Actinide Chemistry Research Articles

Determination of Uranium Central-Field Covalency with 3d4f Resonant Inelastic X-ray Scattering.

Understanding the nature of metal-ligand bonding is a major challenge in actinide chemistry. We present a new experimental strategy for addressing this challenge using actinide 3d4f resonant inelastic X-ray scattering (RIXS). Through a systematic study of uranium(IV) halide complexes, [UX6]2-, where X = F, Cl, or Br, we identify RIXS spectral satellites with relative energies and intensities that relate to the extent of uranium-ligand bond covalency. By analyzing the spectra in combination with ligand field density functional theory we find that the sensitivity of the satellites to the nature of metal-ligand bonding is due to the reduction of 5f interelectron repulsion and 4f-5f spin-exchange, caused by metal-ligand orbital mixing and the degree of 5f radial expansion, known as central-field covalency. Thus, this study furthers electronic structure quantification that can be obtained from 3d4f RIXS, demonstrating it as a technique for estimating actinide-ligand covalency.

The role of modern spectroscopy and chromatography in actinide and lanthanide chemistry for nuclear forensics

The field of nuclear forensics relies heavily on the accurate identification and characterization of actinides and lanthanides to trace the origins and history of nuclear materials. Modern spectroscopy and chromatography play a pivotal role in this domain, offering precise analytical techniques to detect and quantify these elements in various samples. This review provides a comprehensive examination of the contributions of spectroscopy and chromatography to actinide and lanthanide chemistry within the context of nuclear forensics. It begins with an overview of the fundamental principles of these methods, followed by detailed discussions on the specific spectroscopic techniques such as mass spectrometry (including ICP-MS and TIMS), X-ray absorption spectroscopy (XANES and EXAFS), and laser-induced breakdown spectroscopy (LIBS). The chromatographic methods covered include high-performance liquid chromatography (HPLC), gas chromatography (GC), and capillary electrophoresis (CE). The integration of these techniques for enhanced forensic analysis is explored through case studies and examples demonstrating their combined use. Furthermore, recent advances, innovations, and future trends in the field are discussed, highlighting the move towards automation and high-throughput analysis. The review concludes with an exploration of the analytical challenges and limitations, along with recommendations for future research directions. This comprehensive review underscores the critical role of advanced spectroscopy and chromatography in advancing the capabilities of nuclear forensics, ensuring robust and reliable forensic investigations.

Preparation of Neptunyl and Plutonyl Acetates To Access Nonaqueous Transuranium Coordination Chemistry.

Uranyl diacetate dihydrate is a useful reagent for the preparation of uranyl (UO22+) coordination complexes, as it is a well-defined stoichiometric compound featuring moderately basic acetates that can facilitate protonolysis reactivity, unlike other anions commonly used in synthetic actinide chemistry such as halides or nitrate. Despite these attractive features, analogous neptunium (Np) and plutonium (Pu) compounds are unknown to date. Here, a modular synthetic route is reported for accessing stoichiometric neptunyl(VI) and plutonyl(VI) diacetate compounds that can serve as starting materials for transuranic coordination chemistry. The new NpO22+ and PuO22+ complexes, as well as a corresponding molecular UO22+ complex, are isomorphous in the solid state, and in solution show similar solubility properties that facilitate their use in synthesis. In both solid and solution state, the +VI oxidation state (O.S.) is maintained, as demonstrated by vibrational and optical spectroscopy, confirming that acetate anions stabilize the oxidizing, high-valent +VI states of Np and Pu as they do for the more stable U(VI). All three acetate salts readily react with a model diprotic ligand, affording incorporation of U(VI), Np(VI), and Pu(VI) cores into molecular coordination compounds that occurs concomitantly with elimination of acetic acid; the new complexes are high-valent, yet overall charge neutral, facilitating entry into nonaqueous chemistry by rational synthesis. Computational studies reveal that the dianionic ligand framework assists in stabilizing the +VI O.S. via donation to the 5f shells of the actinides, highlighting the potential usefulness of protonolysis reactivity toward preparation of stabilized high-valent transuranic species.

