Since the 1940s, the synthesis and separation of alkyluranides have been studied mainly by centrifugal separation of uranium compounds. For example, in 1956, Gilman et al. Developed a method to separate organic uranium compounds, but failed^[1]. Tetramethyluranium compounds are extremely unstable due to the unsaturated coordination of the uranium center, while benzyl ligands can improve the stability of uranium compounds through a multidentate coordination mode^[2]. Giannini et al. Prepared stable tetrabenzylzirconium by the reaction of benzylmagnesium chloride with ZrCl₄^[3]. Ballard's group has obtained a series of stable transition metal alkyl and benzyl compound MR₄（M=Zr,Ti,Hf;R=Me,benzyl,4-CH₃-benzyl,4-CH₃O-benzyl,4-F-benzyl,2-Me-allyl） with obvious metal-saturated carbon interaction^[4][5]. The study of the interaction between these alkyl and benzyl saturated carbon ligands and transition metals provides a basis for the development of corresponding uranium compounds.

Compared with transition metal ions, uranium metal ions have larger ionic radius (six-coordinated U^Ⅲ:1.17Å,U^Ⅳ:1.03Å,U^Ⅴ:0.90Å,U^Ⅵ:0.87Å）) and higher coordination number, which can coordinate with ligands with greater steric hindrance^[6]. Uranium atoms can coordinate with ligands containing oxygen, nitrogen and carbon to form a wide variety of organo-uranium compounds^{[7⇓⇓⇓~11]}. Uranium carbon compounds are mainly coordinated by carbene carbon, and uranium compounds formed by saturated carbon are relatively few, mainly including benzyl and alkyl ligands^[12]. The 5 f and 6 d orbitals of U are mainly involved in the bonding of saturated carbon uranium compounds, and the bonding properties are mainly covalent or ionic. In this paper, the structures of compounds formed by coordination of uranium ions with saturated carbon in different valence States and the properties of U-C bond are reviewed.

The synthesis conditions of trivalent uranium compounds are harsh and their thermal stability is poor, so there are few studies on trivalent uranium carbon compounds^[13,14]. It has been found that saturated coordination of the central uranium ion or the use of sterically hindered ligands can stabilize uranium carbide compounds, such as trimethylsilyl （SiMe₃）, cyclopentadiene and benzene^[15,16].

Pentavalent uranium is prone to disproportionation in aqueous solution to form tetravalent uranium and hexavalent uranium, so there are very few reports on pentavalent uranium-carbon complexes. At present, there are very few neutral uranium (Ⅴ) complexes with U-C σ bond reported, and lithium cations can be used to stabilize saturated pentavalent uranium anion complexes. As early as 1977, Wilkinson et al. Reported the Li₃-U^ⅤR₈·3dioxane[R=Me,CH₂CMe₃,CH₂SiMe₃] of octaalkyluranate (Ⅴ) complexes with good thermal stability^[55]. In 2011, Hayton's group prepared [Li(THF)₄][U^Ⅴ(CH₂SiMe₃)₆] (Figure 11A), and X-ray diffraction analysis showed that the crystal belonged to the monoclinic space group P2_1/c^[56]. Six CH₂SiMe₃ ligands coordinate to the U (Ⅴ) center in an octahedral geometry. The U-C bond length is 2.451 (6) Å, and the theoretical calculation results show that the U-C σ bond is mainly formed by the uranium 5F orbital^[57]. Meanwhile, the research group also prepared a similar pentavalent uranium complex [Li(DME)₃][U^Ⅴ(O^tBu)₂(CH₂SiMe₃)₄] crystal (Figure 11B)^[56]. By X-ray diffraction analysis, the crystal belongs to the tetragonal space group P4_2/mmc and is also in an octahedral configuration, with four CH₂SiMe₃ ligands located in the same plane, while two O^tBu are located at the two apices of the octahedron, where the U-C and U-O bond lengths are 2.42 (2) and 2.053 (8) Å, respectively.

Hexavalent uranyl ion exists widely in the environment [UO₂]²⁺, but the complex formed by hexavalent uranium and saturated carbon has been studied relatively little. In 2013, Hayton et al. Obtained the crystal structure of the stable complex [Li(DME)_1.5]₂[U^ⅥO₂(CH₂SiMe₃)₄] through the interaction of four CH₂SiMe₃ ligands with uranyl ions (Figure 12A)^[58]. The complex has a near-octahedral configuration with C₂ symmetry and U-C bond lengths of 2.497 (6) and 2.481 (6) Å, respectively. The U-C bond in [Li(DME)_1.5]₂[U^ⅥO₂(CH₂SiMe₃)₄] is longer than that in the hexaalkyl complex U^Ⅵ(CH₂SiMe₃)₆（ Fig. 12 B) due to the participation of the uranyl oxygen atom. In 2013, Schelter's group prepared the U^ⅥO(Me)[N-(SiMe₃)₂]₃ crystal (Figure 12c), with the oxygen atom and methyl group located at both ends of the U atom, and the distance between uranium and methyl carbon U-C was 2.343 (4) Å^[59].

