A covalent chemical bond carries the connotation of overlap of atomic orbitals between bonded atoms, leading to a buildup of the electron density in the internuclear region. Stabilization of the ...valence 5f orbitals as the actinide series is crossed leads, in compounds of the minor actinides americium and curium, to their becoming approximately degenerate with the highest occupied ligand levels and hence to the unusual situation in which the resultant valence molecular orbitals have significant contributions from both actinide and the ligand yet in which there is little atomic orbital overlap. In such cases, the traditional quantum-chemical tools for assessing the covalency, e.g., population analysis and spin densities, predict significant metal–ligand covalency, although whether this orbital mixing is really covalency in the generally accepted chemical view is an interesting question. This review discusses our recent analyses of the bonding in AnCp3 and AnCp4 (An = Th–Cm; Cp = η5-C5H5) using both the traditional tools and also topological analysis of the electron density via the quantum theory of atoms-in-molecules. I will show that the two approaches yield rather different conclusions and suggest that care must be taken when using quantum chemistry to assess metal–ligand covalency in this part of the periodic table. The implications of this work for minor actinide partitioning from nuclear wastes are discussed; minor actinide extractant ligands based on nitrogen donors have received much attention in recent years, as have comparisons of the extent of covalency in actinide–nitrogen bonding with that in analogous lanthanide systems via quantum-chemical studies employing the traditional tools for assessing the covalency.
The 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.
Sweet seventeen: The theoretically predicted compounds AcHe173+, ThHe174+, and PaHe174+ have stable, one‐shell true minimum structures with the highest coordination number yet reported. See DFT optimized structure; actinium atom blue, helium atoms yellow.
Abstract
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
15
N nuclear magnetic resonance spectroscopy, and find an exceptional nitride chemical shift and chemical shift anisotropy. This redefines the
15
N 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
15
N chemical shift-bond order correlations, and reframe our understanding of actinide chemical bonding to guide future studies.
FEUDAL (f's essentially unaffected, d's accommodate ligands) is a longstanding bonding model in actinide chemistry, in which metal-ligand binding uses 6d-orbitals, with the 5f remaining non-bonding. ...The inverse-trans-influence (ITI) is a case where the model may break down, and it has been suggested that ionic and covalent effects work synergistically in the ITI. Here, we report an experimentally grounded computational study that quantitatively explores the ITI, and in particular the structure-directing role of f-orbital covalency. Strong donor ligands generate a cis-ligand-directing electrostatic potential (ESP) at the metal centre. When f-orbital participation, via overlap-driven covalency, becomes dominant via short actinide-element distances, this ionic ESP effect is overcome, favouring a trans-ligand-directed geometry. This study contradicts the accepted ITI paradigm in that here ionic and covalent effects work against each other, and suggests a clearly non-FEUDAL, structure-directing role for the f-orbitals.
Recent developments in the chemistry of the transuranic elements are surveyed, with particular emphasis on computational contributions. Examples are drawn from molecular coordination and ...organometallic chemistry, and from the study of extended solid systems. The role of the metal valence orbitals in covalent bonding is a particular focus, especially the consequences of the stabilization of the 5f orbitals as the actinide series is traversed. The fledgling chemistry of transuranic elements in the +II oxidation state is highlighted. Throughout, the symbiotic interplay of experimental and computational studies is emphasized; the extraordinary challenges of experimental transuranic chemistry afford computational chemistry a particularly valuable role at the frontier of the periodic table.
Getting very radioactive: Recent developments in the fascinating and fledgling chemistry of the transuranic elements are surveyed, with particular focus on the role played by computational chemistry.
The 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.
Echte Minimumstrukturen von AcHe173+, ThHe174+ und PaHe174+ mit der höchsten bekannten Koordinationszahl in einer Schale werden auf der Grundlage von Dichtefunktionalrechnungen postuliert. Siehe optimierte Struktur: Actinium blau, Helium gelb.
Si in sight: A one‐pot, single‐step synthesis of an acyclic silylsilylene, Si{Si(SiMe3)3}{N(SiMe3)Dipp} (Dipp=2,6‐iPr2C6H3), from a silicon(IV) starting material is reported, together with evidence ...for a mechanism involving alkali metal silylenoid intermediates.
The enthalpies of the reactions AnO2(+)→ AnO(+) + O and AnO2(+) + H2O → AnO2(H2O)(+), and those of the isomerisation of the latter to AnO(OH)2(+), have been calculated for An = Pa-Pu. The data match ...previous experimental and computational values very closely, and the computed enthalpy for the isomerisation of PaO2(H2O)(+) to PaO(OH)2(+), requested by the authors of Inorg. Chem., 2015, 54, 7474, is found to be 0.8 kJ mol(-1). The NPA, NBO and QTAIM approaches are used to probe covalency in the An-Oyl bond of AnO2(H2O)(+), and all metrics agree that these bonds become increasingly covalent as the 5f series is crossed, providing rationalisation for the increasingly endothermic isomerisation reactions. QTAIM analysis indicates that the An=O and An-OH bonds in the oxide hydroxide isomers also become increasingly covalent as the 5f series is crossed.
Abstract
Covalency involving the 5f orbitals is regularly invoked to explain the reactivity, structure and spectroscopic properties of the actinides, but the ionic versus covalent nature of ...metal-ligand bonding in actinide complexes remains controversial. The tetrakis 2,6-di-
tert
-butylphenoxide complexes of Th, U and Np form an isostructural series of crystal structures containing approximately tetrahedral MO
4
cores. We show that up to 3 GPa the Th and U crystal structures show negative linear compressibility as the OMO angles distort. At 3 GPa the angles snap back to their original values, reverting to a tetrahedral geometry with an abrupt shortening of the M-O distances by up to 0.1 Å. The Np complex shows similar but smaller effects, transforming above 2.4 GPa. Electronic structure calculations associate the M-O bond shortening with a change in covalency resulting from increased contributions to the M-O bonding by the metal 6d and 5f orbitals, the combination promoting MO
4
flexibility at little cost in energy.
Abstract
A range of reasons has been suggested for why many low-coordinate complexes across the periodic table exhibit a geometry that is bent, rather a higher symmetry that would best separate the ...ligands. The dominating reason or reasons are still debated. Here we show that two pyramidal UX
3
molecules, in which X is a bulky anionic ligand, show opposite behaviour upon pressurisation in the solid state. UN″
3
(
UN3
, N″ = N(SiMe
3
)
2
) increases in pyramidalization between ambient pressure and 4.08 GPa, while U(SAr)
3
(
US3
, SAr = S-C
6
H
2
-
t
Bu
3
−2,4,6) undergoes pressure-induced planarization. This capacity for planarization enables the use of X-ray structural and computational analyses to explore the four hypotheses normally put forward for this pyramidalization. The pyramidality of
UN3
, which increases with pressure, is favoured by increased dipole and reduction in molecular volume, the two factors outweighing the slight increase in metal-ligand agostic interactions that would be formed if it was planar. The ambient pressure pyramidal geometry of
US3
is favoured by the induced dipole moment and agostic bond formation but these are weaker drivers than in
UN3
; the pressure-induced planarization of
US3
is promoted by the lower molecular volume of
US3
when it is planar compared to when it is pyramidal.
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