Hydrolysis of Metal Dioxides Differentiates d-block from f-block Elements: Pa(V) as a 6d Transition Metal; Pr(V) as a 4f “Lanthanyl”

How novel is protactinium: Insights into the structure and properties of (PaO)2(SO4)3(H2O)2

Because of the scarcity of protactinium and the challenges associated with its separation and crystallization, even in sulfuric acid media where protactinium is relatively stable, there has been an incomplete understanding of the structural features of protactinium complexes. The characterization of protactinium sulfate complexes has been limited to those in solution, which have left key details unaddressed since the 1960s. This report describes a synthetic strategy to crystallize a protactinium complex using boric acid and sulfuric acid. Herein, the authors detail the results of hydrothermal synthesis and the single-crystal analysis of a novel protactinium sulfate complex, (PaO)2(SO4)3(H2O)2. This work has elucidated structural features, providing groundwork for accurate computational analysis and clarifying previously unknown details on the coordination, denticity, and binding properties of protactinium sulfate complexes.

Prediction of Redox Potentials for Ac, Th, and Pa in Aqueous Solution

Density functional theory in conjunction with small core pseudopotentials and the associated basis sets was used to calculate potentials for multiple redox couples, covering a range of oxidation states for Ac (0 to III), Th (0 to IV), and Pa (0 to V) in aqueous solution. Solvation effects were incorporated using a supermolecule-continuum approach, with 30 water molecules representing two solvation shells, and the COSMO and SMD implicit solvation models. The calculated geometries for Ac(III), Th(IV), and Pa(V) were in reasonable agreement with the available experimental data. Using the COSMO model with the B3LYP functional, the calculated redox potentials were within ±0.2 V from experiment for most redox couples. Several pathways were explored for the Pa(V/IV) redox couple for different forms of Pa(V) and Pa(IV). Most Pa(V/IV) redox couples have very similar potentials, ranging from 0 to -0.4 V up to a pH of 1.4. At pH = 1.4, the potentials shift to values that are more negative than -0.7 V, reflecting the growing unfavorable nature of the redox process at higher pH levels. The calculated values for An(III/II) potentials were consistent with prior estimates and the available experimental data. The predicted redox potentials for An(II/I) were highly negative, as expected. For An(I/0) potentials, Th and Pa exhibited positive values, contrasting with the negative values calculated for Ac. The An+m/An(0) potentials agreed better with the experimental data when using the COSMO solvation model as compared to the SMD model.

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.

Reactions of gas-phase uranyl formate/acetate anions: reduction of carboxylate ligands to aldehydes by intra-complex hydride attack

In a previous study, electrospray ionization, collision-induced dissociation (CID), and gas-phase ion-molecule reactions were used to create and characterize ions derived from homogeneous precursors composed of a uranyl cation (UVIO22+) coordinated by either formate or acetate ligands [E. Perez, C. Hanley, S. Koehler, J. Pestok, N. Polonsky and M. Van Stipdonk, Gas phase reactions of ions derived from anionic uranyl formate and uranyl acetate complexes, J. Am. Soc. Mass Spectrom., 2016, 27, 1989-1998]. Here, we describe a follow-up study of anionic complexes that contain a mix of formate and acetate ligands, namely [UO2(O2C-CH3)2(O2C-H)]- and [UO2(O2C-CH3)(O2C-H)2]-. Initial CID of either anion causes decarboxylation of a formate ligand to create carboxylate-coordinated U-hydride product ions. Subsequent CID of the hydride species causes elimination of acetaldehyde or formaldehyde, consistent with reactions that include intra-complex hydride attack upon bound acetate or formate ligands, respectively. Density functional theory (DFT) calculations reproduce the experimental observations, including the favored elimination of formaldehyde over acetaldehyde by hydride attack during CID of [UO2(H)(O2C-CH3)(O2C-H)]-. We also discovered that MSn CID of the acetate-formate complexes leads to generation of the oxyl-methide species, [UO2(O)(CH3)]-, which reacts with H2O to generate [UO2(O)(OH)]-. DFT calculations support the observation that formation of [UO2(O)(OH)]- by elimination of CH4 is favored over H2O addition and rearrangement to create [UO2(OH)2(CH3)]-.

Stability of the protactinium(V) mono-oxo cation probed by first-principle calculations

This study explores the distinctive behavior of protactinium (Z=91) within the actinide series. In contrast to neighboring elements like uranium or plutonium, protactinium in the pentavalent state diverges by not forming the typical dioxo protactinyl moiety PaO2 + in aqueous phase. Instead, it manifests as a monooxo PaO3+ cation or a Pa5+ . Employing first-principle calculations with implicit and explicit solvation, we investigate two stoichiometrically equivalent neutral complexes: PaO(OH)2 (X)(H2 O) and Pa(OH)4 (X), where X represents various monodentate and bidentate ligands. Calculating the Gibbs free energy for the reaction PaO(OH)2 (X)(H2 O)→Pa(OH)4 (X), we find that the PaO(OH)2 (X)(H2 O) complex is stabilized with Cl- , Br- , I- , NCS- , NO3 - , and SO4 2- ligands, while it is not favored with OH- , F- , and C2 O4 2- ligands. Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) methods reveal the Pa mono-oxo bond as a triple bond, with significant contributions from the 5f and 6d shells. Covalency of the Pa mono-oxo bond increases with certain ligands, such as Cl- , Br- , I- , NCS- , and NO3 - . These findings elucidate protactinium's unique chemical attributes and provide insights into the conditions supporting the stability of relevant complexes.

