Actinide Surprises With Covalent Bonds

The conventional view of actinides is that they’re basically heavy lanthanides, with bonding driven by ionic interactions. A new study of a californium borate complex demonstrates that that view may be wrong. Wrap a Cf(III) center in polyborate chains and it looks much like a transition metal with covalent bonds to ligands, according to a team led by Thomas E. Albrecht-Schmitt of Florida State University; Laura Gagliardi of the University of Minne­sota, Twin Cities; and David A. Dixon of the University of Alabama, Tuscaloosa (Nat. Chem. 2014, DOI: 10.1038/nchem.1896). The researchers reacted ²⁴⁹CfCl₃ with molten boric acid to produce Cf[B₆O₈(OH)₅] microcrystals that glow green. Spectroscopic and computational analyses indicate that the Cf–O bonds contain significant covalent character, with electron donation from oxygen 2p orbitals to californium 6d, 7p, and 5f orbitals. Previous work on Pu(III), Am(III), and Cm(III) borates and various lanthanide compounds showed different structures and either no covalency or far weaker covalency than that observed in Cf[B₆O₈(OH)₅]. The ability to form covalently bonded actinide compounds could change approaches to separating and storing nuclear waste, Albrecht-Schmitt says.