Review of actinide core-level photoemission

Photoelectron spectroscopy allows for the investigation of the electronic structure and chemical bonding of actinide elements and their compounds, providing insights into oxidation states, chemical environments, and electronic configurations. This knowledge can aid in comprehending reactivity, stability, and other properties of actinide materials, which is essential for ensuring safe handling, storage, and disposal in nuclear applications. We have reviewed a number of results in actinide core-level photoemission studies, with a particular focus on x-ray photoemission spectroscopy (XPS) techniques. Actinides, due to their inherent radioactivity, have not been as well studied with XPS as have other segments of the periodic table. Given the inherent safety concerns, equipment requirements, and short isotopic lifetimes associated with actinide research, we outline the strategies and precautions necessary for conducting successful and safe XPS experiments on these elements. Core-level photoemission can be a powerful proven tool for investigating the electronic structure, chemical bonding behaviors, and physical properties of actinides, providing valuable insights into an incredibly complex behavior of these systems. We highlight key findings from recent studies that demonstrate the potential of core-level photoemission in uncovering the unique properties of actinides and their compounds. Finally, we identify current knowledge gaps and future research directions that could enhance our understanding of actinide chemistry and physics.

Structural evolution and electronic properties of anionic plutonium-doped oxygen clusters

Plutonium oxides are important in actinide chemistry and nuclear industry activities, and the demand and interest in enriching knowledge of plutonium chemistry is growing increasingly significant. However, unlike uranium oxide clusters, few studies related to the geometry and bonding properties of anionic plutonium oxide clusters have been reported. Here we explored the geometries, photoelectron spectra, and electronic properties of anionic plutonium oxide clusters by means of unbiased structural searches coupled with density-functional theory (DFT). The results show that the PuO6− cluster with a high D2h symmetry formed by a linear O-Pu-O plutonyl unit with two O2 units exhibits excellent stability. In addition, the HOMO–LUMO gap value of PuO6− cluster are 4.35 eV and 3.41 eV for the α- and β-orbitals, respectively, as well as the vertical detachment energy (VDE) of 4.20 eV. In the analysis of molecular orbitals and adaptive natural density partitioning in oxygen-rich systems, the σ bonds between plutonium atom and oxygen ligands, primarily from the Pu-5f and O-2p orbitals, are responsible for the goodish stability of PuO6− cluster. We hope that the current work can enrich the geometrical configuration of plutonium oxide oxygen clusters and provide reference information for the subsequent research work.

On the aromaticity of actinide compounds.

The chemistry of actinides has flourished since the late 2010s with the synthesis of new actinide complexes and clusters. On the theoretical side, a range of tools is available for the characterization of these heavy element-containing compounds, but discrepancies in the assessment of aromaticity using different tools have led to controversies. In this Perspective, we examine the origin of controversies relating to the aromaticity of metallic compounds, with a focus on actinides. The aromaticity of actinides is important, not because these molecules are numerous or have a special role in catalysis or reactivity, but because this topic pushes theories of aromaticity to their limits. Owing to its reference independence, the magnetic criterion of aromaticity has been the most popular choice for the characterization of the aromaticity of metallic compounds, including actinide compounds. Through examination of several case studies, we show why this criterion might be misleading for metallic species and explain how findings relating to actinide compounds could reshape theories of aromaticity, not just for actinides but perhaps also for well-known hydrocarbons.

Progress in Nonaqueous Molecular Uranium Chemistry: Where to Next?

There is long-standing interest in nonaqueous uranium chemistry because of fundamental questions about uranium's variable chemical bonding and the similarities of this pseudo-Group 6 element to its congener d-block elements molybdenum and tungsten. To provide historical context, with reference to a conference presentation slide presented around 1988 that advanced a defining collection of top targets, and the challenge, for synthetic actinide chemistry to realize in isolable complexes under normal experimental conditions, this Viewpoint surveys progress against those targets, including (i) CO and related π-acid ligand complexes, (ii) alkylidenes, carbynes, and carbidos, (iii) imidos and terminal nitrides, (iv) homoleptic polyalkyls, -alkoxides, and -aryloxides, (v) uranium-uranium bonds, and (vi) examples of topics that can be regarded as branching out in parallel from the leading targets. Having summarized advances from the past four decades, opportunities to build on that progress, and hence possible future directions for the field, are highlighted. The wealth and diversity of uranium chemistry that is described emphasizes the importance of ligand-metal complementarity in developing exciting new chemistry that builds our knowledge and understanding of elements in a relativistic regime.

Complexation of Hexavalent Neptunium(VI) with Oxydiacetic Acid and Its Amide Derivatives in Aqueous Solution: Spectrophotometry and DFT Calculations.

