Compression of curium pyrrolidine-dithiocarbamate enhances covalency.

Curium is unique in the actinide series because its half-filled 5f 7 shell has lower energy than other 5f n configurations, rendering it both redox-inactive and resistant to forming chemical bonds that engage the 5f shell1-3. This is even more pronounced in gadolinium, curium's lanthanide analogue, owing to the contraction of the 4f orbitals with respect to the 5f orbitals4. However, at high pressures metallic curium undergoes a transition from localized to itinerant 5f electrons5. This transition is accompanied by a crystal structure dictated by the magnetic interactions between curium atoms5,6. Therefore, the question arises of whether the frontier metal orbitals in curium(III)-ligand interactions can also be modified by applying pressure, and thus be induced to form metal-ligand bonds with a degree of covalency. Here we report experimental and computational evidence for changes in the relative roles of the 5f/6d orbitals in curium-sulfur bonds in [Cm(pydtc)4]- (pydtc, pyrrolidinedithiocarbamate) at high pressures (up to 11 gigapascals). We compare these results to the spectra of [Nd(pydtc)4]- and of a Cm(III) mellitate that possesses only curium-oxygen bonds. Compared with the changes observed in the [Cm(pydtc)4]- spectra, we observe smaller changes in the f-f transitions in the [Nd(pydtc)4]- absorption spectrum and in the f-f emission spectrum of the Cm(III) mellitate upon pressurization, which are related to the smaller perturbation of the nature of their bonds. These results reveal that the metal orbital contributions to the curium-sulfur bonds are considerably enhanced at high pressures and that the 5f orbital involvement doubles between 0 and 11 gigapascal. Our work implies that covalency in actinides is complex even when dealing with the same ion, but it could guide the selection of ligands to study the effect of pressure on actinide compounds.

Synthesis and Crystallographic Characterization of a Reduced Bimetallic Yttrium ansa-Metallocene Hydride Complex, [K(crypt)][(µ-CpAn)Y(µ-H)]2 (CpAn = Me2Si[C5H3(SiMe3)-3]2), with a 3.4 Å Yttrium-Yttrium Distance.

The reduction of a bimetallic yttrium ansa-metallocene hydride was examined to explore the possible formation of Y-Y bonds with 4d1 Y(II) ions. The precursor [CpAnY(µ-H)(THF)]2 (CpAn = Me2Si[C5H3(SiMe3)-3]2) was synthesized by hydrogenolysis of the allyl complex CpAnY(η3-C3H5)(THF), which was prepared from (C3H5)MgCl and [CpAnY(µ-Cl)]2. Treatment of [CpAnY(µ-H)(THF)]2 with excess KC8 in the presence of one equivalent of 2.2.2-cryptand (crypt) generates an intensely colored red-brown product crystallographically identified as [K(crypt)][(µ-CpAn)Y(µ-H)]2. The two rings of each CpAn ligand in the reduced anion [(µ-CpAn)Y(µ-H)]21- are attached to two yttrium centers in a "flyover" configuration. The 3.3992(6) and 3.4022(7) Å Y···Y distances between the equivalent metal centers within two crystallographically independent complexes are the shortest Y···Y distances observed to date. Ultraviolet-visible (UV-visible)/near infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy support the presence of Y(II), and theoretical analysis describes the singly occupied molecular orbital (SOMO) as an Y-Y bonding orbital composed of metal 4d orbitals mixed with metallocene ligand orbitals. A dysprosium analogue, [K(18-crown-6)(THF)2][(µ-CpAn)Dy(µ-H)]2, was also synthesized, crystallographically characterized, and studied by variable temperature magnetic susceptibility. The magnetic data are best modeled with the presence of one 4f9 Dy(III) center and one 4f9(5dz2)1 Dy(II) center with no coupling between them. CASSCF calculations are consistent with magnetic measurements supporting the absence of coupling between the Dy centers.

Co-Crystallization of Plutonium(III) and Plutonium(IV) Diglycolamides with Pu(III) and Pu(IV) Hexanitrato Anions: A Route to Redox Variants of [PuIII,IV(DGA)3][PuIII,IV(NO3)6]x.

