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 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 to Cf°. Here we show that a Cf crown-ether complex, Cf(18-crown-6)I, can be prepared using an Al/Hg amalgam as a reductant. Spectroscopic evidence shows that Cf can be quantitatively reduced to Cf, and rapid radiolytic re-oxidation in solution yields co-crystallized mixtures of Cf and Cf 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.