Unfolding the solution coordination chemistry of high-valent transuranium elements with the "CHON"-type ligands is important to understand the fundamental chemistry of actinides and to design more efficient extractants for partitioning of transuranium elements in advanced nuclear fuel cycles. Here, the complexation of a hexavalent neptunyl ion (NpO22+ or Np(VI)) with oxydiacetic acid (ODA) has been systematically investigated in comparison with its amide analogues N,N-dimethyl-3-oxa-glutaramic acid (DMOGA) and N,N,N',N'-tetramethyl-3-oxa-glutaramide (TMOGA) both experimentally and computationally. The formation of both 1:1 and 1:2 complexes between Np(VI) and the three ligands was identified by spectrophotometry, and their stability constants were obtained and compared with those of hexavalent U(VI) and Pu(VI). The corresponding bonding nature is elucidated by using energy decomposition analysis (EDA), electrostatic potential (ESP), ELF contours, and natural orbitals for chemical valence (NOCV) methods, which shows that the Np-O bonds are essentially ionic in character and the unoccupied 6d orbitals of Np play a key role in enhancing the covalent interactions between Np(VI) and the three ligands.

N-Aryloxide-Amidinate Thorium Complexes.

An N-aryloxide-amidine ligand (1), [ONNO] ligand, integrating phenoxide (PhO-) and amidine ligands through methylene linkers, was employed in actinide chemistry. Upon reaction of the deprotonated ligand with ThCl4(DME)2 in ether, the corresponding dimer complex 2 was obtained. Upon treatment of 2 with KCp* (Cp* = Cp(Me)5) in tetrahydrofuran, the corresponding {[ONNO]ThIVCp*(LiCl)}2 (4) was obtained. In complex 2, the two ArO- arms bonded from the same ligand to different ThIV centers. In contrast, both ArO- arms coordinated to the same metal center in 4. Notably, when a mixture of 2 and bipyridine was treated with one or two equiv of KC8, the [ONNO]ThIV-bipyridyl•̅ radical dimer complex (5) and [ONNO]ThIV-bipyridyl2- dianionic dimer species (6) were obtained, respectively.

Highly selective separation of tetravalent plutonium from complex system with novel phenylpyridine diamide ligands.

In this study, three phenylpyridine diamide ligands, namely, 2,2'-((pyridine-2,6-diylbis(3,1-phenylene))bis(oxy))bis(N,N-diethylacetamide) (PPEA, L1), 2,2'-((pyridine-2,6-diylbis(3,1-phenylene))bis(oxy))bis(N-ethyl-N-phenylacetamide) (PEPA, L2), and 2,2'-(((4-phenylpyridine-2,6-diyl)bis(3,1-phenylene))bis(oxy))bis(N,N-dioctylacetamide) (PPOA, L3), were synthesized and explored for the solvent extraction of Pu(iv) in a HNO3 medium using 1-(trifluoromethyl)-3-nitrobenzene as the diluent. The effects of HNO3 concentration, extractant concentration, and temperature on the Pu(iv) extraction efficiency were studied. All three extractants displayed high selectivity for Pu(iv) over other metals such as U(vi), Np(v), Am(iii), and various fission elements. At 3 M HNO3, the distribution ratio for Pu(iv) reached 27.18, in contrast to 1.11, 0.3, and 0.03 for U(vi), Np(v), Am(iii), respectively. Slope analysis and UV titration revealed the formation of 1 : 1 Pu(NO3)4/ligand complexes during extraction. The extraction reactions had negative Gibbs free energies, indicating the spontaneous nature of Pu(iv) extraction at room temperature. Furthermore, the extractants demonstrated good stripping ability and reusability, and their radiolytic stability was reasonable up to an absorbed dose of 100 kGy, underscoring their potential for practical applications. Overall, this study broadens our understanding of actinide-diamide ligand coordination and actinide chemistry during coordination, paving the way for the design and synthesis of new extractants.

Structures of Th4+ aqueous solutions: insights from AIMD and metadynamics simulations.

