Compression of curium pyrrolidine-dithiocarbamate enhances covalency

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 shell^1-3. 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 atoms^5,6. 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.