Compounds featuring unsupported metal–metal bonds between actinide elements remain highly sought after yet confined experimentally to inert gas matrix studies. Notwithstanding this paucity, actinide–actinide bonding has been the subject of extensive computational research. In this contribution, high level quantum chemical calculations at both the scalar and spin–orbit levels are used to probe the Th–Th bonding in a range of zero valent systems of general formula LThThL. Several of these compounds have very short Th–Th bonds arising from a new type of Th–Th quadruple bond with a previously unreported electronic configuration featuring two unpaired electrons in 6d-based δ bonding orbitals. H3AsThThAsH3 is found to have the shortest Th–Th bond yet reported (2.590 Å). The Th2 unit is a highly sensitive probe of ligand electron donor/acceptor ability; we can tune the Th–Th bond from quadruple to triple, double and single by judicious choice of the L group, up to 2.888 Å for singly-bonded ONThThNO.

AbstractThe geometries and electronic structures of molecular ions featuring He atoms complexed to actinide cations are explored computationally using density functional and coupled cluster theories. A new record coordination number is established, as AcHe173+, ThHe174+, and PaHe174+ are all found to be true geometric minima, with the He atoms clearly located in the first shell around the actinide. Analysis of AcHen3+ (n=1–17) using the quantum theory of atoms in molecules (QTAIM) confirms these systems as having closed shell, charge-induced dipole bonding. Excellent correlations (R2>0.95) are found between QTAIM metrics (bond critical point electron densities and delocalization indices) and the average Ac−He distances, and also with the incremental He binding energies.

AbstractThe geometries and electronic structures of molecular ions featuring He atoms complexed to actinide cations are explored computationally using density functional and coupled cluster theories. A new record coordination number is established, as AcHe173+, ThHe174+, and PaHe174+ are all found to be true geometric minima, with the He atoms clearly located in the first shell around the actinide. Analysis of AcHen3+ (n=1–17) using the quantum theory of atoms in molecules (QTAIM) confirms these systems as having closed shell, charge-induced dipole bonding. Excellent correlations (R2>0.95) are found between QTAIM metrics (bond critical point electron densities and delocalization indices) and the average Ac−He distances, and also with the incremental He binding energies.

A systematic computational study of organoactinide complexes of the form [LAnX]n+ has been carried out using density functional theory, the quantum theory of atoms in molecules (QTAIM) and Ziegler-Rauk energy decomposition analysis (EDA) methods. The systems studied feature L = trans-calix[2]benzene[2]pyrrolide, An = Th(IV), Th(III), U(III) and X = BH4, BO2C2H4, Me, N(SiH3)2, OPh, CH3, NH2, OH, F, SiH3, PH2, SH, Cl, CH2Ph, NHPh, OPh, SiH2Ph, PHPh2, SPh, CPh3, NPh2, OPh, SiPh3 PPh2, SPh. The PBE0 hybrid functional proved most suitable for geometry optimisations based on comparisons with available experimental data. An–X bond critical point electron densities, energy densities and An–X delocalisation indices, calculated with the PBE functional at the PBE0 geometries, are correlated with An–X bond energies, enthalpies and with the terms in the EDA. Good correlations are found between energies and QTAIM metrics, particularly for the orbital interaction term, provided the X ligand is part of an isoelectronic series and the number of open shell electrons is low (i.e. for the present Th(IV) and Th(III) systems).

The interactions between water and the actinide oxides UO2 and PuO2 are important both fundamentally and when considering the long-term storage of spent nuclear fuel. However, experimental studies in this area are severely limited by the intense radioactivity of plutonium, and hence, we have recently begun to investigate these interactions computationally. In this paper, we report the results of plane-wave density functional theory calculations of the interaction of water with the {111}, {110}, and {100} surfaces of UO2 and PuO2, using a Hubbard-corrected potential (PBE + U) approach to account for the strongly correlated 5f electrons. We find a mix of molecular and dissociative water adsorption to be most stable on the {111} surface, whereas the fully dissociative water adsorption is most stable on the {110} and {100} surfaces, leading to a fully hydroxylated monolayer. From these results, we derive water desorption temperatures at various pressures for the different surfaces. These increase in the order {111} < {110} < {100}, and these data are used to propose an alternative interpretation for the two experimentally determined temperature ranges for water desorption from PuO2.

Reaction of Ce(NO3)3(THF)4 with Li3(THF)3(NN′3) (NN′3 = N(CH2CH2NR)3, R = SitBuMe2) in Et2O, in the presence of 12-crown-4, results in the formation of [Li(12-crown-4)][(NN′3)Ce(O)] (1) in 36% yield. This transformation proceeds via formation of a Ce(III) nitrate intermediate, [Li(12-crown-4)][(NN′3)Ce(κ2-O2NO)] (2), which undergoes inner sphere nitrate reduction. In addition, reaction of 1 with tBuMe2SiCl results in the formation of (NN′3)Ce(OSitBuMe2) (3), confirming the nucleophilic character of its oxo ligand. Natural bond orbital and quantum theory of atoms-in-molecules data reveal the Ce–O interaction in 1 to be significantly covalent, and strikingly similar to analogous U–O bonding.

