Surprise findings about lanthanide and actinide chemical bonding could have far-reaching implications

Research published in Nature Communications has resulted in a surprise finding about the nature of lanthanide and actinide chemical bonding, suggesting that an important but previously niche bonding model may be general

A cerium(IV) bis(carbene) complex that exhibits the inverse-trans-influence

The trans-influence is a well-established concept with the majority of metals. Conversely, for heavy elements like uranium the inverse-trans-influence (ITI) operates. Here, ligands that would normally avoid being trans-destabilised preferentially reside trans, and this actually mutually reinforces them. However, the ITI was limited to high oxidation state actinides, e.g. {UO2}2+.

These moieties are important environmentally, but the universal view was that the ITI is a niche concept because it seemed to be restricted to high oxidation state actinides. So whether this was a niche rule or hinted at a broader concept in f-block chemistry, in an area of few rules, had been an enduring question.

To probe whether the ITI has broader relevance, the team targeted lower metal oxidation state complexes, in this case +4, with actinides and lanthanides. In order to do this the team had to match the metal-ligand frontier orbital energies so C-based ligands were needed.

Through an interdisciplinary study spanning Manchester, Lancaster, Nottingham, and Grenoble beamline the team were able to fully characterise cerium, uranium, and thorium complexes with C=M=C cores and computational work suggests that the ITI does operate in these complexes. The most notable observation is the fact that the two trans carbon atoms are bonded very closely to the metals, and this is a tell-tale sign of the ITI.

The combination of finding the ITI in +4 metal ion complexes as well as a lanthanide as well as actinide complexes leads to the conclusion that this phenomenon may be of broader scope than previously thought.

Citation: M. Gregson, E. Lu, D. P. Mills, F. Tuna, E. J. L. McInnes, C. Hennig, A. C. Scheinost, J. McMaster, W. Lewis, A. J. Blake, A. Kerridge, S. T. Liddle, Nat. Comm., 2017, 8, 14137.

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