N,N,N',N'-tetramethyl diglycolamide (TMDGA), a methylated variant of the diglycolamide extractants being proposed as curium holdback reagents in advanced used nuclear fuel reprocessing technologies, has been crystallized with plutonium, a transuranic actinide that has multiple accessible oxidation states. Two plutonium TMDGA complexes, [PuIII(TMDGA)3][PuIII(NO3)6] and[PuIV(TMDGA)3][PuIV(NO3)6]2·0.75MeOH, were crystallized through solvent diffusion of a reaction mixture containing plutonium(III) nitrate and TMDGA. The sample was then partially oxidized by air to yield [PuIV(TMDGA)3][PuIV(NO3)6]2·0.75MeOH. Single-crystal X-ray diffraction reveals that the multinuclear systems crystallize with hexanitrato anionic species, providing insight into the first solid-state isolation of the elusive trivalent plutonium hexanitrato species. Crystallography data show a change in geometry around the TMDGA metal center from Pu3+ to Pu4+, with the symmetry increasing approximately from C4v to D3h. These complexes provide a rare opportunity to investigate the bond metrics of plutonium in two different oxidation states with similar coordination environments. Further, these new structures provide insight into the potential chemical and structural differences arising from the radiation-induced formation of transient tetravalent curium oxidation states in used nuclear fuel reprocessing streams.

Radium Revisited: Revitalization of the Coordination Chemistry of Nature's Largest +2 Cation.

The crystallization, single crystal structure, and Raman spectroscopy of Ra(NO3)2 have been investigated by experimentation and theory, which represent the first pure radium compound characterized by single crystal X-ray diffraction. The Ra2+ centers are bound by six chelating nitrate anions to form an anticuboctahedral geometry. The Raman spectrum acquired from a single crystal of Ra(NO3)2 generally occurs at a lower frequency than found in Ba(NO3)2, as expected. Computational studies on Ra(NO3)2 provide an estimation of the bond orders via Wiberg bond indices and indicate that Ra-O interactions are weak with values of 0.025 and 0.026 for Ra-O bonds. Inspection of natural bond orbitals and natural localized molecular orbitals suggest negligible orbital mixing. However, second-order perturbation interactions show that donation from the lone pairs of the nitrate oxygen atoms to the 7s orbitals of Ra2+ stabilizes each Ra-O interaction by ca. 5 kcal mol-1.

Impact of lanthanide ion complexation and temperature on the chemical reactivity of N,N,N',N'-tetraoctyl diglycolamide (TODGA) with the dodecane radical cation.

The impact of trivalent lanthanide ion complexation and temperature on the chemical reactivity of N,N,N',N'-tetraoctyl diglycolamide (TODGA) with the n-dodecane radical cation (RH˙+) has been measured by electron pulse radiolysis and evaluated by quantum mechanical calculations. Additionally, Arrhenius parameters were determined for the reaction of the non-complexed TODGA ligand with the RH˙+ from 10-40 °C, giving the activation energy (Ea = 17.43 ± 1.64 kJ mol-1) and pre-exponential factor (A = (2.36 ± 0.05) × 1013 M-1 s-1). The complexation of Nd(III), Gd(III), and Yb(III) ions by TODGA yielded [LnIII(TODGA)3(NO3)3] complexes that exhibited significantly increased reactivity (up to 9.3× faster) with the RH˙+, relative to the non-complexed ligand: k([LnIII(TODGA)3(NO3)3] + RH˙+) = (8.99 ± 0.93) × 1010, (2.88 ± 0.40) × 1010, and (1.53 ± 0.34) × 1010 M-1 s-1, for Nd(III), Gd(III), and Yb(III) ions, respectively. The rate coefficient enhancement measured for these complexes exhibited a dependence on atomic number, decreasing as the lanthanide series was traversed. Preliminary reaction free energy calculations-based on a model [LnIII(TOGDA)]3+ complex system-indicate that both electron/hole and proton transfer reactions are energetically unfavorable for complexed TODGA. Furthermore, complementary average local ionization energy calculations showed that the most reactive region of model N,N,N',N'-tetraethyl diglycolamide (TEDGA) complexes, [LnIII(TEGDA)3(NO3)3], toward electrophilic attack is for the coordinated nitrate (NO3-) counter anions. Therefore, it is possible that radical reactions with the complexed NO3- counter anions dominate the differences in rates seen for the [LnIII(TODGA)3(NO3)3] complexes, and are likely responsible for the reported radioprotection in the presence of TODGA complexes.

Influence of Outer-Sphere Anions on the Photoluminescence from Samarium(II) Crown Complexes.

