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

Nature Reviews Chemistry volume 8, pages 701–712 (2024)Cite this article

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Abstract

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.

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Fig. 1: Correlation of the calculated 77Se NMR chemical shifts with the QTAIM delocalization index.
Fig. 2: X-ray absorption and emission process in uranyl.
Fig. 3: Magnetic circular dichroism spectroscopy of pentavalent uranium hexachloride.

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Authors and Affiliations

  1. Department of Chemistry, The University of Manchester, Manchester, UK

    Nikolas Kaltsoyannis

  2. Department of Chemistry, The University of Lancaster, Lancaster, UK

    Andrew Kerridge

Authors
  1. Nikolas Kaltsoyannis
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  2. Andrew Kerridge
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Contributions

N.K. wrote the ‘Introduction’ section, and researched and wrote the ‘NMR’ section. A.K. wrote the ‘Electronic structure methods and analysis’ section, and researched and wrote the ‘X-ray spectroscopy’ and ‘Emerging approaches’ sections. Both authors wrote the ‘Conclusions and outlook’ section. Both authors reviewed and edited the manuscript before and after submission.

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Correspondence to Nikolas Kaltsoyannis or Andrew Kerridge.

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Nature Reviews Chemistry thanks Congqing Zhu, Korey Carter and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Glossary

Ab Initio ligand field theory

A theoretical approach for describing the detailed effects of the chemical environment on the electronic structure of the central ion of a coordination complex, taking into account both zero field and crystal field splitting, as well as spin–orbit coupling.

Bond critical point

In the context of the QTAIM (see below), a point in which the gradient of the electronic density vanishes, and two of the eigenvalues of the Hessian are negative and the other positive. Also known as a (3,−1) critical point. It is used to assess the degree of electronic charge accumulation in the internuclear region between two atoms, and it is thus a measure of covalency.

Delocalization index

In the context of the QTAIM, a measure of the number of electrons shared between two atoms in a molecule, derived from the second-order reduced density matrix. Formally, the covariance of the electron populations of the two atoms. It is used as a measure of covalency and bond order.

High energy resolution fluorescence detected–X-ray absorption spectroscopy

(HERFD-XAS). A technique for the extraction of X-ray absorption spectroscopy data from resonant inelastic X-ray scattering maps that takes advantage of the longer core–hole lifetime of an intermediate state to improve resolution.

Hyperfine sublevel correlation

A two-dimensional electron paramagnetic resonance correlation technique used to resolve overlapping signals from different ions in a molecule.

Interacting quantum atoms

An extension of QTAIM that partitions the total energy of molecular systems into intra-atomic and inter-atomic terms to provide information on binding and stability, amongst many other chemical properties.

Mayer bond order

An analysis technique that can be considered an extension of the Wiberg bond index (see below), incorporating non-orthogonality in the atomic basis.

Natural bond orbitals

Orbitals obtained from a decomposition of the density matrix focusing on the maximization of localized one-centre and two-centre regions in order to recover the closest approximation of a Lewis-type description of electronic structure as possible.

Natural localized molecular orbitals

(NLMOs). Orbitals encompassing the characteristics of natural bond orbitals extended to incorporate acceptor–donor delocalization. NLMOs can be considered 'semi-localized', lying between natural bond orbitals and molecular orbitals in terms of localization.

Natural transition orbitals

Orbitals obtained from the diagonalization of the transition density matrix. They are used to express the electronic excitation in terms of a minimal number of orbital transitions.

Quantum theory of atoms in molecules

(QTAIM). An analysis method based on the electron density as a fundamental quantity. It characterizes a molecule in terms of a set of critical points (satisfying the Poincaré–Hopf relationship) and contiguous, space-filling atomic basins, from which chemical properties can be unambiguously derived.

Wiberg bond index

(WBI). An analysis technique for obtaining an approximation of the formal bond order between bound atoms based on summation of the square of off-diagonal elements of the density matrix expressed over an orthogonal atomic basis.

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Kaltsoyannis, N., Kerridge, A. Understanding covalency in molecular f-block compounds from the synergy of spectroscopy and quantum chemistry. Nat Rev Chem 8, 701–712 (2024). https://doi.org/10.1038/s41570-024-00641-y

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