Chemists have created a neptunium-oxygen bond that behaves differently from the equivalent uranium bond, overturning long-held assumptions about how heavy radioactive elements form chemical connections. This matters because our understanding of chemical bonding for elements beyond uranium—neptunium, plutonium, and others—remains almost entirely theoretical. The few known compounds are dominated by actinyls, molecules where the metal is tightly clamped between two oxygen atoms in a way that masks its true bonding behaviour. Without experimental data on isolated bonds, scientists cannot test whether periodic trends hold for these elements, nor answer fundamental questions about how covalent their bonds really are. The team’s 2022 discovery that a neptunium-oxo bond is *more* covalent than its uranium counterpart suggests the textbook picture is incomplete. This is fundamental science with no immediate practical application. However, a better grasp of actinide bonding could eventually inform the design of extractant molecules for nuclear waste clean-up, where selectivity depends on subtle differences in covalency between elements. The project also addresses a looming skills shortage by training UK technical staff in handling radioactive materials at Europe’s specialist facilities.
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Context: Metal-ligand multiple-bonding is a cornerstone of chemistry that underpins structure, bonding, reactivity, and catalysis. Isolated metal-ligand multiple bonds have been studied up to uranium, but transuranium-ligand multiple bonding is embryonic and dominated by actinyls, MO2n+ (M = uranium, neptunium, plutonium), and a single neptunium-bis(imido) actinyl analogue. Thus, it has not been possible to study transuranium-ligand multiple bonds in isolation, free of the strong inverse-trans-influence effects that dominate actinyls, but this is vital to do to assess actinide periodicity and answer enduring basic questions about actinide bond covalency. Challenge Addressed: Historically, the paucity of experimental data has held the area back because computational predictions are very difficult to be sure of in a relativistic regime and conducting experimental research in this area requires access to specialist facilities. This project seeks to build on our preliminary result, joint with the European Commission Joint Research Centre at Karlsruhe (JRC), of a neptunium-mono(oxo) complex (Nature Chemistry, 2022, 14, 342-349). That study demonstrated that our approach works, and contrary to expectations, based on the commonly accepted picture of actinide bonding, this neptunium complex was found to be more covalent than the analogous uranium-mono(oxo) complex. This provides a tantalising hint that neptunium chemistry may prove to be even more diverse than the already rich uranium-chemistry and an opportunity to make a long overdue step-change in the area. Aims and Objectives: Our hypothesis is that our preliminary result shows that the area can at last undergo a long overdue expansion, and that through our combination of people, facilities, approach, and ambition we can take the necessary step-change to sustainably elaborate transuranium science. Thus, our aim is to extend out, to realise new high-value transuranium-ligand multiple bonds and in-depth analyses to transform our understanding of actinide periodic trends and hence redefine the state-of-the-art. This will be achieved by delivering the following objectives with verifiable deliverables and milestones: Prepare a range of neptunium-group 16 multiple bonds. Prepare a range of neptunium-nitrogen multiple bonds. Conduct in-depth structural, spectroscopic, magnetic, and computational characterisation. Benefits: This work will make use of the world-leading JRC and KIT Light Source facilities to give unprecedented insight into transuranium electronic structure, bonding, and periodic trend data, which will transform our understanding of these elements and their chemical bonding. It is widely accepted that advances in separations science requires a better understanding of chemical bonding as extractant selectivity originates from covalency differences. Hence, in time the framework of understanding that this work generates might find use in generating new ideas to addressing future separations, e.g. waste clean-up. This project benefits from having a Senior Experimental Officer at the UoM who is a transuranium specialist. His placement at the JRC will promote mobility, putting a member of technical staff at the heart of delivering internationally leading science, thus developing his career in-line with the progressive UKRI agenda to realise greater research prominence and recognition for UKHE technical staff. He will bring best practice from the JRC back to the UK, enhancing the UK’s skills and knowledge for handling radionuclides and glove box techniques, addressing a looming, recognised skills-shortage in this area by disseminating that know-how to new generations of nuclear workers through our Centre for Radiochemistry Research (see LoS). This will benefit UK science and key nuclear sector stakeholders against fierce international competition.
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