results in increasing U═E distances and E-U-N_amide angles. These changes have two principal effects on the resulting electronic structure: (1) the increasing U═E distances decreases the energy of the f_σ orbital, which is primarily σ* with respect to the U═E bond, and (2) the increasing E-U-N_amide angles increases the energy of f_δ, due to increasing antibonding interactions with the amide ligands. As a result of the latter change, the electronic ground state for complexes 1 and 2 is primarily f_φ in character, whereas the ground state for complex 3 is primarily f_δ.

Trends in the Electronic Structure and Chemical Bonding of a Series of Porphyrinoid-Uranyl Complexes

In this paper, we have explored the relativistic density functional theory study on a series of deprotonated porphyrinoid (Lⁿ) complexes of uranyl to investigate the geometrical structures and chemical bonding. The ligands bound with uranyl in the 1:1 complexes [UO₂(Lⁿ)]^x (n = 4, 5, 6; x = 0, -1, -2), showing more thermodynamic stability for "in-cavity" structures of L⁵ and L⁶ than that of the "side-on" structure of L⁴ and an increase in stability with the increase of negative charges, L^2- < L^3- < L^4-. Among the six ligands, the cyclo[6]pyrrole presents the best selectivity toward uranyl. Based on chemical bonding analyses, the U-N_L bond in the in-cavity complexes adopts a typical dative N_L → U bond with mainly ionic bonding and significant covalency, which comes from the significant orbital interaction of U 5f_ϕ6d_δ7s hybrid AOs and N_L 2p-based MOs. This work provides a systematic understanding of the coordination chemistry in uranyl pyrrole-containing macrocycle complexes and the nature of chemical bonding in such systems, which may provide inspirations for the future design of synthetic targets that could be relevant to actinide separations or in the remediation of spent nuclear fuel.

Aqueous Speciation of Tetravalent Actinides in the Presence of Chloride and Nitrate Ligands

Speciation of hexachloride tetravalent uranium, neptunium, and plutonium species in aqueous media has been investigated using density functional theory in the presence of inner sphere ligands such as chloride, nitrate, and solvent molecules. All possible structures with the formula [An^IV(Cl)_x(H₂O)_y(NO₃)_z]^4-x-z (An = U, Np, and Pu; x = 0-6, y = 0-8, and z = 0-6) were considered to explore the speciation chemical space of each actinide. The nature of the mixed-ligand complexes present in solution is controlled by the concentration of free ligands in solution. A low chloride concentration is suitable to drive the speciation away from the highly thermodynamically stable hexachloride species. Furthermore, the formation of dimeric species can proceed through both olation and oxolation mechanisms. Oxolation is preferred for monomers that contain fewer water ligands, while olation becomes favorable for complexes with more water ligands.

Uranyl Analogue Complexes—Current Progress and Synthetic Challenges

Uranyl ions, {UO2}n+ (n = 1, 2), display trans, strongly covalent, and chemically robust U-O multiple bonds, where 6d, 5f, and 6p orbitals play important roles. The synthesis of isoelectronic analogues of uranyl has been of interest for quite some time, mainly with the purpose of unveiling covalence and 5f-orbital participation in bonding. Significant advances have occurred in the last two decades, initially marked by the synthesis of uranium(VI) bis(imido) complexes, the first analogues with a {RNUNR}2+ core, later followed by the synthesis of unique trans-{EUO}2+ (E = S, Se) complexes, and recently highlighted by the synthesis of the first complexes featuring a linear {NUN} moiety. This review covers the synthesis, structure, bonding, and reactivity of uranium complexes containing a linear {EUE}n+ core (n = 0, 1, 2), isoelectronic to uranyl ions, {OUO}n+ (n = 1, 2), incorporating σ- and π-donating ligands that can engage in uranium–ligand multiple bonding, where oxygen may be replaced by heavier chalcogenido, imido, nitride, and carbene ligands, or by a transition metal. It focuses on synthetic methods of well-defined molecular uranium species in the condensed phase but also references gas-phase and low-temperature-matrix experiments, as well as computational studies that may lead to valuable insights.

