Covalent bond shortening and distortion induced by pressurization of thorium, uranium, and neptunium tetrakis aryloxides.

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 MO4 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 MO4 flexibility at little cost in energy.

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.

29Si NMR Spectroscopy as a Probe of s- and f-Block Metal(II)-Silanide Bond Covalency.

We report the use of 29Si NMR spectroscopy and DFT calculations combined to benchmark the covalency in the chemical bonding of s- and f-block metal-silicon bonds. The complexes [M(SitBu3)2(THF)2(THF)x] (1-M: M = Mg, Ca, Yb, x = 0; M = Sm, Eu, x = 1) and [M(SitBu2Me)2(THF)2(THF)x] (2-M: M = Mg, x = 0; M = Ca, Sm, Eu, Yb, x = 1) have been synthesized and characterized. DFT calculations and 29Si NMR spectroscopic analyses of 1-M and 2-M (M = Mg, Ca, Yb, No, the last in silico due to experimental unavailability) together with known {Si(SiMe3)3}--, {Si(SiMe2H)3}--, and {SiPh3}--substituted analogues provide 20 representative examples spanning five silanide ligands and four divalent metals, revealing that the metal-bound 29Si NMR isotropic chemical shifts, δSi, span a wide (∼225 ppm) range when the metal is kept constant, and direct, linear correlations are found between δSi and computed delocalization indices and quantum chemical topology interatomic exchange-correlation energies that are measures of bond covalency. The calculations reveal dominant s- and d-orbital character in the bonding of these silanide complexes, with no significant f-orbital contributions. The δSi is determined, relatively, by paramagnetic shielding for a given metal when the silanide is varied but by the spin-orbit shielding term when the metal is varied for a given ligand. The calculations suggest a covalency ordering of No(II) > Yb(II) > Ca(II) ≈ Mg(II), challenging the traditional view of late actinide chemical bonding being equivalent to that of the late lanthanides.

Polarised covalent thorium(IV)- and uranium(IV)-silicon bonds.

We report the synthesis and characterisation of isostructural thorium(iv)- and uranium(iv)-silanide actinide (An) complexes, providing an opportunity to directly compare Th-Si and U-Si chemical bonds. Quantum chemical calculations show significant and surprisingly similar An%:Si%, 7s-, 6d-, and 5f-orbital contributions from both elements in polarised covalent An-Si bonds, and marginally greater covalency in the U-Si vs. Th-Si linkages.

Quantum chemical topology and natural bond orbital analysis of M-O covalency in M(OC6H5)4 (M = Ti, Zr, Hf, Ce, Th, Pa, U, Np).

Covalency is complex yet central to our understanding of chemical bonding, particularly in the actinide series. Here we assess covalency in a series of isostructural d and f transition element compounds M(OC6H5)4 (M = Ti, Zr, Hf, Ce, Th, Pa, U, Np) using scalar relativistic hybrid density functional theory in conjunction with the Natural Bond Orbital (NBO), quantum theory of atoms in molecules (QTAIM) and interacting quantum atoms (IQA) approaches. The IQA exchange-correlation covalency metric is evaluated for the first time for actinides other than uranium, in order to assess its applicability in the 5f series. It is found to have excellent correlation with NBO and QTAIM covalency metrics, making it a promising addition to the computational toolkit for analysing metal-ligand bonding. Our range of metrics agree that the actinide-oxygen bonds are the most covalent of the elements studied, with those of the heavier group 4 elements the least. Within the early actinide series, Th stands apart from the other three elements considered, being consistently the least covalent.

Emergence of the structure-directing role of f-orbital overlap-driven covalency.

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.

Computational analysis of M-O covalency in M(OC6H5)4 (M = Ti, Zr, Hf, Ce, Th, U).

A series of compounds M(OC6H5)4 (M = Ti, Zr, Hf, Ce, Th, U) is studied with hybrid density functional theory, to assess M-O bond covalency. The series allows for the comparison of d and f element compounds that are structurally similar. Two well-established analysis methods are employed: Natural Bond Orbital and the Quantum Theory of Atoms in Molecules. A consistent pattern emerges; the U-O bond is the most covalent, followed by Ce-O and Th-O, with those involving the heavier transition metals the least so. The covalency of the Ti-O bond differs relative to Ce-O and Th-O, with the orbital-based method showing greater relative covalency for Ti than the electron density-based methods. The deformation energy of r(M-O) correlates with the d orbital contribution from the metal to the M-O bond, while no such correlation is found for the f orbital component. f orbital involvement in M-O bonding is an important component of covalency, facilitating orbital overlap and allowing for greater expansion of the electrons, thus lowering their kinetic energy.

Balancing Exchange Mixing in Density-Functional Approximations for Iron Porphyrin.

Predicting the correct ground-state multiplicity for iron(II) porphyrin, a high-spin quintet, remains a significant challenge for electronic-structure methods, including commonly employed density functionals. An even greater challenge for these methods is correctly predicting favorable binding of O2 to iron(II) porphyrin, due to the open-shell singlet character of the adduct. In this work, the performance of a modest set of contemporary density-functional approximations is assessed and the results interpreted using Bader delocalization indices. It is found that inclusion of greater proportions of Hartree-Fock exchange, in hybrid or range-separated hybrid functionals, has opposing effects; it improves the ability of the functional to identify the ground state but is detrimental to predicting favorable dioxygen binding. Because of the uncomplementary nature of these properties, accurate prediction of both the relative spin-state energies and the O2 binding enthalpy eludes conventional density-functional approximations.

Effect of amino acid ligands on the structure of iron porphyrins and their ability to bind oxygen.

Density functional theory is used to study a series of model iron porphyrins in the gas phase. In the first part of this study, three range-separated hybrid density functionals developed by Chai and Head-Gordon were assessed; ωB97, ωB97X, and ωB97XD. The effects of including full Hartree-Fock exchange at long-range and dispersion corrections are reported with respect to the geometries and binding energies of oxygen to the iron porphyrin systems. The functionals all correctly predict the quintet ground state for the deoxy-iron porphyrins, where typically hybrid functionals fail and predict a triplet ground state. Including dispersion in ωB97XD is shown to give the best results for the O2 binding energy and geometrical parameters. The second part of the study employs ωB97XD to study iron porphine systems with different amino acids in the axial position. Geometrical parameters are reported and compared to experimental data, where available. Binding energies of the systems with oxygen are also reported and discussed.

Self-assembling ADADA helices formed by hydrogen bonding.

A computational investigation of the electronic properties of an experimentally prepared ADADA helix indicates that the helix is held together with four strong hydrogen bonds as well as many other weak interactions. Determination of the electronic energy changes, as well as thermodynamic parameters, suggests that helix formation is a favorable process, driven by the formation of the hydrogen bonds. For instance, the unsubstituted helix has an electronic binding energy of -85.8 kJ/mol. Furthermore, the strength of binding can be tuned to some extent by the careful selection of substituents. The hydrogen bonds are strengthened when the pyridine ring (H-bond acceptor) is substituted with an electron-donating group such as an amine, while electron-withdrawing groups on the thiazine ring (H-bond donor) are preferred. The most significant enhancement in binding is achieved when the helix is constructed from monomers that consist of contiguous hydrogen-bond acceptors or donors. This so-called AAAAA-DDDDD helix exhibits a binding energy almost 4-fold greater than that of the unsubstituted ADADA helix at -335.4 kJ/mol, a dramatic improvement over the ADADA helix.