Hydrolysis Reactions of the High Oxidation State Dimers Th2O4, Pa2O5, U2O6, and Np2O6. A Computational Study

The energetics of the hydrolysis reactions for high oxidation states of the dimeric actinide species Th2IVO4, Pa2VO5, and U2VIO6 were calculated at the CCSD(T) level and those for triplet Np2VIO6 at the B3LYP level. Hydrolysis is initiated by the formation of a Lewis acid/base adduct with H2O (physisorbed product), followed by a proton transfer to form a dihydroxide molecule (chemisorbed product); this process was repeated until the initial actinide oxide is fully hydrolyzed. For Th2O4, hydrolysis (chemisorption) by the initial and subsequent H2O molecules prefers proton transfer to terminal oxo groups before the bridge oxo groups. The overall Th2O4 hydration pathway is exothermic with chemisorbed products preferred over the physisorption products, and the fully hydrolyzed Th2(OH)8 can form exothermically. Hydrolysis of Pa2O5 forms isomers of similar energies with no initial preference for bridge or terminal hydroxy groups. The most exothermic hydrolysis product for Pa is Pa2O(OH)8 and the most stable species is Pa2O(OH)8(H2O). Hydrolysis of U2O6 and Np2O6 with strong [O═An═O]2+ actinyl groups occurs first at the bridging oxygens rather than at the terminal oxo groups. The U2O6 and Np2O6 pathways predict hydrated products to be more favored than hydrolyzed products, as more H2O molecules are added. The stability of the U and Np clusters is predicted to decrease with increasing number of hydroxyl groups. The most stable species on the hydration reaction coordinate for U and Np is An2O3(OH)6(H2O).

Reactions of Thorium Oxide Clusters with Water: The Effects of Oxygen Content

Thorium oxide has many important applications in industry. In this article, theoretical calculations have been carried out to explore the hydrolysis reactions of the ThO_n (n=1-3) clusters. The reaction mechanisms of the O-deficient ThO and the O-rich ThO₃ are compared with the stoichiometric ThO₂ . The theoretical results show good agreement with the prior experiments. It is shown that the hydrolysis mainly occurred on the singlet potential surface. The overall reactions consist of two hydrolysis steps which are all favourable in energy. The effects of oxygen content on the hydrolysis are elucidated. Interestingly, among them, the peroxo group O₂ ^2- in ThO₃ is converted to the HOO- ligand, behaving like the terminal O^2- in the hydrolysis which is transformed into the HO- groups. In addition, natural bond orbital (NBO) analyses were employed to further understand the bonding of the pertinent species and to interpret the differences in hydrolysis.

Protactinium and Actinium Monohydrides: A Theoretical Study on Their Spectroscopic and Thermodynamic Properties

Spectroscopic and thermodynamics properties including bond dissociation energies (BDEs), adiabatic electron affinities (AEAs), and ionization energies (IEs) have been predicted for AcH and PaH using the Feller-Peterson-Dixon composite approach. Comparisons with previous studies on ThH and UH were performed to identify possible trends in the actinide series. Multireference CASPT2 calculations were used to predict the spin-orbit effects and obtain potential energy curves for the low-lying Ω states around the equilibrium distance as well as the vertical detachment energies (VDEs) from AcH^- and PaH^- to excited states of the neutral species. The calculated AEA for AnH (An = Ac, Th, Pa, U) showed that the AEA increases from AcH (0.425 eV) to ThH (0.820 eV) and decreases to PaH (0.781 eV) and to UH (0.457 eV), whereas the IE values are 5.887 eV (AcH), 6.181 eV (ThH), 6.204 eV (PaH), and 6.182 eV (UH). The ground state of AcH, AcH^-, PaH, and PaH^- are predicted to be¹Σ⁺₀,²Π_3/2, ³H₄, and ⁴I_9/2, respectively. The BDEs for AcH and PaH are 276.4 and 237.2 kJ/mol, and those for AcH^- and PaH^- are 242.8 and 239.8 kJ/mol, respectively. The natural bond analysis shows a significant ionic character, An⁺H^-, in the bonding of the neutral hydrides.

Hydrolysis of Small Oxo/Hydroxo Molecules Containing High Oxidation State Actinides (Th, Pa, U, Np, Pu): A Computational Study

The energetics of hydrolysis reactions for high oxidation states of oxo/hydroxo monomeric actinide species (Th^IVO₂, Pa^IVO₂, U^IVO₂, Pa^VO₂(OH), U^VO₂(OH), U^VIO₃, Np^VIO₃, Np^VIIO₃(OH), and Pu^VIIO₃(OH)) were calculated at the CCSD(T) level. The first step is the formation of a Lewis acid/base adduct with H₂O (hydration), followed by a proton transfer to form a dihydroxide molecule (hydrolysis); this process is repeated until all oxo groups are hydrolyzed. The physisorption (hydration) for each H₂O addition was predicted to be exothermic, ca. -20 kcal/mol. The hydrolysis products are preferred energetically over the hydration products for the +IV and +V oxidation states. The compounds with An^VI are a turning point in terms of favoring hydration over hydrolysis. For An^VIIO₃(OH), hydration products are preferred, and only two waters can bind; the complete hydrolysis process is now endothermic, and the oxidation state for the An in An(OH)₇ is +VI with two OH groups each having one-half an electron. The natural bond order charges and the reaction energies provide insights into the nature of the hydrolysis/hydration processes. The actinide charges and bond ionicity generally decrease across the period. The ionic character decreases as the oxidation state and coordination number increase so that covalency increases moving to the right in the actinide period.