In recent years, actinide metal multiple bonding compounds have attracted much attention due to their unique bonding characteristics and properties. With the in-depth understanding of the complex electronic structure of actinide metals and their special properties, some progress has been made in the synthesis and research of actinide metal multiple bonding compounds. However, the synthesis and separation of actinide compounds are very challenging, especially for transuranium compounds, so it is very important and critical to study the structure and bonding properties of actinide compounds theoretically. The electronic structure of actinides is very complex due to the strong relativistic effect, spin-orbit coupling effect and electron correlation effect. On the other hand, actinides have more valence electrons and similar orbital energy levels, which makes actinides show rich bonding characteristics^[60,61]. The 5 f orbitals of actinides are more ductile than the 4 f orbitals of lanthanides, so actinides are more likely to bond with different ligands. With the rapid development of theoretical methods and high-performance computers, great progress has been made in the theoretical calculation and simulation of actinide systems^[62]. At present, there is little theoretical work on the bonding properties of U and saturated carbon. Here, we illustrate the progress of the theory of compounds containing U-C bonds (saturated and unsaturated carbon).

Hayton et al. Used DFT theory to study the electronic structures of [U^Ⅳ(CH₂SiMe₃)₅]^-, [Li(THF)₄][U^Ⅴ(CH₂SiMe₃)₆] and U^Ⅵ(CH₂SiMe₃)₆. The results of Mulliken population analysis were that the contribution of U 5F was 19%, 29% and 35% in the three compounds, respectively, indicating that with the increase of oxidation state, the participation of 5 f valence orbital in U-C bonding was strengthened, and the contribution of 6 d and 7 s remained basically unchanged in the three compounds^[56]. The Gagliardi group used the CASSCF/CASPT2/SO method to confirm the two electronic States of CUO molecule, ¹Σ⁺ and the triplet state ³Φ in ³Φ(Ω=2), as the ground state^[63]. Andrews group used CASPT2 method to explore the electronic structure of uranium carbide molecules UC and CUC. The calculation shows that the ground state of UC and CUC is quintet and triplet linear, the U ≡ C bond length is 1.855 and 1.840 840 Å, and the effective bond order is 2.82 and 2.83, respectively^[64]. Li Jun's group reported a series of uranium carbide X₃U≡CH(X=F,Cl,Br) and F₃U≡CF, DFT calculations show that the ground state of the series of compounds is singlet, the electron-withdrawing group helps to stabilize U (Ⅵ), and the ligand with less electronegativity increases the effective overlap between the 6d orbital of U and the 2p orbital of C, resulting in a shorter U ≡ C bond length^[65]. Subsequently, Li Jun et al. analyzed the molecular orbital theory and valence bond theory of CUO molecule, and showed that the chemical bond between U-C is close to the quadruple bond, as shown in Fig. 13, which proved for the first time that carbon atoms can form quadruple bonds in actinide compounds^[66]. Wang Zhigang's group used density functional theory calculations to study the structural UC₂ isomers of two typical configurations, linear and triangular, adsorbed on the surface of graphene, which have strong interactions^[67]. But also the U 5F orbital of the triangular UC₂ is easier to bind to graphene than the 5 f orbital of the linear CUC. Bowen et al. Used a high-level ab initio method to study the different oxidation States of U molecules UC, UC⁻ and UC⁺, and the NBO calculation results showed that their electronic configurations were 5f²7s¹, 5f²7s² and 5f²7s⁰, respectively.The electronic structure analysis shows that the U-C bond in UC, UC⁻ and UC⁺ is triple bond, and the U-C bond order is 2. 5^[68]. Autschbach et al. Prepared the first U (VI) fullerene compound U^ⅥSc₂C₂@D_5h(6)-C₈₀, in which one C atom was replaced by N atom to obtain a U^ⅤSc₂NC@D_5h(6)-C₈₀ with U (V) center.The natural electronic configurations of U in these two compounds are 5f^3.666d^1.46 and 5f^3.446d^1.16, respectively, which have large deviations from the electronic configurations 5f⁰6d⁰ and 5f¹6d⁰ of U (Ⅵ) and U (Ⅴ) ions alone, indicating the electron transfer of C/N → U^[69]. Schelter et al. Have systematically studied the structural properties of U^Ⅵ(O)(C≡C-C₆H₄-R)[N(SiMe₃)₂]₃(R=NMe₂,OMe,Me,Ph,H,Cl) by using density functional theory, and the calculation results show that the electron-donating group and the electron-withdrawing group make the U-C bond shorter and longer, respectively^[70]. Kaltsoyannis group theoretically studied the molecular orbital, spin and electron density of trivalent actinide compound AnCp₃(An=Th-Cm), and found that the interaction between An and Cp is ionic, and its ionicity increases with the increase of atomic number^[71]. Batista's group has found two new orbital bonding modes in actinide metallocyclopropenes (Fig. 14 a) and actinide metallocene compounds (Fig. 14 B) (Pa-Pu), as shown in Fig. 14C and d, which are combined in the form of "head-to-side" δ and "side-to-side" φ antibonding, respectively, mainly due to the interaction between the 5 f orbital of the actinide metal and the "side" of the C 2p orbital in propylene or allene^[72].