Lanthanide Complexes Containing a Terminal Ln═O Oxo Bond: Revealing Higher Stability of Tetravalent Praseodymium versus Terbium

We report on the reactivity of gas-phase lanthanide-oxide nitrate complexes, [Ln(O)(NO₃)₃]^- (denoted LnO²⁺), produced via elimination of NO₂^• from trivalent [Ln^III(NO₃)₄]^- (Ln = Ce, Pr, Nd, Sm, Tb, Dy). These complexes feature a Ln^III-O^• oxyl, a Ln^IV═O oxo, or an intermediate Ln^III/IV oxyl/oxo bond, depending on the accessibility of the tetravalent Ln^IV state. Hydrogen atom abstraction reactivity of the LnO²⁺ complexes to form unambiguously trivalent [Ln^III(OH)(NO₃)₃]^- reveals the nature of the oxide bond. The result of slower reactivity of PrO²⁺ versus TbO²⁺ is considered to indicate higher stability of the tetravalent praseodymium-oxo, Pr^IV═O, versus Tb^IV═O. This is the first report of Pr^IV as more stable than Tb^IV, which is discussed with respect to ionization potentials, standard electrode potentials, atomic promotion energies, and oxo bond covalency via 4f- and/or 5d-orbital participation.

Relativistic Orbital-Optimized Density Functional Theory for Accurate Core-Level Spectroscopy

Core-level spectra of 1s electrons of elements heavier than Ne show significant relativistic effects. We combine advances in orbital-optimized density functional theory (OO-DFT) with the spin-free exact two-component (X2C) model for scalar relativistic effects to study K-edge spectra of third period elements. OO-DFT/X2C is found to be quite accurate at predicting energies, yielding a ∼0.5 eV root-mean-square error versus experiment with the modern SCAN (and related) functionals. This marks a significant improvement over the >50 eV deviations that are typical for the popular time-dependent DFT (TDDFT) approach. Consequently, experimental spectra are quite well reproduced by OO-DFT/X2C, sans empirical shifts for alignment. OO-DFT/X2C combines high accuracy with ground state DFT cost and is thus a promising route for computing core-level spectra of third period elements. We also explored K and L edges of 3d transition metals to identify limitations of the OO-DFT/X2C approach in modeling the spectra of heavier atoms.

Relativistic Effects in Modeling the Ligand K-Edge X-ray Absorption Near-Edge Structure of Uranium Complexes

Accurate modeling of the complex electronic structure of actinide complexes requires full inclusion of relativistic effects. In this study, we examine the effect of explicit inclusion of spin-orbit coupling (SOC) versus scalar relativistic effects on the predicted spectra for heavy-element complexes. In this study, we employ a relativistic two-component Hamiltonian in the X2C form with all of the electrons in the system being considered explicitly to compare and contrast with previous studies that included the relativistic effects by means of relativistic effective core potentials (RECPs). A few uranium complexes are chosen as model systems. Comparison of the computed Cl K-edge X-ray absorption spectra with experimental data shows significantly improved agreement when a variational relativistic treatment of SOC is performed. In particular, we note the importance of SOC terms to obtain not only correct transition energies but also correct intensities for these heavy-element complexes because of the redistribution of ligand bonding character among the valence MOs. While RECPs generally agree well with all-electron scalar relativistic calculations, there are some differences in the predicted spectra of open-shell systems. These methods are still suitable for broad application to analyze the qualitative nature of transitions in X-ray absorption spectra, but caution is recommended for quantitative analysis, as SOC can be non-negligible for both open- and closed-shell heavy-element systems.

A Combined Experimental and Theoretical Investigation of Arene-Supported Actinide and Ytterbium Tetraphenolate Complexes

Modular tetraphenolate ligands tethered with a protective arene platform (para-phenyl or para-terphenyl) are used to support mononuclear An(IV) (An = Th, U) complexes with an exceptionally large and open axial coordination site at the metal. The base-free complexes and a series of neutral donor adducts were synthesized and characterized by spectroscopies and single-crystal X-ray diffraction. Anionic Th(IV) -ate complexes with an additional axial aryloxide ligand were also synthesized and characterized. The para-phenyl-tethered mononuclear complexes exhibit rare An(IV)-arene interactions, and the An(IV)-arene distance broadly increases with axial donor strength. The para-terphenyl-tethered complexes have almost no interaction with the arene base, isolating the central metal cation. Computational analysis of the mononuclear complexes and their reduced analogues, and Yb(III) congeners, as well as the effect of additional donor ligand binding, seek to elucidate the electronic structure of the metal-arene interactions and establish whether they, or their reduced or oxidized counterparts, could function as molecular qubits.

