Divalent and trivalent gas-phase coordination complexes of californium: evaluating the stability of Cf(ii)

Abstract

The divalent oxidation state is increasingly stable relative to the trivalent state for the later actinide elements, with californium the first actinide to exhibit divalent chemistry under moderate conditions. Although there is evidence for divalent Cf in solution and solid compounds, there are no reports of discrete complexes in which Cf^II is coordinated by anionic ligands. Described here is the divalent Cf methanesulfinate coordination complex, Cf^II(CH₃SO₂)₃⁻, prepared in the gas phase by reductive elimination of CH₃SO₂ from Cf^III(CH₃SO₂)₄⁻. Comparison with synthesis of the corresponding Sm and Cm complexes reveals reduction of Cf^III and Sm^III, and no evidence for reduction of Cm^III. This reflects the comparative 3+/2+ reduction potentials: Cf³⁺ (−1.60 V) ≈ Sm³⁺ (−1.55 V) ≫ Cm³⁺ (−3.7 V). Association of O₂ to the divalent complexes is attributed to formation of superoxides, with recovery of the trivalent oxidation state. The new gas-phase chemistry of californium, now the heaviest element to have been studied in this manner, provides evidence for Cf^II coordination complexes and similar chemistry of Cf and Sm.