Nuclear fuel is both the densest form of energy in its virgin state and, once used, one of the most hazardous materials known to humankind. Though commonly viewed as a waste—with over 300,000 tons stored worldwide and an additional 7–11,000 tons accumulating annually—spent nuclear fuel (SNF) represents a significant potential source of scarce, valuable strategic materials. Beyond the major (U and Pu) and minor (Np, Am, and Cm) actinides, which can be used to generate further energy, resources including the rare earth elements (Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, and Tb), platinum group metals, (Ru, Rh, Pd, and Ag), noble gases (He, Kr, and Xe), and a range of isotopes useful for medical and energy generation purposes are also produced during fission. One reason for the accumulation of so much SNF is the low uptake of SNF recycle (or reprocessing), primarily due to the high capital and operational costs alongside concerns regarding proliferation and wastes generated. This study will highlight the predominantly overlooked potential for the recovery of strategic materials from SNF, which may offset costs and facilitate advanced waste management techniques for minimised waste volumes, thus increasing the sustainability of the nuclear fuel cycle on the path towards Net Zero. Potential challenges in the implementation of this concept will also be identified.

The hypervalent adducts of SiF₄, trans-[SiF₄(R₃PO)₂] (R = Me, Et or Ph), cis-[SiF₄{R₂P(O)CH₂P(O)R₂}] (R = Me or Ph), cis-[SiF₄(pyNO)₂] and trans-[SiF₄(DMSO)₂] have been prepared from SiF₄ and the ligands in anhydrous CH₂Cl₂, and characterised by microanalysis, IR and VT multinuclear (¹H, ¹⁹F, ³¹P) NMR spectroscopy. The NMR studies show extensive dissociation at ambient temperatures in non-coordinating solvents, but mixtures of cis and trans isomers of the monodentate ligand complexes were identified at low temperatures. Crystal structures are reported for trans-[SiF₄(R₃PO)₂] (R = Me or Ph), and cis-[SiF₄(pyNO)₂]. The GeF₄ analogues cis-[GeF₄{R₂P(O)(CH₂)_nP(O)R₂}] (R = Me or Ph, n = 1; R = Ph, n = 2) were similarly characterised and the structures of cis-[GeF₄{R₂P(O)CH₂P(O)R₂}] (R = Me or Ph) determined. The reaction of R₃AsO (R = Me or Ph) with SiF₄ does not give simple adducts, but forms [R₃AsOH]⁺ cations as fluorosilicate salts. SiF₄ adducts of some ether ligands (including THF, 12-crown-4) were also characterised by ¹⁹F NMR spectroscopy in solution at low temperatures (∼190 K), but are fully dissociated at room temperature. Attempts to isolate, or even to identify, SiF₄ adducts with phosphine or thioether ligands in solution at 190 K were unsuccessful, contrasting with the recent isolation and detailed characterisation of GeF₄ analogues. The chemistry of SiF₄ with these oxygen donor ligands, and with soft donors (P, As, S or Se), is compared and contrasted with those of GeF₄, SnF₄ and SiCl₄. The key energy factors determining stability of these complexes are discussed.

The reaction of AlCl₃ with Me₂E (E = S, Se or Te) or ⁿBu₂E (E = Se or Te) in CH₂Cl₂ under rigorously anhydrous conditions gave the pseudo-tetrahedral complexes [AlCl₃(R₂E)]. The [AlX₃(Me₂E)] (X = Br or I, E = S; X = Br, E = Te) were made from toluene solution since attempted syntheses in CH₂Cl₂ resulted in substantial chloride incorporation. The synthesis of [(AlCl₃)₂{o-C₆H₄(CH₂SEt)₂}], in which the ligand bridges two tetrahedral aluminium centres, and of the six-coordinate trans-[AlX₂{MeE(CH₂)₂EMe}₂][AlX₄] (X = Cl or Br, E = S, and X = Cl, E = Se) and cis-[AlI₂{MeS(CH₂)₂SMe}₂][AlI₄] are reported. The tripodal thioether forms [AlCl₃{MeC(CH₂SMe)₃}], which is a chain polymer with κ²-coordinated ligand and a tbp arrangement at Al(III). Chalcogenoether macrocycle complexes [AlCl₃([9]aneS₃)], [AlCl₂([14]aneS₄)][AlCl₄] and [AlCl₂([16]aneSe₄)] [AlCl₄] are also described. All complexes were characterised by microanalysis, IR and multinuclear NMR (¹H, ²⁷Al, ⁷⁷Se or ¹²⁵Te) spectroscopy as appropriate. In CH₂Cl₂ solution [AlCl₃(Me₂S)] with added Me₂S forms [AlCl₃(Me₂S)₂], and the [AlX₂{MeS(CH₂)₂SMe}₂][AlX₄] exist as mixtures of cis and trans isomers which undergo rapid exchange at ambient temperatures. X-Ray crystal structures are reported for [AlCl₃(Me₂Se], [AlX₃(Me₂Te)] (X = Cl or Br), trans-[AlCl₂{MeE(CH₂)₂EMe}₂][AlCl₄] (E = S or Se), cis-[AlI₂{MeS(CH₂)₂SMe}₂][AlI₄], [AlCl₃{MeC(CH₂SMe)₃}], and for the sulfonium salt [Me₂SH][AlCl₄]. The aluminium halide chalcogenoether chemistry is compared with the corresponding gallium and indium systems, and the relative Lewis acidities of the metals discussed. Attempts to use [AlCl₃(ⁿBu₂E)] (E = Se or Te) as LPCVD reagents to form aluminium chalcogenide films were unsuccessful.