Our group has also been committed to the theoretical study of multiple bonds in actinide compounds, and has systematically explored and studied the structure and bonding properties of a series of actinide (Th-Am) compounds, including actinide-14 main group compounds, actinide-15 main group compounds, actinide-16 main group compound, actinide-17 main group compound and actinide-transition metal compounds^{[73⇓~75][76,77][78,79][80,81][82,83]}. For example, based on the first L-U≡N(L=N(CH₂CH₂NSiPrⁱ₃)₃) compound with a terminal U ≡ N triple bond assembled by Liddle's project, we used density functional theory to study the bonding properties of L-An-N (An = Pa-Pu)^[84,85][76]. The results show that the An-N multiple bond contains one σ bond and two π bonds (Fig. 15), with significant covalent interactions. With the increase of atomic number, the covalency of An-N decreases, and the contribution of An 5F orbital to the bonding increases gradually, while the contribution of An 6d orbital shows the opposite trend. Subsequently, we further explored the electronic structures of the fifth main group uranium compounds L-U-E(L=N(CH₂CH₂NSiPrⁱ₃)₃,E=N,P,As,Sb and Bi) and the bonding properties of their U-E multiple bonds, which also contain one σ bond and two π bonds, and the covalency of U-E bonds decreases with the increase of the charge number of the fifth main group nucleus^[77]. At the same time, the analysis of bonding composition and covalency reveals that the contribution of U 5F orbital to σ and π bonding is greater than that of 6d orbital, and the contribution of U 5F orbital to π bonding is greater than that of σ bonding. These works help us to understand the electronic structure of actinide and main group element compounds, reveal the bonding nature of actinide multiple bonds, and enrich our understanding of bonding. Based on the obtained UL₂(L=C(PPh₂NMes)₂) of uranium-carbon carbene compounds, the corresponding analogues of Ce, Th, U, Np and Pu have been systematically studied by relativistic quantum chemical methods^[73]. The differences and nature of bonding between lanthanide metals and actinide metals, as well as between pre-actinide metals and post-actinium metals with carbon carbene are compared and studied, as shown in Figure 16. For ML₂(M=Ce,Th,U,Np and Pu), the metallic valence electrons reside mainly in the d and f orbitals. In addition, we also theoretically explored the electronic structure of a series of divalent actinide compounds [An^IICp'₃]⁻(An=Th-Am,Cp'=[η⁵-C₅H₄(SiMe₃)]⁻) and the interaction between actinide and Cp 'ring^[86]. The molecular orbital analysis shows that the interaction between the divalent actinide ion and Cp 'ligand mainly comes from the 5F orbital of the actinide and the π orbital of the Cp' ring, and the interaction between the actinide atom and Cp 'ligand decreases with the increase of the atomic number of the actinium. These works can help to understand the unique bonding characteristics of actinide multiple bonds, explain the complex electronic structure changes and trends of actinide series compounds, clarify the construction characteristics of compounds containing special actinide metal multiple bonds, and provide theoretical guidance and support for experimental research.

With the development of advanced instrumentation (such as high sensitive X-ray diffraction and high resolution mass spectrometry) and the development of high-precision quantum calculation methods, people have a more comprehensive understanding of the electronic structure and properties of actinides, and the study of uranium compounds has also been unprecedented. For example, ligands containing N, O atoms and carbon carbene ligands can form stable compounds with uranium atoms under certain conditions, but saturated carbon atoms without lone pairs of electrons are difficult to form stable compounds with uranium atoms. How to control the experimental conditions and find new ligands to synthesize stable compounds of uranium and saturated carbon will be the focus of future research, which can enrich the types of actinide chemistry and have a deeper understanding of actinide chemistry. In addition, the experimental and theoretical studies on the nature of U-C bonding need to be further deepened. Gagliardi et al. Reviewed the new progress of actinide computational chemistry and the study of gaseous spectra of actinide small molecules by multi-configuration quantification methods, and also summarized the multiple bond properties of diuranium molecules and other diactinide molecules^[87]. Li Jun's group reviewed the theoretical research work and progress of multiple bonds formed between actinides and main group elements, and the theoretical research progress of fluorescence spectra of actinides^[88][89]. In this paper, the experimental and theoretical progress of actinide-transition metal compounds and actinide-main group metal compounds are reviewed^[61]. These reviews and summaries are helpful to understand the design and synthesis of actinide complexes, the unique electronic structure and properties of actinide, and the bonding rules of actinides.