Curium is unique in the actinide series because its half-filled 5f 7 shell has lower energy than other 5f n configurations, rendering it both redox-inactive and resistant to forming chemical bonds that engage the 5f shell1,2,3. This is even more pronounced in gadolinium, curium’s lanthanide analogue, owing to the contraction of the 4f orbitals with respect to the 5f orbitals4. However, at high pressures metallic curium undergoes a transition from localized to itinerant 5f electrons5. This transition is accompanied by a crystal structure dictated by the magnetic interactions between curium atoms5,6. Therefore, the question arises of whether the frontier metal orbitals in curium(iii)–ligand interactions can also be modified by applying pressure, and thus be induced to form metal–ligand bonds with a degree of covalency. Here we report experimental and computational evidence for changes in the relative roles of the 5f/6d orbitals in curium–sulfur bonds in [Cm(pydtc)4]− (pydtc, pyrrolidinedithiocarbamate) at high pressures (up to 11 gigapascals). We compare these results to the spectra of [Nd(pydtc)4]− and of a Cm(iii) mellitate that possesses only curium–oxygen bonds. Compared with the changes observed in the [Cm(pydtc)4]− spectra, we observe smaller changes in the f–f transitions in the [Nd(pydtc)4]− absorption spectrum and in the f–f emission spectrum of the Cm(iii) mellitate upon pressurization, which are related to the smaller perturbation of the nature of their bonds. These results reveal that the metal orbital contributions to the curium–sulfur bonds are considerably enhanced at high pressures and that the 5f orbital involvement doubles between 0 and 11 gigapascal. Our work implies that covalency in actinides is complex even when dealing with the same ion, but it could guide the selection of ligands to study the effect of pressure on actinide compounds.
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Crystallographic information files have been deposited in the Cambridge Crystallographic Data Centre (CCDC; https://www.ccdc.cam.ac.uk/structures/) with deposition numbers 1927752 (Cm-1), 1927751 (Cm-2) and 1930252 (Nd-1).
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All experimental studies and the high-pressure molecular geometry and electronic structure calculations were supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Elements Chemistry Program under award numbers DE-FG02-13ER16414 (T.E.A.-S.) and DE-SC0001136 (J.A.). Theoretical studies after high-pressure evaluation were supported by the US Department of Energy through the Center for Actinide Science and Technology (CAST) funded by the Energy Frontiers Research Program under award number DE-SC0016568. We are grateful for the assistance and supervision of the Office of Environmental Health and Safety at Florida State University, specifically J. A. Johnson and A. L. Gray of the Office of Radiation Safety for their facilitation of these studies. D.-C.S., X.W., E.Z. and J.A. thank the Center for Computational Research (CCR) at the University at Buffalo for providing computational resources. E.Z. acknowledges the NSF (DMR-1827815) for financial support.
The authors declare no competing interests.
Peer review information Nature thanks Olle Eriksson, Stephen Moggach and Tonya Vitova for their contribution to the peer review of this work.
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Extended data figures and tables
a, f–f transitions in Cm-1 spectra show large bathochromic shifts as pressure is applied; however, the pydtc− ligand-based broadband absorption quickly obscures the Cm3+ transitions. b, In Cm-2, these transitions are not competing with any broadband ligand-based transitions; however, the f–f transitions show much smaller shifts up to high pressures than those observed for Cm-1. c, The shifts observed in Nd-1, the Nd3+ analogue of Cm-1, are more similar to those of Cm-2 than those of Cm-1.
a, b, Phosphorescence spectra of Cm-2 as a function of temperature (a) and pressure (b) (λex = 420 nm). c, d, Emission decay curves (λex = 443 nm, λem = 609 nm) for Cm-1 (c) and Cm-2 (d) (λex = 434 nm, λem = 596 nm). Even at low temperatures the Stark levels are not clearly resolved in Cm-2. Pressure does not induce considerable broadening, and only minor bathochromic shifts are observed. Cm-1 has a weighted lifetime of 92 μs and Cm-2 has a lifetime of 18 μs. The longer lifetime of Cm-1 compared to Cm-2 is reflected in the increased intensity of the emission.
a, NLMOs (isosurface of ±0.02) obtained from the MC-pDFT+SO/tPBE wavefunctions of the lowest-energy components of the 8S7/2 ground state and the photoluminescent 6D7/2 state of Cm-1 at 0 GPa (left) and 11 GPa (right). NLMO compositions (averaged over the eight Cm–S bonds), aggregate Cm Wiberg bond index (WBI) and NLMO/NPA-based effective bond order (EBONLMO/NPA, obtained from the NLMO shared occupancies and bonding–antibonding overlap) (NPA, natural population analysis). b, NLMOs, aggregate Cm WBI and EBONLMO/NPA of the 8S7/2 ground state of Cm-2 (theoretical model).
a, b, The reaction of Cm3+ before (a) and after (b) the addition of [NH4][pydtc]. c, Single crystal upon excitation with 420-nm light at 20 °C (bottom), −80 °C (middle) and −180 °C (top). d, e, Cm-1 crystals before (d) and during (e) excitation with 420-nm light. f, g, Single crystals before (f) and during (g) irradiation with 420-nm light. h, The reaction vial of Cm-2. i, Cm-2 crystal under 420-nm light at 20 °C (left), −80 °C (middle) and −180 °C (right).
Geometry of Cm-2 units (nine-coordinated Cm by three mellitate ligands and four water ligands) at 0 GPa (left, wire frame) and 7 GPa (right, wire frame) superimposed with model geometry Cm-2′ (‘balls & sticks’). The model geometries retain the original Cm coordination while substituting the mellitate C6(CO2)4 terminals by CF3. The model geometries were used for MC-pDFT+SO calculations and DFT ground-state bonding analyses.
Two different pressures are shown, 0 GPa (red line) and 8.0 GPa (blue line). Inset, total ligand-field splitting of the 8S7/2 term (left; red, 0 GPa; blue 8 GPa) and total splitting of the first excited 6D7/2 term (right; red, 0 GPa; blue, 8 GPa; black, calculated energies).
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Sperling, J.M., Warzecha, E.J., Celis-Barros, C. et al. Compression of curium pyrrolidine-dithiocarbamate enhances covalency. Nature 583, 396–399 (2020). https://doi.org/10.1038/s41586-020-2479-2
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