Three samarium(II) crown ether complexes, [Sm(15-crown-5)2]I2 (1), [Sm(15-crown-5)2]I2·CH3CN (2), and [Sm(benzo-15-crown-5)2]I2 (3), have been prepared via the reaction of SmI2 with the corresponding crown ether in either THF or acetonitrile in good to moderate yields. The compounds have been characterized by single crystal X-ray diffraction and a variety of spectroscopic techniques. In all cases, the Sm(II) centers are sandwiched between two crown ether molecules and are bound by the five etheric oxygen atoms from each crown ether to yield 10-coordinate environments. Despite the higher symmetry crystal class of 1 (R3c), the samarium center resides on a general position, whereas in 2 and 3 (both in P21/c) the metal centers lie upon inversion centers. Moreover, the complexes in 2 and 3 are approximated well by D5d symmetry. The molecule in 1, however, is distorted from idealized D5d symmetry, and the crown ethers are more puckered than observed in 2 and 3. All three complexes luminesce in the NIR at low temperatures. However, the nature of the luminescence differs between the three compounds. 1 exhibits broadband photoluminescence at 20 °C but at low temperatures transitions to narrow peaks. 2 only exhibits nonradiative decay at 20 °C and at low temperatures retains a mixture of broadband and fine transitions. Finally, 3 displays broadband luminescence regardless of temperature. Spin-orbit (SO) CASSCF calculations reveal that the outer-sphere iodide anions influence whether broadband luminescence from 5d → 4f or fine 4f → 4f transitions occur through the alteration of symmetry around the metal centers and the nature of the excited states as a function of temperature.

Understanding the Stabilization and Tunability of Divalent Europium 2.2.2B Cryptates.

Lanthanides such as europium with more accessible divalent states are useful for studying redox stability afforded by macrocyclic organic ligands. Substituted cryptands, such as 2.2.2B cryptand, that increase the oxidative stability of divalent europium also provide coordination environments that support synthetic alterations of Eu(II) cryptate complexes. Two single crystal structures were obtained containing nine-coordinate Eu(II) 2.2.2B cryptate complexes that differ by a single coordination site, the occupation of which is dictated by changes in reaction conditions. A crystal structure containing a [Eu(2.2.2B)Cl]+ complex is obtained from a methanol-THF solvent mixture, while a methanol-acetonitrile solvent mixture affords a [Eu(2.2.2B)(CH3OH)]2+ complex. While both crystals exhibit the typical blue emission observed in most Eu(II) containing compounds as a result of 4f65d1 to 4f7 transitions, computational results show that the substitution of a Cl- anion in the place of a methanol molecule causes mixing of the 5d excited states in the Eu(II) 2.2.2B cryptate complex. Additionally, magnetism studies reveal the identity of the capping ligand in the Eu(II) 2.2.2B cryptate complex may also lead to exchange between Eu(II) metal centers facilitated by π-stacking interactions within the structure, slightly altering the anticipated magnetic moment. The synthetic control present in these systems makes them interesting candidates for studying less stable divalent lanthanides and the effects of precise modifications of the electronic structures of low valent lanthanide elements.

Influence of uranyl complexation on the reaction kinetics of the dodecane radical cation with used nuclear fuel extraction ligands (TBP, DEHBA, and DEHiBA).

Specialized extractant ligands - such as tri-butyl phosphate (TBP), N,N-di-(2-ethylhexyl)butyramide (DEHBA), and N,N-di-2-ethylhexylisobutryamide (DEHiBA) - have been developed for the recovery of uranium from used nuclear fuel by reprocessing solvent extraction technologies. These ligands must function in the presence of an intense multi-component radiation field, and thus it is critical that their radiolytic behaviour be thoroughly evaluated. This is especially true for their metal complexes, where there is negligible information on the influence of complexation on radiolytic reactivity, despite the prevalence of metal complexes in used nuclear fuel reprocessing solvent systems. Here we present a kinetic investigation into the effect of uranyl (UO22+) complexation on the reaction kinetics of the dodecane radical cation (RH˙+) with TBP, DEHBA, and DEHiBA. Complexation had negligible effect on the reaction of RH˙+ with TBP, for which a second-order rate coefficient (k) of (1.3 ± 0.1) × 1010 M-1 s-1 was measured. For DEHBA and DEHiBA, UO22+ complexation afforded an increase in their respective rate coefficients: k(RH˙+ + [UO2(NO3)2(DEHBA)2]) = (2.5 ± 0.1) × 1010 M-1 s-1 and k(RH˙+ + [UO2(NO3)2(DEHiBA)2]) = (1.6 ± 0.1) × 1010 M-1 s-1. This enhancement with complexation is indicative of an alternative RH˙+ reaction pathway, which is more readily accessible for [UO2(NO3)2(DEHBA)2] as it exhibited a much larger kinetic enhancement than [UO2(NO3)2(DEHiBA)2], 2.6× vs. 1.4×, respectively. Complementary quantum mechanical calculations suggests that the difference in reaction kinetic enhancement between TBP and DEHBA/DEHiBA is attributed to a combination of reaction pathway (electron/hole transfer vs. proton transfer) energetics and electron density distribution, wherein attendant nitrate counter anions effectively 'shield' TBP from RH˙+ electron transfer processes.

