Probing the Softer-acid Nature of 102No2+ Compared to Alkaline Earth Metal ions via Synergistic Extraction with Sulfur Donor on Crown Ether Supported Resin

Abstract

Abstract

Unlike other f block elements that typically exhibit +3 or higher oxidation states, nobelium (No, atomic number = 102) exists predominantly as the divalent cation in aqueous solutions. While No2+ has long been considered chemically analogous to Ca2+ and Sr2+, strong relativistic effects are predicted to alter its reactivity. In our previous study, the soft metal ion behavior of No2+ was demonstrated through complexation with crown ethers. Prompted by this implication, in this study, we performed online solid-liquid extraction experiments of 255No by employing diethyl dithiophosphate (DEDTP−) as a soft sulfur-donor ligand to probe the softer Lewis acid character of No relative to alkaline earth metals via No–S bonding. Here we report the results of the experiments and relativistic DFT calculations. In the Sr resin/HDEDTP (aqueous) system, No2+ behaves in a manner distinct from Ca2+ and Sr2+, resembling Ba2+ and Ra2+. No2+ exhibits a significantly higher extraction efficiency in the HDEDTP system than Sr2+, confirming enhanced soft Lewis acid character. The computed Gibbs free energies of extraction-complex formation are consistent with the experimental ΔG trend, validating the theoretical model. The optimized geometry revealed a No–S bond shorter than ionic radius trends, indicating stronger bonding. We attribute this distinctive behavior to unique electronic states of No. Specifically, higher electron gain in the d-orbital, stronger relativistic 7s stabilization, and closer 5f energy matching synergistically enhance covalent character of No–S bonding.