Isolation of a californium(II) crown-ether complex

The actinides, from californium to nobelium (Z = 98-102), are known to have an accessible +2 oxidation state. Understanding the origin of this chemical behaviour requires characterizing Cf-II materials, but investigations are hampered by the fact that they have remained difficult to isolate. This partly arises from the intrinsic challenges of manipulating this unstable element, as well as a lack of suitable reductants that do not reduce Cf-III to Cf-degrees. Here we show that a Cf-II crown-ether complex, Cf(18-crown-6)I-2, can be prepared using an Al/Hg amalgam as a reductant. Spectroscopic evidence shows that Cf-III can be quantitatively reduced to Cf-II, and rapid radiolytic re-oxidation in solution yields co-crystallized mixtures of Cf-II and Cf-III complexes without the Al/Hg amalgam. Quantum-chemical calculations show that the Cf-ligand interactions are highly ionic and that 5f/6d mixing is absent, resulting in weak 5f -> 5f transitions and an absorption spectrum dominated by 5f -> 6d transitions.