Softly does it
Published in the Nature Communications, recent work by Liz Wildman, a PhD student in the Liddle group, has reported the first examples of thorium-arsenic multiple bonds outside of cryogenic matrix isolation experiments
Nuclear power generates long-lived radioactive waste. In order to find ways of separating, recycling and reducing the volume of nuclear waste, research has focussed on developing our understanding of how elements like thorium and uranium interact with elements that might be integrated into extraction agents.
There is mounting evidence that softer elements like As and S could be effective in separations, but our understanding of the bonding to these elements to actinides is sparse. Therefore, the team targeted thorium-arsenic bonds, and in particular thorium-arsenic multiple bonds because these had been restricted to spectroscopic studies in frozen argon matrices at 5 K.
Building on prior thorium-phosphorus work from the group (Nat. Comm. 2016, 7, 12884), a family of thorium-arsenic compounds exhibiting Th-AsH2, Th=AsH, Th-As(H)-Th, and Th=As=Th linkages were prepared.
Apart from developing the synthetic methodologies to prepare these linkages, a computational study shed intriguing light on their chemical bonding. The chemical bonding of thorium vs uranium is often portrayed as a straight comparison of 6d vs 5f character, but other 7s and 7p valence orbitals do not feature in such descriptions. However, for the arsenic complexes significant intrusion of 7s character in the Th-As bonds was found, and this comes at the expense of 5f character. This contrasts to Th-P systems where 7s intrudes at the expense of 6d character.
Why these observations should be the case is currently not understood, but it suggests that more families of related molecules need to be synthesised and studied to more fully understand the phenomena which might be exploitable in extraction processes in the future.
Citation: E. P. Wildman, G. Balázs, A. J. Wooles, M. Scheer, S. T. Liddle, Nat. Comm., 2017, 8, 14769.