Stable Chelation of the Uranyl Ion by Acyclic Hexadentate Ligands: Potential Applications for 230U Targeted α-Therapy

Uranium-230 is an α-emitting radionuclide with favorable properties for use in targeted α-therapy (TAT), a type of nuclear medicine that harnesses α particles to eradicate cancer cells. To successfully implement this radionuclide for TAT, a bifunctional chelator that can stably bind uranium in vivo is required. To address this need, we investigated the acyclic ligands H2dedpa, H2CHXdedpa, H2hox, and H2CHXhox as uranium chelators. The stability constants of these ligands with UO22+ were measured via spectrophotometric titrations, revealing log βML values that are greater than 18 and 26 for the "pa" and "hox" chelators, respectively, signifying that the resulting complexes are exceedingly stable. In addition, the UO22+ complexes were structurally characterized by NMR spectroscopy and X-ray crystallography. Crystallographic studies reveal that all six donor atoms of the four ligands span the equatorial plane of the UO22+ ion, giving rise to coordinatively saturated complexes that exclude solvent molecules. To further understand the enhanced thermodynamic stabilities of the "hox" chelators over the "pa" chelators, density functional theory (DFT) calculations were employed. The use of the quantum theory of atoms in molecules revealed that the extent of covalency between all four ligands and UO22+ was similar. Analysis of the DFT-computed ligand strain energy suggested that this factor was the major driving force for the higher thermodynamic stability of the "hox" ligands. To assess the suitability of these ligands for use with 230U TAT in vivo, their kinetic stabilities were probed by challenging the UO22+ complexes with the bone model hydroxyapatite (HAP) and human plasma. All four complexes were >95% stable in human plasma for 14 days, whereas in the presence of HAP, only the complexes of H2CHXdedpa and H2hox remained >80% intact over the same period. As a final validation of the suitability of these ligands for radiotherapy applications, the in vivo biodistribution of their UO22+ complexes was determined in mice in comparison to unchelated [UO2(NO3)2]. In contrast to [UO2(NO3)2], which displays significant bone uptake, all four ligand complexes do not accumulate in the skeletal system, indicating that they remain stable in vivo. Collectively, these studies suggest that the equatorial-spanning ligands H2dedpa, H2CHXdedpa, H2hox, and H2CHXhox are highly promising candidates for use in 230U TAT.

X-ray absorption spectra of f-element complexes: insight from relativistic multiconfigurational wavefunction theory

X-ray absorption near edge structure (XANES) spectroscopy, coupled with ab initio calculations, has emerged as the state-of-the-art tool for elucidating the metal-ligand bonding in f-element complexes. This highlight presents recent efforts in calculating XANES spectra of lanthanide and actinide compounds with relativistic multiconfiguration wavefunction approaches that account for differences in donation bonding in the ground state (GS) versus a core-excited state (ES), multiplet effects, and spin-orbit-coupling. With the GS and ES wavefunctions available, including spin-orbit effects, an arsenal of chemical bonding tools that are popular among chemists can be applied to rationalize the observed intensities in terms of covalent bonding.

Covalency between the uranyl ion and dithiophosphinate by sulfur K-edge X-ray absorption spectroscopy and density functional theory