Evidence of Alpha Radiolysis in the Formation of a Californium Nitrate Complex.

Well-characterized complexes of transplutonium elements are scarce because of the experimental challenges of working with these elements and the rarity of the isotopes. This leads to a lack of structural and spectroscopic data needed to understand the nature of chemical bonds in these compounds. In this work, the synthesis of Cf(DOPOq )2 (NO3 )(py) (DOPOq =2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate; py=pyridine) is reported, in which the nitrate anion is hypothesized to form through the α-radiolysis-induced reaction of pyridine and/or the ligand. Computational analysis of the electronic structure of the complex reveals that the CfIII -ligand interactions are largely ionic.

Synthesis, Spectroscopy, and Theoretical Details of Uranyl Schiff-Base Coordination Complexes.

Two uranyl Schiff-base coordination complexes, UO2L(MeOH) and UO2Cl2(H2L) {L = N,N'-bis[(4,4'-diethylamino)salicylidene]-1,2-phenylenediamine}, have been synthesized that feature a rigid phenyl backbone. These complexes have been characterized by structural, spectroscopic, and theoretical analysis to offer an electronic structure basis to explain the bonding parameters and stability. Single-crystal X-ray analysis reveals that UO2L(MeOH) adopts the typical "soft taco confirmation" characteristic of uranyl salophen complexes, whereas UO2Cl2(H2L) features an unusual neutral ligand coordination that contains an internal hydrogen bond between the phenol and imine. Rate constants calculated from electrochemical experiments confirm a quasi-reversible UO22+/UO2+ couple. Single-configurational and multiconfigurational methods were used to explore the bonding in UO2L(MeOH) and UO2Cl2(H2L). For UO2Cl2(H2L), the U-Cl bond exhibits more covalent contributions than U-OL.

Exploring the Oxidation States of Neptunium with Schiff Base Coordination Complexes.

A pair of neptunium Schiff base coordination complexes, NpVIO2L(MeOH) and NpIVL2 {H2L = N,N'-bis[(4,4'-diethylamino)salicylidene]-1,2-phenylenediamine}, have been synthesized and analyzed by several characterization methods including single-crystal X-ray diffraction, electronic absorption, 1H NMR, cyclic voltammetry, and theoretical interpretation. Structural analysis reveals that NpVIO2L(MeOH) and NpIVL2 are isomorphous with the previously reported UVIO2L(MeOH) and MIVL2 (M = Pu, Ce, U, Th) complexes, respectively, allowing for a direct comparison across the series. The reduction of NpVIO2L(MeOH) in situ or direct synthesis from a (NpVO2)+ source shows evidence of a pentavalent neptunyl (NpVO2L)xn- species as determined by UV/vis/NIR and 1H NMR spectroscopy. The synthesis of (NpVO2L)xn- directly from a (NpVO2)+ starting material gives a similar spectrum. Theoretical analysis offers insight into the electronic structure for a better understanding of the bonding patterns and relative stability of the different oxidation states. Computational results show that the Np-L covalent interactions in NpIVL2 are similar to those in the NpVIO2L(MeOH) complex, indicating that neither the presence of the axial oxo ligands nor the oxidation state significantly modify the nature of the Np-L bonds.

Origin of Bond Elongation in a Uranium(IV) cis-Bis(imido) Complex.

The activation of U-N multiple bonds in an imido analogue of the uranyl ion is accomplished by using a system that is very electron-rich with sterically encumbering ligands. Treating the uranium(VI) trans-bis(imido) UI2(NDIPP)2(THF)3 (DIPP = 2,6-diisopropylphenyl and THF = tetrahydrofuran) with tert-butyl(dimethylsilyl)amide (NTSA) results in a reduction and rearrangement to form the uranium(IV) cis-bis(imido) [U(NDIPP)2(NTSA)2]K2 (1). Compound 1 features long U-N bonds, pointing toward substantial activation of the N═U═N unit, as determined by X-ray crystallography and 1H NMR, IR, and electronic absorption spectroscopies. Computational analyses show that uranium(IV)-imido bonds in 1 are significantly weakened multiple bonds due to polarization toward antibonding and nonbonding orbitals. Such geometric control has important effects on the electronic structures of these species, which could be useful in the recycling of spent nuclear fuels.

