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Physical Chemistry

Actinide Surprises With Covalent Bonds

Green californium borate complex is unique among actinides, lanthanides

by Jyllian Kemsley
March 31, 2014 | APPEARED IN VOLUME 92, ISSUE 13

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Credit: Nat. Chem.
The layered structure of Cf[B6O8(OH)5] is shown with CfO8 polyhedrons in orange, BO4 tetrahedrons in light green, and BO3 triangles in dark green.
09213-scicon-Cfboratelayerscxd.jpg
Credit: Nat. Chem.
The layered structure of Cf[B6O8(OH)5] is shown with CfO8 polyhedrons in orange, BO4 tetrahedrons in light green, and BO3 triangles in dark green.

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.

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