The dithiophosphinic acids (HS2PR2) have been used for the selective separation of trivalent actinides (AnIII) from lanthanides (LnIII) over the past decades. The substituents on the dithiophosphinic acids dramatically impact the separation performance, but the mechanism is still open for debate. In this work, two dithiophosphinic acids with significantly different AnIII/LnIII separation performance, i.e. diphenyl dithiophosphinic acid (HS2PPh2) and bis(ortho-trifluoromethylphenyl) dithiophosphinic acid [HS2P(o-CF3C6H4)2], are employed to understand the substituent effect on the bonding covalency between the S2PR2- anions (R = Ph and o-CF3C6H4) and the uranyl ion by sulfur K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory calculations. The two UO2(S2PR2)(EtOH) complexes display similar XAS spectra, in which the first pre-edge feature with an intensity of 0.16 is entirely attributed to the transitions from S 1s orbitals to the unoccupied molecular orbitals due to the mixing between U 5f and S 3p orbitals. The Mulliken population analysis indicates that the amount of \% S 3p character in these orbitals is essentially identical for the UO2(S2PPh2)2(EtOH) and UO2[S2P(o-CF3C6H4)2]2(EtOH) complexes, which is lower than that in the U 6d-based orbitals. The essentially identical covalency in U-S bonds for the two UO2(S2PR2)2(EtOH) complexes are contradictory to the significantly different AnIII/LnIII separation performance of the two dithiophosphinic acids, thus the covalency seems to be unable to account for substituent effects in the AnIII/LnIII separation by the dithiophosphinic acids. The results in this work provide valuable insight into the understanding of the mechanism in the AnIII/LnIII separation by the dithiophosphinic acids.

A theoretical investigation of uranyl covalency via symmetry-preserving excited state structures

The structures of electronically excited states of uranyl are probed via density-based analysis to deepen understanding of uranium bonding.

Hexanitrato complexes and hybrid double perovskites of Am3+ and Cm3+

Isostructural, homoleptic twelve-coordinate nitrato complexes of the 5f-ions Am and Cm are reported that occur as hybrid double perovskites.

[AnI3(THF)4] (An = Np, Pu) Preparation Bypassing An0 Metal Precursors: Access to Np3+/Pu3+ Nonaqueous and Organometallic Complexes

Direct comparison of homologous molecules provides a foundation from which to elucidate both subtle and patent changes in reactivity patterns, redox processes, and bonding properties across a series of elements. While trivalent molecular U chemistry is richly developed, analogous Np or Pu research has long been hindered by synthetic routes often requiring scarcely available metallic-phase source material, high-temperature solid-state reactions producing poorly soluble binary halides, or the use of pyrophoric reagents. The development of routes to nonaqueous Np³⁺/Pu³⁺ from widely available precursors can potentially transform the scope and pace of research into actinide periodicity. Here, aqueous stocks of An⁴⁺ (An = Np, Pu) are dehydrated to well-defined [AnCl₄(DME)₂] (DME = 1,2-dimethoxyethane), and then a single-step halide exchange/reduction employing Me₃SiI produces [AnI₃(THF)₄] (THF = tetrahydrofuran) in a high to nearly quantitative crystalline yield (with I₂ and Me₃SiCl as easily removed byproducts). We demonstrate the synthetic utility of these An-iodide molecules, prepared by metal⁰-free routes, through characterization of archetypal complexes including the tris-silylamide, [Np{N(SiMe₃)₂}₃], and bent metallocenes, [An(C₅Me₅)₂(I)(THF)] (An = Np, Pu)─chosen because both motifs are ubiquitous in Th, U, and lanthanide research. The synthesis of [Np{N(Se═PPh₂)₂}₃] is also reported, completing an isomorphous series that now extends from U to Am and is the first characterized Np³⁺-Se bond.

Involvement of 5f Orbitals in the Covalent Bonding between the Uranyl Ion and Trialkyl Phosphine Oxide: Unraveled by Oxygen K-Edge X-ray Absorption Spectroscopy and Density Functional Theory

