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The nature of the interaction between actinide ions and ligands is of great significance in the application of the separation of actinide groups. Therefore, we have been devoted to this aspect in recent years. In this article, the coordination structure, stability and bonding properties of the complexes formed by uranyl and macrocyclic ligands are reviewed. First of all, our study found that the coordination structure of actinyl macrocyclic complexes depends on the size of the macrocyclic ligands; secondly, in the case of “side-on” coordination, uranyl tends to form a 1∶2 complex, while in the case of “insertion” structure, uranyl tends to form a 1∶1 complex; thirdly, it has been found that the ligands with ring size of 18 have excellent coordination ability when combined with uranyl, and ［CHP］4- has the strongest binding ability; finally, this stability is mainly due to the greater covalence of U-N than that of U-O and U-S. These structural rules and bonding characteristics provide important theoretical guidance for the experimental screening of reasonable and efficient ligands for the separation of actinides.

A gas-phase uranyl peroxide dimer supported by three 12-crown-4 ether (12C4) ligands, [(UO2)2(O2)(12C4)3)]2+ (A), was prepared by electrospray ionization. Density functional theory (DFT) indicates a structure with two terminal 12C4 and the third 12C4 bridging the uranium centers. Collision induced dissociation (CID) of A resulted in elimination of the bridging 12C4 to yield a uranyl peroxide dimer with two terminal donor ligands, [(12C4)(UO2)(O2)(UO2)(12C4)]2+ (B). Remarkably, CID of B resulted in elimination of the bridging peroxide concomitant with reduction of U(VI) to U(V) in C, [(12C4)(UO2)(UO2)(12C4)]2+. DFT studies indicate that in C there is direct interaction between the two UO2+ species, which can thus be considered as a so-called cation-cation interaction (CCI). This formal CCI, induced by tetradentate 12C4 ligands, corresponds to destruction of the linear uranyl moieties and creation of bridging U-O-U oxo-bonds. On the basis of the structural rearrangement to achieve the structurally extreme CCI interaction, it is predicted also to be accessible for PaO2+ but is less feasible for transuranic actinyls.

The following gas-phase uranyl/12-crown-4 (12C4) complexes were synthesized by electrospray ionization: [UO2(12C4)2]2+ and [UO2(12C4)2(OH)]+. Collision-induced dissociation (CID) of the dication resulted in [UO2(12C4-H)]+ (12C4-H is a 12C4 that has lost one H), which spontaneously adds water to yield [UO2(12C4-H)(H2O)]+. The latter has the same composition as complex [UO2(12C4)(OH)]+ produced by CID of [UO2(12C4)2(OH)]+ but exhibits different reactivity with water. The postulated structures as isomeric [UO2(12C4-H)(H2O)]+ and [UO2(12C4)(OH)]+ were confirmed by comparison of infrared multiphoton dissociation (IRMPD) spectra with computed spectra. The structure of [UO2(12C4-H)]+ corresponds to cleavage of a C-O bond in the 12C4 ring, with formation of a discrete U-Oeq bond and equatorial coordination by three intact ether moieties. Comparison of IRMPD and computed IR spectra furthermore enabled assignment of the structures of the other complexes. Theoretical studies of the chemical bonding features of the complexes provide an understanding of their stabilities and reactivities. The results reveal bonding and structures of the uranyl/12C4 complexes and demonstrate the synthesis and identification of two different isomers of gas-phase uranyl coordination complexes.

The concept of oxidation state (OS) is based on the concept of Lewis electron pairs, in which the bonding electrons are assigned to the more electronegative element. This approach is useful for keeping track of the electrons, predicting chemical trends, and guiding syntheses. Experimental and quantum-chemical results reveal a limit near +8 for the highest OS in stable neutral chemical substances under ambient conditions. OS=+9 was observed for the isolated [IrO4 ]+ cation in vacuum. The prediction of OS=+10 for isolated [PtO4 ]2+ cations is confirmed computationally for low temperatures only, but hasn't yet been experimentally verified. For high OS species, oxidation of the ligands, for example, of O-2 with formation of . O-1 and O-O bonds, and partial reduction of the metal center may be favorable, possibly leading to non-Lewis type structures.

In-cavity complexes and their bonding features between thio-crown (TC) ethers and f-elements are unexplored so far. In this paper, actinyl(VI) (An = U, Np, Pu, Am, and Cm) complexes of TC ethers have been characterized using relativistic density functional theory. The TC ether ligands include tetrathio-12-crown-4 (12TC4), pentathio-15-crown-5 (15TC5), and hexathio-18-crown-6 (18TC6). On the basis of the calculations, it is found that the "double-decker" sandwich structure of AnO2(12TC4)22+ and "side-on" structure AnO2(12TC4)2+ are changed to "insertion" structures for AnO2(15TC5)2+ and AnO2(18TC6)2+ due to increased size of the TC ether ligands. The actinyl monocyclic TC ether complexes are found to exhibit conventional conformations, with typical An-Oactinyl and An-Sligand distances and angles. Chemical bonding analyses by Weinhold's natural population analysis (NPA), natural localized molecular orbital (NLMO), and energy decomposed analysis (EDA), show that a typical ionic An-Sligand bond with the extent of covalent interaction between the An and S atoms primarily attributable to the degree of radial distribution of the S 3p atomic orbitals. The similarity and difference of the oxo-crown and TC ethers as ligands for actinide coordination chemistry are discussed. As soft S-donor ligands, TC ethers may be candidate ligands for actinide recognition and extraction.

The concept of oxidation state (OS) is based on the concept of Lewis electron pairs, in which the bonding electrons are assigned to the more electronegative element. This approach is useful for keeping track of the electrons, predicting chemical trends, and guiding syntheses. Experimental and quantum-chemical results reveal a limit near +8 for the highest OS in stable neutral chemical substances under ambient conditions. OS=+9 was observed for the isolated [IrO4 ]+ cation in vacuum. The prediction of OS=+10 for isolated [PtO4 ]2+ cations is confirmed computationally for low temperatures only, but hasn't yet been experimentally verified. For high OS species, oxidation of the ligands, for example, of O-2 with formation of . O-1 and O-O bonds, and partial reduction of the metal center may be favorable, possibly leading to non-Lewis type structures.

An inverse sandwich structure has been computationally predicted for uranium boride and extended to the series of actinide elements (An) from Th to Cm. The electronic structure and chemical bonding of these novel compounds have been analyzed using density functional theory and multireference wave-function based methods. We report the trends in electronic structure and bonding for An2B8, and found that (d–π)π and (d–p)δ are the most important factors in the stability of An2B8. The (f–p)δ bond provides extra stabilization for Pa2B8 and U2B8, owing to the extensive interactions of An–B8–An, resulting in a short distance for the Pa–Pa and U–U bonds. [ABSTRACT FROM AUTHOR]

Understanding of the nature and extent of chemical bonding in uranyl coordination complexes is crucial for the design of new ligands for nuclear waste separation, uranium extraction from seawater, and other applications. We report here the synthesis, infrared spectroscopic characterization, and quantum chemical studies of a molecular uranyl-di-15-crown-5 complex. The structure and bonding of this unique complex featuring a distinctive 6-fold coplanar coordination staggered sandwich structure and an unusual non-perpendicular orientation of the uranyl moiety are evaluated using density functional theory and chemical bonding analyses. The results provide fundamental understanding of the coordination interaction of uranyl with oxygen-donor ligands.