Structure and Characterization of an Americium Bis(O,O'-diethyl)dithiophosphate Complex.

A facile synthesis of an americium complex with a sulfur-donor ligand has been developed, allowing characterization of americium bonding from multiple perspectives via several techniques. Reaction of 243Am with S2P(OEt)2- yields the tetrakis complex [Am(S2P(OEt)2)4]- that can be crystallized as the tetraphenylarsonium salt. Structures obtained from single crystal X-ray diffraction show bond length discrepancies from the neodymium analogue consistent with the soft-donor bond enhancement common to actinides. Solid state optical spectroscopy confirms interaction of the ligand with 5f orbitals. 31P nuclear magnetic reflects the minor paramagnetism of Am(III). Computational investigations through CASSCF calculations, ligand-field density functional theory, and quantum chemical topological analysis allow a quantification of covalency or orbital interaction effects via total energy density and nephelauxetic parameters, both of which indicate greater covalency in the americium species than in the neodymium analogue or the americium aquo complex.

Trivalent f-Element Squarates, Squarate-Oxalates, and Cationic Materials, and the Determination of the Nine-Coordinate Ionic Radius of Cf(III).

The synthesis, structure, and solid-state UV-vis-NIR spectroscopy of four new f-element squarates, M2(C4O4)3(H2O)4 (M = Eu, Am, Cf) and Sm(C4O4)(C4O3OH)(H2O)2·0.5H2O, four new cationic lanthanide squarate chlorides, [M4(C4O4)5(H2O)12]Cl2·5H2O (M = Eu, Dy, Ho Er), and two new actinide squarate oxalates, M2(C4O4)2(C2O4)(H2O)4 (M = Am, Cf), are presented. All of the metal centers are trivalent. Single-crystal X-ray diffraction analysis reveals that M2(C4O4)3(H2O)4 and Sm(C4O4)(C4O3OH)(H2O)2·0.5H2O have a two-dimensional sheet structure constructed from MO7(H2O)2 monocapped square-antiprismatic (coordination number (CN) = 9) metal centers and SmO6(H2O)2 square-antiprismatic (CN = 8) metal centers, respectively, whereas M2(C4O4)2(C2O4)(H2O)4 have a three-dimensional (3D) structure constructed from MO7(H2O)2 monocapped square-antiprismatic (CN = 9) metal centers. Additionally, the cationic framework materials [M4(C4O4)5(H2O)12]Cl2·5H2O have a 3D structure constructed from two crystallographically unique MO5(H2O)3 square-antiprismatic (CN = 8) metal centers. In these structures, the squarate ligands bind to the metal centers with varying coordination modes and denticities. The results of this study provide another example of the nonparallel chemistry between the lanthanides and transplutonium elements. From the crystallographic data for the isotypic series M2(C4O4)3(H2O)4 (M = La-Nd, Sm, Eu) and the linear regression fit to a plot of the unit cell volume as a function of the cube of the ionic radius, the nine-coordinate ionic radius of Cf 3+ was determined to be 1.127 ± 0.003 Å. Finally, computational analysis of the americium and californium complexes M2(C4O4)3(H2O)4 and M2(C4O4)2(C2O4)(H2O)4 reveals three important attributes: (i) the 5f orbitals are nonbonding in all cases, with the bonding differences occurring with the empty 6d orbitals; (ii) the Cf complexes exhibit more covalent character than their Am counterparts; and (iii) there is more covalent character in the squarate-oxalate complexes than in the squarate complexes.

Origin of Selectivity of a Triazinyl Ligand for Americium(III) over Neodymium(III).

[M(EtBTP)3 ][BPh4 ]3 ⋅3 CH3 CN (M=Nd, Am; EtBTP=2,6-bis(5,6-diethyl-1,2,4-triazin-3-yl)pyridine) have been synthesized from reactions of MCl3 ⋅n H2 O with EtBTP in acetonitrile followed by anion metathesis. Structural analysis reveals that these compounds contain M3+ cations bound by tridentate EtBTP ligands to create a tricapped trigonal prismatic geometry around the metal centers. Collection of high-resolution, single-crystal X-ray diffraction data also allowed reduction in bond lengths esd's, such that a slight contraction of Δ=0.0158(18) Šin the Am-N versus Nd-N bond lengths was observed, even though these cations ostensibly have matching ionic radii. Theoretical evaluation revealed enhanced metal-ligand bonding through back donation in the [Am(EtBTP)3 ]3+ complex that is absent in [Nd(EtBTP)3 ]3+ .