Monodentate organophosphorus ligands have been used for the extraction of the uranyl ion (UO₂²⁺) for over half a century and have exhibited exceptional extractability and selectivity toward the uranyl ion due to the presence of the phosphoryl group (O═P). Tributyl phosphate (TBP) is the extractant of the world-renowned PUREX process, which selectively recovers uranium from spent nuclear fuel. Trialkyl phosphine oxide (TRPO) shows extractability toward the uranyl ion that far exceeds that for other metal ions, and it has been used in the TRPO process. To date, however, the mechanism of the high affinity of the phosphoryl group for UO₂²⁺ remains elusive. We herein investigate the bonding covalency in a series of complexes of UO₂²⁺ with TRPO by oxygen K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory (DFT) calculations. Four TRPO ligands with different R substituents are examined in this work, for which both the ligands and their uranyl complexes are crystallized and investigated. The study of the electronic structure of the TRPO ligands reveals that the two TRPO molecules, irrespective of their substituents, can engage in σ- and π-type interactions with U 5f and 6d orbitals in the UO₂Cl₂(TRPO)₂ complexes. Although both the axial (O_yl) and equatorial (O_eq) oxygen atoms in the UO₂Cl₂(TRPO)₂ complexes contribute to the X-ray absorption, the first pre-edge feature in the O K-edge XAS with a small intensity is exclusively contributed by O_eq and is assigned to the transition from O_eq 1s orbitals to the unoccupied molecular orbitals of 1b_1u + 1b_2u + 1b_3u symmetries resulting from the σ- and π-type mixing between U 5f and O_eq 2p orbitals. The small intensity in the experimental spectra is consistent with the small amount of O_eq 2p character in these orbitals for the four UO₂Cl₂(TRPO)₂ complexes as obtained by Mulliken population analysis. The DFT calculations demonstrate that the U 6d orbitals are also involved in the U-TRPO bonding interactions in the UO₂Cl₂(TRPO)₂ complexes. The covalent bonding interactions between TRPO and UO₂²⁺, especially the contributions from U 5f orbitals, while appearing to be small, are sufficiently responsible for the exceptional extractability and selectivity of monodentate organophosphorus ligands for the uranyl ion. Our results provide valuable insight into the fundamental actinide chemistry and are expected to directly guide actinide separation schemes needed for the development of advanced nuclear fuel cycle technologies.

Exceptional uranium(VI)-nitride triple bond covalency from 15N nuclear magnetic resonance spectroscopy and quantum chemical analysis

Determining the nature and extent of covalency of early actinide chemical bonding is a fundamentally important challenge. Recently, X-ray absorption, electron paramagnetic, and nuclear magnetic resonance spectroscopic studies have probed actinide-ligand covalency, largely confirming the paradigm of early actinide bonding varying from ionic to polarised-covalent, with this range sitting on the continuum between ionic lanthanide and more covalent d transition metal analogues. Here, we report measurement of the covalency of a terminal uranium(VI)-nitride by 15N nuclear magnetic resonance spectroscopy, and find an exceptional nitride chemical shift and chemical shift anisotropy. This redefines the 15N nuclear magnetic resonance spectroscopy parameter space, and experimentally confirms a prior computational prediction that the uranium(VI)-nitride triple bond is not only highly covalent, but, more so than d transition metal analogues. These results enable construction of general, predictive metal-ligand 15N chemical shift-bond order correlations, and reframe our understanding of actinide chemical bonding to guide future studies.

Competitive Coordination of Chloride and Fluoride Anions Towards Trivalent Lanthanide Cations (La3+ and Nd3+ ) in Molten Salts

Molten salt electrolysis is a vital technique to produce high-purity lanthanide metals and alloys. However, the coordination environments of lanthanides in molten salts, which heavily affect the related redox potential and electrochemical properties, have not been well elucidated. Here, the competitive coordination of chloride and fluoride anions towards lanthanide cations (La³⁺ and Nd³⁺ ) is explored in molten LiCl-KCl-LiF-LnCl₃ salts using electrochemical, spectroscopic, and computational approaches. Electrochemical analyses show that significant negative shifts in the reduction potential of Ln³⁺ occur when F^- concentration increases, indicating that the F^- anions interact with Ln³⁺ via substituting the coordinated Cl^- anions, and confirm [LnCl_x F_y ]^3-x-y (y_max =3) complexes are prevailing in molten salts. Spectroscopic and computational results on solution structures further reveal the competition between Cl^- and F^- anions, which leads to the formation of four distinct Ln(III) species: [LnCl₆ ]^3- , [LnCl₅ F]^3- , [LnCl₄ F₂ ]^3- and [LnCl₄ F₃ ]^4- . Among them, the seven-coordinated [LnCl₄ F₃ ]^4- complex possesses a low-symmetry structure evidenced by the pattern change of Raman spectra. After comparing the polarizing power (Z/r) among different metal cations, it was concluded that Ln-F interaction is weaker than that between transition metal and F^- ions.

