₁₀₂No²⁺ as a soft metal ion: Extraction of ²⁵⁵No using crown ethers

The chemical properties of transfermium elements with atomic numbers >100 are largely unknown. Element 102, nobelium, is an actinide and forms stable +2 valent cations in aqueous solutions, unlike other heavy actinide. In previous experiments, No²⁺ exhibits similar behavior to alkaline earth metal ions, particularly Ca²⁺ and Sr²⁺, suggesting hard-acid character. In this study, the soft metal ion behavior of No²⁺ was demonstrated through complex with crown ethers. In the solid-liquid extraction of ²⁵⁵No using di-^tBu-dicyclohexano-18-crown-6 in HNO₃, No²⁺ was extracted as much as Sr²⁺. Density functional theory calculations confirmed higher stability of the No²⁺-DtBuCH18C6 complex, with covalent bonding involving the 6d orbitals. Theoretical studies also revealed strong relativistic effects on the atomic orbitals of No²⁺, rendering No²⁺ a soft ion similar to Pb²⁺.