Isolation of a Californium(II) Crown-Ether Complex

Abstract Californium ( Z = 98) is the first member of the actinide series displaying metastability of the 2 + oxidation state. Understanding the origin of this chemical behavior requires characterizing Cf II materials, but isolating a complex with this state has remained elusive. The source of its inaccessibility 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 0 . Herein 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. While spectroscopic evidence shows that Cf III can be quantitatively reduced to Cf II , rapid radiolytic re-oxidation back to the Cf III parent occurs and co-crystallized mixtures of Cf II and Cf III complexes are isolated if the crystallization is not conducted over the Al/Hg amalgam. Quantum chemical calculations show that the Cf‒ligand interactions are highly ionic and that 5 f /6 d mixing is absent, resulting in remarkably weak 5 f →5 f transitions and an absorption spectrum dominated by 5 f →6 d transitions.