Near-infrared C-term MCD spectroscopy of octahedral uranium(V) complexes

C-term magnetic circular dichroism (MCD) spectroscopy is a powerful method for probing d-d and f-f transitions in paramagnetic metal complexes. However, this technique remains underdeveloped both experimentally and theoretically for studies of U(v) complexes of Oh symmetry, which have been of longstanding interest for probing electronic structure, bonding, and covalency in 5f systems. In this study, C-term NIR MCD of the Laporte forbidden f-f transitions of [UCl6]- and [UF6]- are reported, demonstrating the significant fine structure resolution possible with this technique including for the low energy Γ7 → Γ8 transitions in [UF6]-. The experimental NIR MCD studies were further extended to [U(OC6F5)6]-, [U(CH2SiMe3)6]-, and [U(NC(tBu)(Ph))6]- to evaluate the effects of ligand-type on the f-f MCD fine structure features. Theoretical calculations were conducted to determine the Laporte forbidden f-f transitions and their MCD intensity experimentally observed in the NIR spectra of the U(v) hexahalide complexes, via the inclusion of vibronic coupling, to better understand the underlying spectral fine structure features for these complexes. These spectra and simulations provide an important platform for the application of MCD spectroscopy to this widely studied class of U(v) complexes and identify areas for continued theoretical development.

Structural and Spectroscopic Comparison of Soft-Se vs. Hard-O Donor Bonding in Trivalent Americium/Neodymium Molecules

Covalency is often considered to be an influential factor in driving An³⁺ vs. Ln³⁺ selectivity invoked by soft donor ligands. This is intensely debated, particularly the extent to which An³⁺ /Ln³⁺ covalency differences prevail and manifest as the f-block is traversed, and the effects of periodic breaks beyond Pu. Herein, two Am complexes, [Am{N(E=PPh₂ )₂ }₃ ] (1-Am, E=Se; 2-Am, E=O) are compared to isoradial [Nd{N(E=PPh₂ )₂ }₃ ] (1-Nd, 2-Nd) complexes. Covalent contributions are assessed and compared to U/La and Pu/Ce analogues. Through ab initio calculations grounded in UV-vis-NIR spectroscopy and single-crystal X-ray structures, we observe differences in f orbital involvement between Am-Se and Nd-Se bonds, which are not present in O-donor congeners.

Efficient discrimination of transplutonium actinides by in vivo models

Transplutonium actinides are among the heaviest elements whose macroscale chemical properties can be experimentally tested. Being scarce and hazardous, their chemistry is rather unexplored, and they have traditionally been considered a rather homogeneous group, with most of their characteristics extrapolated from lanthanide surrogates. Newly emerged applications for these elements, combined with their persistent presence in nuclear waste, however, call for a better understanding of their behavior in complex living systems. In this work, we explored the biodistribution and excretion profiles of four transplutonium actinides (248Cm, 249Bk, 249Cf and 253Es) in a small animal model, and evaluated their in vivo sequestration and decorporation by two therapeutic chelators, diethylenetriamine pentaacetic acid and 3,4,3-LI(1,2-HOPO). Notably, the organ deposition patterns of those transplutonium actinides were element-dependent, particularly in the liver and skeleton, where lower atomic number radionuclides showed up to 7-fold larger liver/skeleton accumulation ratios. Nevertheless, the metal content in multiple organs was significantly decreased for all tested actinides, particularly in the liver, after administering the therapeutic agent 3,4,3-LI(1,2-HOPO) post-contamination. Lastly, the systematic comparison of the radionuclide biodistributions showed discernibly element-dependent organ depositions, which may provide insights into design rules for new bio-inspired chelating systems with high sequestration and separation performance.