Solution chemistry of actinide ions is critical to understanding the solvation behaviors and hydrolysis process. Using tetravalent thorium ion Th4+ as a representative example, we investigate the local structures and dynamic behaviors of hydrated Th4+ ions by ab initio molecular dynamics (AIMD) simulations using the recently developed norm-conserving pseudopotentials and basis sets optimized for actinides (J.-B. Lu et al., J. Chem. Theory Comput. 2021, 17, 3360-3371). AIMD simulations reveal two distinct solvation shells, with the first shell comprising 9 water molecules at approximately rTh-O = 2.50 Å and exhibiting a tricapped trigonal prism geometry. These conclusions are confirmed through metadynamics simulations and further structural analysis. AIMD simulations also show the slight effect of temperature and counterions on the structure of the solution. The structured solvation shells of the highly charged Th4+ ion with the specific geometry, distinct from the structure of liquid water, lead to corresponding structural changes in the hydrogen bond network in water. Additionally, beyond the solvent-shared ion pair (SIP) state observed in the unbiased AIMD simulations, the metadynamics simulations reconstruct a two-dimensional free energy surface that clearly indicates the potential stability of the contact ion pair (CIP) state in the system with Cl- as a counterion. The findings in this work provide insights into the solution chemistry of actinides and serve as a reference for studying other actinide solution systems.

Extension of the D3 and D4 London dispersion corrections to the full actinides series.

Efficient dispersion corrections are an indispensable component of modern density functional theory, semi-empirical quantum mechanical, and even force field methods. In this work, we extend the well established D3 and D4 London dispersion corrections to the full actinides series, francium, and radium. To keep consistency with the existing versions, the original parameterization strategy of the D4 model was only slightly modified. This includes improved reference Hirshfeld atomic partial charges at the ωB97M-V/ma-def-TZVP level to fit the required electronegativity equilibration charge (EEQ) model. In this context, we developed a new actinide data set called AcQM, which covers the most common molecular actinide compound space. Furthermore, the efficient calculation of dynamic polarizabilities that are needed to construct CAB6 dispersion coefficients was implemented into the ORCA program package. The extended models are assessed for the computation of dissociation curves of actinide atoms and ions, geometry optimizations of crystal structure cutouts, gas-phase structures of small uranium compounds, and an example extracted from a small actinide complex protein assembly. We found that the novel parameterizations perform on par with the computationally more demanding density-dependent VV10 dispersion correction. With the presented extension, the excellent cost-accuracy ratio of the D3 and D4 models can now be utilized in various fields of computational actinide chemistry and, e.g., in efficient composite DFT methods such as r2SCAN-3c. They are implemented in our freely available standalone codes (dftd4, s-dftd3) and the D4 version will be also available in the upcoming ORCA 6.0 program package.

Multiconfigurational actinide nitrides assisted by double Möbius aromaticity.

Understanding the bonding nature between actinides and main-group elements remains a key challenge in actinide chemistry due to the involvement of f orbitals. Herein, we propose a unique "aromaticity-assisted multiconfiguration" (AAM) model to elucidate the bonding nature in actinide nitrides (An2N2, An = Ac, Th, Pa, U). Each planar four-membered An2N2 with equivalent An-N bonds possesses four delocalized π electrons and four delocalized σ electrons, forming a new family of double Möbius aromaticity that contributes to the molecular stability. The unprecedented aromaticity further supports actinide nitrides to exhibit multiconfigurational characters, where the unpaired electrons (2, 4 or 6 in naked Th2N2, Pa2N2 or U2N2, respectively) either are spin-free and localized on metal centres or form metal-ligand bonds. High-level multiconfigurational computations confirm an open-shell singlet ground state for actinide nitrides, with small energy gaps to high spin states. This is consistent with the antiferromagnetic nature observed experimentally in uranium nitrides. The novel AAM bonding model can be authenticated in both experimentally identified compounds containing a U2N2 motif and other theoretically modelled An2N2 clusters and is thus expected to be a general chemical bonding pattern between actinides and main-group elements.

Synthesis and Characterization of Cerium-Oxo Clusters Capped by Acetylacetonate.

