How strong uranium bonds influence ligand exchange: Insights from AMS bonding analysis

In most transition-metal complexes, a strongly bound ligand weakens the bond directly opposite to it. Uranium can behave differently. Through the inverse trans-influence (ITI), a strongly donating ligand can instead strengthen the bond on the opposite side of the metal center. Understanding this unusual bonding effect is important for predicting the structure, stability, and reactivity of actinide complexes.

In a recent Inorganic Chemistry study, Ressnik and co-workers combined nuclear magnetic resonance (NMR) spectroscopy with density functional theory (DFT) to investigate ligand exchange in four uranium(VI) mono-oxo complexes. Using ADF in the Amsterdam Modeling Suite (AMS), the researchers analyzed how electronic interactions influence uranium–nitrogen bonding and the experimentally measured ligand-exchange barriers.

Variable-temperature and exchange spectroscopy (EXSY) NMR revealed rapid ligand exchange at room temperature, with activation barriers ranging from 7.98 to 9.15 kcal/mol. The calculations showed that uranium–nitrogen bonds opposite the oxo ligand are approximately 0.1 Å shorter and stronger than their equatorial counterparts, confirming the inverse trans-influence.

The researchers used energy decomposition analysis (EDA) and ETS-NOCV, together with natural localized molecular orbital analysis, to identify the electronic origin of this strengthening. The axial bonds exhibit greater orbital interactions and increased uranium f-orbital contributions. Importantly, the calculated bonding characteristics correlate with the experimentally measured exchange barriers, indicating that stronger inverse trans-influence contributes to slower ligand exchange.

The study demonstrates how bonding analysis with AMS can connect electronic structure to experimentally observed molecular dynamics, providing insights that may help guide ligand design and selective separation strategies for actinide chemistry.