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Actinide covalency measured by pulsed electron paramagnetic resonance spectroscopy


Our knowledge of actinide chemical bonds lags far behind our understanding of the bonding regimes of any other series of elements. This is a major issue given the technological as well as fundamental importance of f-block elements. Some key chemical differences between actinides and lanthanides—and between different actinides—can be ascribed to minor differences in covalency, that is, the degree to which electrons are shared between the f-block element and coordinated ligands. Yet there are almost no direct measures of such covalency for actinides. Here we report the first pulsed electron paramagnetic resonance spectra of actinide compounds. We apply the hyperfine sublevel correlation technique to quantify the electron-spin density at ligand nuclei (via the weak hyperfine interactions) in molecular thorium(III) and uranium(III) species and therefore the extent of covalency. Such information will be important in developing our understanding of the chemical bonding, and therefore the reactivity, of actinides.

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Figure 1: Molecular structures and calculated SOMOs of complexes 1 and 2.
Figure 2: EDFS EPR spectra of complexes 1 and 2 at X-band frequency (9.67 GHz).
Figure 3: X-band HYSCORE spectra for complex 1, measured under the conditions in Fig. 2.
Figure 4: X-band HYSCORE data for complex 2, measured under the conditions in Fig. 2.


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This paper is dedicated to D. Gatteschi (University of Florence) on the occasion of his retirement. We acknowledge the Engineering and Physical Sciences Research Council (grant no. EP/K039547/1, EP/L014416/1 and EP/J002208/1), the Nuclear FiRST DTC (doctoral scholarship to A.F.), Marie Curie Actions, The European Union (FP7-PEOPLE-2013-ITN ‘MAGIC’ Initial Training Network for a doctoral scholarship to A.-M.A.) and The University of Manchester, University College London, Lancaster University and the UK National EPR Facility and Service for supporting this work.

Author information




A.F. synthesized and characterized the compounds. F.O. carried out the single-crystal X-ray diffraction analysis. A.-M.A., F.T. and E.J.L.M. collected and interpreted the EPR spectroscopy and magnetic data. R.B. and A.K. performed and interpreted the calculations. D.P.M. provided the initial concept and supervised A.F., D.P.M. and E.J.L.M. wrote the manuscript, with contributions from all the co-authors.

Corresponding authors

Correspondence to Floriana Tuna or Eric J. L. McInnes or David P. Mills.

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The authors declare no competing financial interests.

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Supplementary information

CrystalData.cif Crystallographic data for compound 1, 2 and ([U(Cptt)3(Cl)]) (CIF 2736 kb)

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Formanuik, A., Ariciu, AM., Ortu, F. et al. Actinide covalency measured by pulsed electron paramagnetic resonance spectroscopy. Nature Chem 9, 578–583 (2017).

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