A very special bond

There is something of a mystery surrounding the metallic elements of the actinide series – just how do they share electrons when forming bonds? Thought to represent a middle ground between ionic and covalent bonds – a team from The University of Manchester used electron paramagnetic resonance spectroscopy to explore this sticky enigma

In a paper recently published in Nature Chemistry, a team of chemists from The University of Manchester have reported a new method for the measurement of covalency in actinide compounds, in other words the extent to which electrons are shared between an actinide metal ion and a coordinated ligand atom.

Actinide-element bonds are thought to have very little covalency as the bonding is predominantly electrostatic, with most electron density localised on the coordinated atom. Calculated “polarised covalent” bonding regimes of actinide complexes are between the two extremes of purely ionic and covalent bonding and thus, for validation of computational models in future, more experimental data is urgently needed. Notably for actinides, relativistic effects must be taken into account and calculations tend to be computationally expensive.

The amount of covalency in actinide-element bonds is a fundamentally important matter because the nature of chemical bonding has direct consequences for chemical reactivity. However, there are very few ways of measuring covalency in actinide compounds quantitatively, and our knowledge of bonding involving these elements lags considerably behind that for other parts of the Periodic Table. This is a topic that has bothered researchers for decades and has significant technological implications, for example, in inter-nuclear separation processes in nuclear fuel cycles. The team at The University of Manchester have shown that pulsed electron paramagnetic resonance (EPR) spectroscopy can provide such information. In the study, they used these techniques to measure the interactions between nominally metal-based unpaired electrons and coordinated ligands in molecular thorium and uranium complexes.

The advantage of using EPR over other techniques is its ability to provide detailed information on the environment of unpaired electrons

The advantage of using EPR over other techniques is its ability to provide detailed information on the environment of unpaired electrons. Importantly, this includes interactions with surrounding nuclei, known as ‘hyperfine interactions’. Analysis of hyperfine interactions reveals the extent of delocalisation, i.e. the sharing of electrons between metal and ligand. The team used modern pulsed EPR methods, such as the hyperfine sublevel correlation (HYSCORE) technique, which give much greater spectral resolution than conventional EPR methods, to measure weak hyperfine interactions. Dr Floriana Tuna, a Senior Research Fellow in the School of Chemistry who led the EPR studies, explains: “It’s a difficult task to measure covalency in f-block compounds. We need more hard data, like that provided by this technique – at the moment no other method can provide such detailed information. Better theoretical models are being developed, but they need to be tested against experiment.”

Ana-Maria Ariciu, a PhD student who performed the EPR experiments, adds: “In this study we could define the extent of covalency and also show significant differences between different actinide ions in analogous complexes.” Dr David Mills, a Lecturer in the School of Chemistry who led the synthetic chemistry studies with PhD student Dr Alasdair Formanuik, comments on the wider potential relevance of the work: “The relative extent of covalency in actinide versus lanthanide bonding has been a topic of much debate since the development of nuclear fuel cycles and the necessity to separate chemically similar f-elements in spent fuel for reprocessing, recycling and long-term storage of waste materials. The tailor-made compounds in this paper would never be found in a nuclear fuel cycle, being highly air- and moisture-sensitive, but they do provide useful models for covalency to be measured to produce textbook data.”

The synthesis of such compounds requires specialist techniques and appropriate facilities, such as those in the School of Chemistry at Manchester, which is host to the Centre for Radiochemistry Research. The EPR studies were then performed in the UK National EPR Facility, based a stone’s throw away in the Photon Science Institute at Manchester. Professor Eric McInnes, co-Director of the EPR Facility, comments that: “The large majority of actinide ions are paramagnetic, i.e. have unpaired electrons – this can make use of some techniques, like NMR, difficult. The beauty of EPR is that it is specifically sensitive to these valence electrons.”

Although actinide- and lanthanide-element bonds are predominantly electrostatic, early actinides are often considered to exhibit a greater amount of covalency than their lanthanide counterparts

Now that the team have shown that pulsed EPR measurements on actinide materials are plausible, they are keen to extend this research to a wider range of actinide and lanthanide compounds in order to provide data to develop our understanding of chemical bonding involving f-block elements. Although actinide- and lanthanide-element bonds are predominantly electrostatic, early actinides are often considered to exhibit a greater amount of covalency than their lanthanide counterparts. This is considered to be in part due to the greater radial extension of 5f (actinide) versus 4f (lanthanide) orbitals, which facilitates increased valence orbital interactions of actinides with coordinated ligands.

For the ligands investigated in this paper, they found that a uranium compound exhibited more covalency than an analogous thorium compound, and the thorium compound was more similar to that seen previously by others for an analogous late lanthanide compound (ytterbium). The overall differences in covalency between these three compounds is relatively small and dependent upon the ligand environment. The team intend to use pulsed EPR techniques to study a wider range of f-element compounds in future, as the amount of covalency depends upon the f-element, its oxidation states and ligand environment. This will produce a large amount of textbook data, which in future may be employed to improve computational models to predict f-element reactivity and the fate of f-elements in nuclear fuel cycles.

Professor Eric McInnes (bl), Dr Floriana Tuna (b), Dr David Mills (tr) and Ana-Maria Ariciu (tl) are based at the School of Chemistry of The University of Manchester were involved in the study published in Nature Chemistry.