Heterobimetallic complexes containing short uranium–group 10 metal bonds have been prepared from monometallic IUIV(OArP-κ2O,P)3 (2) {[ArPO]− = 2-tert-butyl-4-methyl-6-(diphenylphosphino)phenolate}. The U–M bond in IUIV(μ-OArP-1κ1O,2κ1P)3M0, M = Ni (3–Ni), Pd (3–Pd), and Pt (3–Pt), has been investigated by experimental and DFT computational methods. Comparisons of 3–Ni with two further U–Ni complexes XUIV(μ-OArP-1κ1O,2κ1P)3Ni0, X = Me3SiO (4) and F (5), was also possible via iodide substitution. All complexes were characterized by variable-temperature NMR spectroscopy, electrochemistry, and single crystal X-ray diffraction. The U–M bonds are significantly shorter than any other crystallographically characterized d–f-block bimetallic, even though the ligand flexes to allow a variable U–M separation. Excellent agreement is found between the experimental and computed structures for 3–Ni and 3–Pd. Natural population analysis and natural localized molecular orbital (NLMO) compositions indicate that U employs both 5f and 6d orbitals in covalent bonding to a significant extent. Quantum theory of atoms-in-molecules analysis reveals U–M bond critical point properties typical of metallic bonding and a larger delocalization index (bond order) for the less polar U–Ni bond than U–Pd. Electrochemical studies agree with the computational analyses and the X-ray structural data for the U–X adducts 3–Ni, 4, and 5. The data show a trend in uranium–metal bond strength that decreases from 3–Ni down to 3–Pt and suggest that exchanging the iodide for a fluoride strengthens the metal–metal bond. Despite short U–TM (transition metal) distances, four other computational approaches also suggest low U–TM bond orders, reflecting highly transition metal localized valence NLMOs. These are more so for 3–Pd than 3–Ni, consistent with slightly larger U–TM bond orders in the latter. Computational studies of the model systems (PH3)3MU(OH)3I (M = Ni, Pd) reveal longer and weaker unsupported U–TM bonds vs 3.

Generalised gradient approximation (PBE) and hybrid (PBE0) density functional theory (DFT) within the periodic electrostatic embedded cluster method have been used to study AnO2 bulk and surfaces (An = U, Np, Pu). The electronic structure has been investigated by examining the projected density of states (PDOS). While PBE incorrectly predicts these systems to be metallic, PBE0 finds them to be insulators, with the composition of the valence and conduction levels agreeing well with experiment. Molecular and dissociative water adsorption on the (111) and (110) surfaces of UO2 and PuO2 has been investigated, with that on the (110) surface being stronger than on the (111). Similar energies are found for molecular and dissociative adsorption on the (111) surfaces, while on the (110) there is a clear preference for dissociative adsorption. Adsorption energies and geometries on the (111) surface of UO2 are in good agreement with recent periodic DFT studies using the GGA+U approach, and our data for dissociative adsorption on the (110) surface of PuO2 match experiment rather well, especially when dispersion corrections are included.The electronic structures of AnO2 (An = U, Np, Pu) are studied computationally with hybrid density functional theory, and the geometries and energetics of water adsorption on the low index surfaces are presented.

Quantum Theory of Atoms-in-Molecules bond critical point and delocalisation index metrics are calculated for the actinide-element bonds in Cs2UO2Cl4, U(Se2PPh2)4 and Np(Se2PPh2)4, in gas-phase, continuum solvent (COSMO) and via the periodic electrostatic embedded cluster method. The effects of the environment are seen to be very minor, suggesting that they do not account for the differences previously observed between the experimental and theoretical QTAIM ρb and ∇2ρb for the U–O bonds in Cs2UO2Cl4. With the exception of the local density approximation, there is only a small dependence of the QTAIM metrics on the exchange–correlation functional employed.Quantum Theory of Atoms-in-Molecules bond critical point and delocalisation index metrics are calculated for a range of actinide-element bonds in gas-phase, continuum solvent and via an electrostatic embedded cluster approach.

A systematic computational study of organoactinide complexes of the form [LAnX]n+ has been carried out using density functional theory, the quantum theory of atoms in molecules (QTAIM) and Ziegler-Rauk energy decomposition analysis (EDA) methods. The systems studied feature L = trans-calix[2]benzene[2]pyrrolide, An = Th(IV), Th(III), U(III) and X = BH4, BO2C2H4, Me, N(SiH3)2, OPh, CH3, NH2, OH, F, SiH3, PH2, SH, Cl, CH2Ph, NHPh, OPh, SiH2Ph, PHPh2, SPh, CPh3, NPh2, OPh, SiPh3 PPh2, SPh. The PBE0 hybrid functional proved most suitable for geometry optimisations based on comparisons with available experimental data. An–X bond critical point electron densities, energy densities and An–X delocalisation indices, calculated with the PBE functional at the PBE0 geometries, are correlated with An–X bond energies, enthalpies and with the terms in the EDA. Good correlations are found between energies and QTAIM metrics, particularly for the orbital interaction term, provided the X ligand is part of an isoelectronic series and the number of open shell electrons is low (i.e. for the present Th(IV) and Th(III) systems).

Compounds featuring unsupported metal–metal bonds between actinide elements remain highly sought after yet confined experimentally to inert gas matrix studies. Notwithstanding this paucity, actinide–actinide bonding has been the subject of extensive computational research. In this contribution, high level quantum chemical calculations at both the scalar and spin–orbit levels are used to probe the Th–Th bonding in a range of zero valent systems of general formula LThThL. Several of these compounds have very short Th–Th bonds arising from a new type of Th–Th quadruple bond with a previously unreported electronic configuration featuring two unpaired electrons in 6d-based δ bonding orbitals. H3AsThThAsH3 is found to have the shortest Th–Th bond yet reported (2.590 Å). The Th2 unit is a highly sensitive probe of ligand electron donor/acceptor ability; we can tune the Th–Th bond from quadruple to triple, double and single by judicious choice of the L group, up to 2.888 Å for singly-bonded ONThThNO.