High-Pressure Studies of Cesium Uranyl Chloride.

The pressure behavior of crystalline Cs2UO2Cl4 has been explored using a diamond anvil cell. The uranyl fluorescence intensity decreases dramatically with increasing pressure. Using the O-U-O symmetric stretching frequency, an apparent linear decrease in bond length with increasing pressure was observed. A linear decrease in fluorescence intensity with increasing pressure was attributed to a large growth in the nonradiative relaxation, likely attributed to increased relaxation through phonon modes. Quantum theory of atoms in molecules calculations and ab initio wave function methods (CASSCF) support the U≡O bond in UO22+ being highly sensitive to the bond distance, but negligibly affected by the U-Cl bond length.

Structure, Spectroscopy, and Theoretical Analysis of Zero- and Three-Dimensional Lithium Plutonium Fluorides: Li4PuF8 and LiPuF5.

The reaction of 242PuO2 with HF and LiF under hydrothermal conditions results in the formation of Li4PuF8 and LiPuF5. These compounds were structurally characterized using single-crystal X-ray diffraction, and UV-vis-near-IR absorption spectroscopy was employed to confirm the oxidation state of the plutonium in the compounds as 4+. The structure of Li4PuF8 consists of [PuF8]4- anions that adopt a bicapped trigonal-prismatic geometry with approximate C2v symmetry. These molecules are bridged by Li+ cations. In contrast, LiPuF5 forms a dense three-dimensional network constructed from [PuF9]5- units that are bridged by F- anions. The Pu4+ cations are found within tricapped trigonal prisms. Extensive theoretical analysis of the electronic and bonding interactions is included with a comparison between the results derived from complete-active-space self-consistent-field at different levels of theory, quantum theory of atoms in molecules, interacting quantum atom, natural localized molecular orbital, and Wiberg bond order analyses. Covalent interactions in these compounds are examined, and intramolecular trends in covalent and electrostatic interactions are discussed.

Molecular and Electronic Structure, and Hydrolytic Reactivity of a Samarium(II) Crown Ether Complex.

The reaction of SmI2 with dibenzo-30-crown-10 (DB30C10), followed by metathesis with [Bu4N][BPh4], allows for the isolation of [SmII(DB30C10)][BPh4]2 as bright-red crystals in good yield. Exposure of [Sm(DB30C10)]2+ to solvents containing trace water results in the conversion to the dinuclear SmIII complex, Sm2(DB30C10)(OH)2I4. Structural analysis of both complexes shows substantial rearrangement of the crown ether from a folded, Pac-Man form with SmII to a twisted conformation with SmIII. The optical properties of [SmII(DB30C10)][BPh4]2 exhibit a strong temperature dependence and change from broad-band absorption features indicative of domination by 5d states to fine features characteristic of 4f → 4f transitions at low temperatures. Examination of the electronic structure of these complexes via ab initio wave function calculations (SO-CASSCF) shows that the ground state of SmII in [SmII(DB30C10)]2+ is a 4f6 state with low-lying 4f55d1 states, where the latter states have been lowered in energy by ∼12 000 cm-1 with respect to the free ion. The decacoordination of the SmII cation by the crown ether is responsible for this alteration in the energies of the excited state and demonstrates the ability to tune the electronic structure of SmII.

Electronic, Magnetic, and Theoretical Characterization of (NH4)4UF8, a Simple Molecular Uranium(IV) Fluoride.

The simple system of tetraammonium octafluorouranate is employed to derive a fundamental understanding of the uranium-fluorine interaction. The structure is composed of isolated molecules, enabling a detailed examination of the U4+ ( f2) ion. Characterization of single-crystals by X-ray diffraction, absorption spectroscopy, and magnetic analysis up to 45 T is combined with extensive theoretical treatment by CASSCF. The influence of different active spaces and representations of the structure is examined in the context of the experimental evidence. The Interacting Quantum Atoms method (IQA) is used to examine the nature of the U-F bond, concluding that there is a non-negligible degree of covalent character (9% of the total bond energy) in [UF8]4-. For the structural and theoretical reasons discussed herein, it is proposed that the structure of (NH4)4UF8 may be appropriately employed as a benchmark compound for future theoretical characterization of U(IV).