Observation of a promethium complex in solution

Lanthanide rare-earth metals are ubiquitous in modern technologies^1–5, but we know little about chemistry of the 61st element, promethium (Pm)⁶, a lanthanide that is highly radioactive and inaccessible. Despite its importance^7,8, Pm has been conspicuously absent from the experimental studies of lanthanides, impeding our full comprehension of the so-called lanthanide contraction phenomenon: a fundamental aspect of the periodic table that is quoted in general chemistry textbooks. Here we demonstrate a stable chelation of the ¹⁴⁷Pm radionuclide (half-life of 2.62 years) in aqueous solution by the newly synthesized organic diglycolamide ligand. The resulting homoleptic Pm^III complex is studied using synchrotron X-ray absorption spectroscopy and quantum chemical calculations to establish the coordination structure and a bond distance of promethium. These fundamental insights allow a complete structural investigation of a full set of isostructural lanthanide complexes, ultimately capturing the lanthanide contraction in solution solely on the basis of experimental observations. Our results show accelerated shortening of bonds at the beginning of the lanthanide series, which can be correlated to the separation trends shown by diglycolamides^9–11. The characterization of the radioactive Pm^III complex in an aqueous environment deepens our understanding of intra-lanthanide behaviour^12–15 and the chemistry and separation of the f-block elements¹⁶.