Bonding Trends in Tetravalent Th-Pu Monosalen Complexes

The synthesis of three complex series of the form [AnCl2 (salen)(Pyx)2 ] (H2 salen=N,N'-bis(salicylidene)ethylenediamine; Pyx=pyridine, 4-methylpyridine, 3,5-dimethylpyridine) with tetravalent early actinides (An=Th, U, Np, Pu) is reported with the goal to elucidate the affinity of these heavy elements for small neutral N-donor molecules. Structure determination by single-crystal XRD and characterization of bulk powders with infrared spectroscopy reveals isostructurality within each respective series and the same complex conformation in all reported structures. Although the trend of interatomic distances for An-Cl and An-N (imine nitrogen of salen or pyridyl nitrogen of Pyx) was found to reflect an ionic behavior, the trend of the An-O distances can only be described with additional covalent interactions for all elements heavier than thorium. All experimental results are supported by quantum chemical calculations, which confirm the mostly ionic character in the An-N and An-Cl bonds, as well as the highest degree of covalency of the An-O bonds. Structurally, the calculations indicate just minor electronic or steric effects of the additional Pyx substituents on the complex properties.

Electronic structures and bonding of the actinide halides An(TRENTIPS)X (An = Th-Pu; X = F-I): a theoretical perspective

To evaluate how halogen and actinide atoms affect the electronic structures and bonding nature, we have theoretically investigated a series of the actinide halides An(TRENTIPS)X (An = Th-Pu; X = F-I); several of them have been synthesized by Liddle's group. The An-X bond distances decrease from An = Th to Pu for the same halides, and the harmonic vibrational frequencies for the An-X bonds are more susceptible to being affected by the halogen atoms. The analyses of bonding nature reveal that the An-X bonds have a certain covalency with a polarized character, and the σ-bonding component in the total orbital contribution is greatly larger than the corresponding π-bonding ones based on the analysis of the NOCVs (the natural orbitals for chemical valence). Furthermore, the electronic structures of the thorium complexes are obviously different from those of the uranium and transuranic analogues due to more valence electrons in Th 6d orbitals. In addition, thermodynamic results suggest that the U(TRENTIPS)Br complex is the most stable and U(TRENTIPS)Cl has the highest reactivity based on the halide exchange reaction of U(TRENTIPS)X complexes using Me3SiX. The reduction ability of the tetravalent An(TRENTIPS)X is sensitive to halogen atoms according to the calculated electron affinity of the An(TRENTIPS)X and the reactions An(TRENTIPS)X + K → An(TRENTIPS) + KX. This work presents the effect of the halogen and the actinide atoms on the structures, bonding nature and redox ability of a series of the tetravalent actinide halides with TREN ligand and facilitates our in-depth understanding of f-block elements, which could provide theoretical guidance for experimental work on actinide halides, especially for the synthetic chemistry of transuranic halides.

Series of Tetravalent Actinide Amidinates: Structure Determination and Bonding Analysis

Two series of isostructural tetravalent actinide amidinates [AnX((S)-PEBA)₃] (An = Th, U, Np; X = Cl, N₃) bearing the chiral (S,S)-N,N'-bis(1-phenylethyl)benzamidinate ((S)-PEBA) ligand have been synthesized and thoroughly characterized in solid and in solution. This study expands the already reported tetravalent neptunium complexes to the lighter actinides thorium and uranium. Furthermore, a rare Ce(IV) amidinate [CeCl((S)-PEBA)₃] was synthesized to compare its properties to those of the analogous tetravalent actinide complexes. All compounds were characterized in the solid state using single-crystal XRD and infrared spectroscopy and in solution using NMR spectroscopy. Quantum chemical bonding analysis including also the isostructural Pa and Pu complexes was used to characterize the covalent contributions to any bond involving the metal cation. Th shows the least covalent character throughout the series, even substantially smaller than for the Ce complex. For U, Np, and Pu, similar covalent bonding contributions are found, but a natural population analysis reveals different origins. The 6d participation is the highest for U and decreases afterward, whereas the 5f participation increases continuously from Pa to Pu.