Cerium-oxo clusters have applications in fields ranging from catalysis to electronics and also hold the potential to inform on aspects of actinide chemistry. Toward this end, a cerium-acetylacetonate (acac1-) monomeric molecule, Ce(acac)4 (Ce-1), and two acac1--decorated cerium-oxo clusters, [Ce10O8(acac)14(CH3O)6(CH3OH)2]·10.5MeOH (Ce-10) and [Ce12O12(OH)4(acac)16(CH3COO)2]·6(CH3CN) (Ce-12), were prepared and structurally characterized. The Ce(acac)4 monomer contains CeIV. Crystallographic data and bond valence summation values for the Ce-10 and Ce-12 clusters are consistent with both clusters having a mixture of CeIII and CeIV cations. Ce L3-edge X-ray absorption spectroscopy, performed on Ce-10, showed contributions from both CeIII and CeIV. The Ce-10 cluster is built from a hexameric cluster, with six CeIV sites, that is capped by two dimeric CeIII units. By comparison, Ce-12, which formed upon dissolution of Ce-10 in acetonitrile, consists of a central decamer built from edge sharing CeIV hexameric units, and two monomeric CeIII sites that are bound on the outer corners of the inner Ce10 core. Electrospray ionization mass spectrometry data for solutions prepared by dissolving Ce-10 in acetonitrile showed that the major ions could be attributed to Ce10 clusters that differed primarily in the number of acac1-, OH1-, MeO1-, and O2- ligands. Small angle X-ray scattering measurements for Ce-10 dissolved in acetonitrile showed structural units slightly larger than either Ce10 or Ce12 in solution, likely due to aggregation. Taken together, these results suggest that the acetylacetonate supported clusters can support diverse solution-phase speciation in organic solutions that could lead to stabilization of higher order cerium containing clusters, such as cluster sizes that are greater than the Ce10 and Ce12 reported herein.

Actinide‐Pnictide Chemistry: A Uranium Primary Alkyl Stibinide and a Diuranium Hexaantimonide‐Tetralithium Zintl Cluster

AbstractStructurally characterised U−Sb bonds are rare. Here, the synthesis and characterisation of [U(TrenDMBS){Sb(H)C(H)(SiMe3)2}] (5, TrenDMBS=N(CH2CH2NSiMe2But)3) and [{U(TrenTIPS)}2{Sb3(μ6‐Li)(μ‐Li[THF]2)3Sb3}] (6, TrenTIPS=N(CH2CH2NSiPri3)3) are reported. Complex 5 is an unprecedented primary stibinide actinide complex and 6 can be formulated as containing a unique triple decker 20 skeletal‐electron distorted closo tricapped Zintl cluster (Sb3Li4Sb3)2− that is analogous to the D3h closo tricapped trigonal prism form of [Ge9]2−. Quantum chemical calculations emphasise polar U−Sb bonding in 5 and 6, and that viewing (Sb3Li4Sb3)2− as a single unit rather than two distinct but contiguous (Sb3)3− units, themselves unprecedented in molecular actinide chemistry, is valid due to the presence of three weak (Sb3)⋯(Sb3) (3, −1)‐bond critical points. Complexes 5 and 6 highlight the challenges of preparing polar U−Sb bonds and the underlying tendency for unpredictable cluster formation with Sb.

Actinide inverse trans influence versus cooperative pushing from below and multi-center bonding

Actinide-ligand bonds with high multiplicities remain poorly understood. Decades ago, an effect known as 6p pushing from below (PFB) was proposed to enhance actinide covalency. A related effect—also poorly understood—is inverse trans influence (ITI). The present computational study of actinide-ligand covalent interactions with high bond multiplicities quantifies the energetic contributions from PFB and identifies a hitherto overlooked fourth bonding interaction for 2nd-row ligands in the studied organometallic systems. The latter are best described by a terminal O/N ligand exhibiting quadruple bonding interactions with the actinide. The 4th interaction may be characterized as a multi-center or charge-shift bond involving the trans ligand. It is shown in this work that the 4th bonding interaction is a manifestation of ITI, assisted by PFB, and provides a long-sought missing piece in the understanding of actinide chemistry.

Insight into the structural ambiguity of actinide(IV) oxalate sheet structures: a case for alternate coordination geometries.

Plutonium(IV) oxalate hexahydrate (Pu(C2O4)2∙6H2O; PuOx) is an important intermediate in the recovery of plutonium from used nuclear fuel. Its formation via precipitation is well studied, yet its crystal structure remains unknown. Instead, the crystal structure of PuOx is assumed to be isostructural with neptunium(IV) oxalate hexahydrate (Np(C2O4)-2∙6H2O; NpOx) and uranium(IV) oxalate hexahydrate (U(C2O4)-2∙6H2O; UOx) despite the high degree of unresolved disorder that exists when determining water positions in the crystal structures of the latter two compounds. Such assumptions regarding the isostructural behavior of the actinide elements have been used to predict the structure of PuOx for use in a wide range of studies. Herein, we report the first crystal structures for PuOx and Th(C2O4)2·6H2O (ThOx). This data, along with new characterization of UOx and NpOx, has resulted in the full determination of the structures and resolution of the disorder around the water molecules. Specifically, we identify the coordination of two water molecules with each metal center, which necessitates a change in oxalate coordination mode from axial to equatorial that has not been reported in the literature. This work exemplifiesthe need to revisit previous assumptions regarding fundamental actinide chemistry, which are heavily relied upon within the current nuclear field.