Theoretical Prediction of the Potential Applications of Phenanthroline Derivatives in Separation of Transplutonium Elements

Recovery of transplutonium elements from adjacent actinides is extremely complicated in spent fuel reprocessing. Uncovering the electronic structures of transplutonium compounds is essential for designing robust ligands for in-group separation of transplutonium actinides. Here, we demonstrate the in-group transplutonium actinides separation ability of the recent developed phenanthroline ligand Et-Tol-DAPhen (N²,N⁹-diethyl-N²,N⁹-di-p-tolyl-1,10-phenanthroline-2,9-dicarboxamide, L_a) and its derivatives (5-bromo-(N²,N⁹-diethyl-N²,N⁹-di-p-tolyl-1,10-phenanthroline-2,9-dicarboxamide, L_b), and 5-(4-(λ¹-oxidaneyl)phenyl)-(N²,N⁹-diethyl-N²,N⁹-di-p-tolyl-1,10-phenanthroline-2,9- dicarboxamide, L_c) through quasi-relativistic density functional theory (DFT). Both electrostatic potential and molecular orbital analyses of the ligands indicate that the electron-donating group substituted ligand L_c is a better electron donor to actinides than L_a and L_b. The possible extracted complexes AnL(NO₃)₃ and [AnL₂(NO₃)]²⁺ (L = L_a, L_b, L_c; An = Am, Cm, Bk, Cf) possess similar structures. Bonding nature analysis validates that the covalent interactions of the metal-ligand bonds are enhanced across actinide series from Am to Cf, which stem from the energy degeneracy of the 5f orbitals of actinides and the 2p orbitals of the ligand coordinating atoms. The L_c ligand displays slightly stronger covalent bonding compared to the other two ligands. Simultaneously, thermodynamic analysis confirms the stronger metal-ligand bonding of the Cf³⁺ complexes and the higher stability of the extraction species with L_c. Consequently, the covalency between the DAPhen derivatives and transplutonium actinides seems to be positively correlated with the extraction ability of these ligands. Nevertheless, these ligands exhibit diverse separation abilities to in-group actinide recovery. Therefore, the enhancement of covalency does not necessarily lead to the improvement of separation ability due to different extraction capabilities. We hope that these results will provide some inspiration for designing novel ligands for in-group transplutonium separation.

π covalency in the halogen bond

Halogen bonds are a highly directional class of intermolecular interactions widely employed in chemistry and chemical biology. This linear interaction is commonly viewed to be analogous to the hydrogen bond because hydrogen bonding models also intuitively describe the σ-symmetric component of halogen bonding. The possibility of π-covalency in a halogen bond is not contemplated in any known models. Here we present evidence of π-covalency being operative in halogen bonds formed between chloride and halogenated triphenylamine-based radical cations. We reach this conclusion through computational analysis of chlorine K-edge X-ray absorption spectra recorded on these halogen bonded pairs. In light of this result, we contend that halogen bonding is better described by analogy to metal coordination bonds rather than hydrogen bonds. Our revised description of the halogen bond suggests that these interactions could be employed to influence the electronic properties of conjugated molecules in unique ways.

f-Orbital Mixing in the Octahedral f2 Compounds UX62- [X = F, Br, Cl, I] and PrCl63

Understanding how interactions between the f orbitals and ligand orbitals in lanthanide and actinide systems affect their physical properties is the central issue in f-element chemistry. A wide variety of approaches including both theoretical and experimental tools have been used to study these relationships. Among the most widely used tools has been crystal field theory (CFT), which bridges theory and experiment in that it is a model based largely on atomic theory that is parametrized using experimental data. Crystal field theory is quite accurate for the lanthanides, due in part to the highly contracted nature of the 4f orbitals. For actinides, crystal field theory is less accurate, potentially due to the treatment of orbital mixing. In CFT, orbital mixing is handled implicitly by allowing the electron repulsion parameters (Slater F^k parameters) and the spin-orbit coupling constant to vary. As a result, orbital mixing in CFT is isotropic in that the F^k parameters and the spin-orbit coupling constant affect all f orbitals equally. This approximation works well for the lanthanides due to the limited degree of orbital mixing in these complexes. In actinide complexes, the 5f orbitals have greater overlap with the ligand orbitals, and this approximation is less accurate than in the lanthanides. Here, we report a modification of CFT that includes the effect of orbital mixing on electron repulsion and spin-orbit coupling for each f orbital. The model is applied to the tetravalent uranium hexahalide dianions and PrCl₆^3- for which the energies of many low-lying excited states are known. The new model generally fits the data as well the traditional CFT although with fewer parameters. However, the new model does not fit the data better than the more complex CFT models of Faucher and co-workers. The results of the model show in detail how changes in overlap and orbital energies influence the energies of the bonding and antibonding orbitals.