The interplay of d and f orbitals in the chemistry of the early actinides: High spin versus low spin

Early actinides lie at the borderline between d- and f-block elements. Here we describe the coordination capabilities of thorium featuring characteristics from both blocks. Thus analysis of the recently synthesized monoanion [Cp3Th]− indicates a diamagnetic singlet d2f0 ground state for the central thorium(II) atom. In this species the trigonal (Cp3)3− ligand system provides a strong ligand field for the thorium d-orbitals accounting for the low spin of this system. This differs from the high spin unambiguous f-element analogue, [Cp3U]−. For the d0f0 thorium(IV) complex (η8-C8H8)2Th both π→d and π→f forward bonding is observed, where a comparison with (η8-C8H8)2U is given. This indicates that a borderline element such as thorium can exhibit either d- or f-block character according to the surrounding ligand field.

Spectroscopic and Electrochemical Investigation of Uranium and Neptunium in Chloride Room-Temperature Ionic Liquids.

To expand the breadth of knowledge of actinide chemistry in molten chloride salts, chloride room-temperature ionic liquids (RTILs) were used to probe the influence of RTIL cation on second-sphere coordination for anionic complexes of uranium and neptunium. Six chloride RTILs were studied to represent a range of cation polarizing strength, size, and charge density to correlate changes in the complex geometry and redox behaviors. Optical spectroscopy indicated that actinides were dissolved at equilibrium as octahedral AnCl62- (An = U, Np) as is observed in comparable high-temperature molten chloride salts. These anionic metal complexes were sensitive to the RTIL cation polarizing strength and hydrogen bond donating strength and displayed varying levels of fine structure and hypersensitive transition splitting depending on the degree of perturbation to the complex's coordination symmetry. Furthermore, voltammetry experiments on the redox-active complexes indicated a stabilizing effect on lower valence actinide oxidation states by more polarizing RTIL cations whereby the measured E1/2 potentials for both U(IV/III) and Np(IV/III) couples shifted positively by about 600 mV across the different systems. These results indicate that more polarizing RTIL cations inductively remove electron density from the actinide metal center over An-Cl-Cation bond networks to stabilize electron-deficient oxidation states. Electron-transfer kinetics were generally much slower than in molten chloride systems, partially due to lower temperatures and higher viscosities in the working systems and showed diffusion coefficients of 1.8 × 10-8 to 6.4 × 10-8 cm2 s-1 for UIV and 4.4 × 10-8 to 8.3 × 10-8 cm2 s-1 for NpIV. We also detect a one-electron oxidation of NpIV that we have attributed to the formation of NpV as NpCl6-. Overall, we observe a coordination environment for the anionic actinide complexes that is susceptible to even small changes in RTIL cation properties.

Are U-U Bonds Inside Fullerenes Really Unwilling Bonds?

Previous characterizations of diactinide endohedral metallofullerenes (EMFs) Th2@C80 and U2@C80 have shown that although the two Th3+ ions form a strong covalent bond within the carbon cage, the interaction between the U3+ ions is weaker and described as an "unwilling" bond. To evaluate the feasibility of covalent U-U bonds, which are neglected in classical actinide chemistry, we have first investigated the formation of smaller diuranium EMFs by laser ablation using mass spectrometric detection of dimetallic U2@C2n species with 2n ≥ 50. DFT, CASPT2 calculations, and MD simulations for several fullerenes of different sizes and symmetries showed that thanks to the formation of strong U(5f3)-U(5f3) triple bonds, two U3+ ions can be incarcerated inside the fullerene. The formation of U-U bonds competes with U-cage interactions that tend to separate the U ions, hindering the observation of short U-U distances in the crystalline structures of diuranium endofullerenes as in U2@C80. Smaller cages like C60 exhibit the two interactions, and a strong triple U-U bond with an effective bond order higher than 2 is observed. Although 5f-5f interactions are responsible for the covalent interactions at distances close to 2.5 Å, overlap between 7s6d orbitals is still detected above 4 Å. In general, metal ions within fullerenes should be regarded as templates in cage formation, not as statistically confined units that have little chance of being observed.