Measuring spectroscopy and magnetism of extracted and intracellular magnetosomes using soft X-ray ptychography.

Characterizing the chemistry and magnetism of magnetotactic bacteria (MTB) is an important aspect of understanding the biomineralization mechanism and function of the chains of magnetosomes (Fe3O4 nanoparticles) found in such species. Images and X-ray absorption spectra (XAS) of magnetosomes extracted from, and magnetosomes in, whole Magnetovibrio blakemorei strain MV-1 cells have been recorded using soft X-ray ptychography at the Fe 2p edge. A spatial resolution of 7 nm is demonstrated. Precursor-like and immature magnetosome phases in a whole MV-1 cell were visualized, and their Fe 2p spectra were measured. Based on these results, a model for the pathway of magnetosome biomineralization for MV-1 is proposed. Fe 2p X-ray magnetic circular dichroism (XMCD) spectra have been derived from ptychography image sequences recorded using left and right circular polarization. The shape of the XAS and XMCD signals in the ptychographic absorption spectra of both sample types is identical to the shape and signals measured with conventional bright-field scanning transmission X-ray microscope. A weaker and inverted XMCD signal was observed in the ptychographic phase spectra of the extracted magnetosomes. The XMCD ptychographic phase spectrum of the intracellular magnetosomes differed from the ptychographic phase spectrum of the extracted magnetosomes. These results demonstrate that spectro-ptychography offers a superior means of characterizing the chemical and magnetic properties of MTB at the individual magnetosome level.

Chemical and Morphological Inhomogeneity of Aluminum Metal and Oxides from Soft X-ray Spectromicroscopy.

Oxygen and aluminum K-edge X-ray absorption spectroscopy (XAS), imaging from a scanning transmission X-ray microscope (STXM), and first-principles calculations were used to probe the composition and morphology of bulk aluminum metal, α- and γ-Al2O3, and several types of aluminum nanoparticles. The imaging results agreed with earlier transmission electron microscopy studies that showed a 2 to 5 nm thick layer of Al2O3 on all the Al surfaces. Spectral interpretations were guided by examination of the calculated transition energies, which agreed well with the spectroscopic measurements. Features observed in the experimental O and Al K-edge XAS were used to determine the chemical structure and phase of the Al2O3 on the aluminum surfaces. For unprotected 18 and 100 nm Al nanoparticles, this analysis revealed an oxide layer that was similar to γ-Al2O3 and comprised of both tetrahedral and octahedral Al coordination sites. For oleic acid-protected Al nanoparticles, only tetrahedral Al oxide coordination sites were observed. The results were correlated to trends in the reactivity of the different materials, which suggests that the structures of different Al2O3 layers have an important role in the accessibility of the underlying Al metal toward further oxidation. Combined, the Al K-edge XAS and STXM results provided detailed chemical information that was not obtained from powder X-ray diffraction or imaging from a transmission electron microscope.

Enhanced Nonadiabaticity in Vortex Cores due to the Emergent Hall Effect.

We present a combined theoretical and experimental study, investigating the origin of the enhanced nonadiabaticity of magnetic vortex cores. Scanning transmission x-ray microscopy is used to image the vortex core gyration dynamically to measure the nonadiabaticity with high precision, including a high confidence upper bound. We show theoretically, that the large nonadiabaticity parameter observed experimentally can be explained by the presence of local spin currents arising from a texture induced emergent Hall effect. This study demonstrates that the magnetic damping α and nonadiabaticity parameter ß are very sensitive to the topology of the magnetic textures, resulting in an enhanced ratio (ß/α>1) in magnetic vortex cores or Skyrmions.

Nanobubbles at Hydrophilic Particle-Water Interfaces.

The puzzling persistence of nanobubbles breaks Laplace's law for bubbles, which is of great interest for promising applications in surface processing, H2 and CO2 storage, water treatment, and drug delivery. So far, nanobubbles have mostly been reported on hydrophobic planar substrates with atomic flatness. It remains a challenge to quantify nanobubbles on rough and irregular surfaces because of the lack of a characterization technique that can detect both the nanobubble morphology and chemical composition inside individual nanobubble-like objects. Here, by using synchrotron-based scanning transmission soft X-ray microscopy (STXM) with nanometer resolution, we discern nanoscopic gas bubbles of >25 nm with direct in situ proof of O2 inside the nanobubbles at a hydrophilic particle-water interface under ambient conditions. We find a stable cloud of O2 nanobubbles at the diatomite particle-water interface hours after oxygen aeration and temperature variation. The in situ technique may be useful for many surface nanobubble-related studies such as material preparation and property manipulation, phase equilibrium, nucleation kinetics, and relationships with chemical composition within the confined nanoscale space. The oxygen nanobubble clouds may be important in modifying particle-water interfaces and offering breakthrough technologies for oxygen delivery in sediment and/or deep water environments.

A Macrocyclic Chelator That Selectively Binds Ln4+ over Ln3+ by a Factor of 1029.

A tetravalent cerium macrocyclic complex (CeLK4) was prepared with an octadentate terephthalamide ligand comprised of hard catecholate donors and characterized in the solution state by spectrophotometric titrations and electrochemistry and in the crystal by X-ray diffraction. The solution-state studies showed that L exhibits a remarkably high affinity toward Ce4+, with log ß110 = 61(2) and ΔG = -348 kJ/mol, compared with log ß110 = 32.02(2) for the analogous Pr3+ complex. In addition, L exhibits an unusual preference for forming CeL4- relative to formation of the analogous actinide complex, ThL4-, which has ß110 = 53.7(5). The extreme stabilization of tetravalent cerium relative to its trivalent state is also evidenced by the shift of 1.91 V in the redox potential of the Ce3+/Ce4+ couple of the complex (measured at -0.454 V vs SHE). The unprecedented behavior prompted an electronic structure analysis using L3- and M5,4-edge X-ray absorption near-edge structure (XANES) spectroscopies and configuration interaction calculations, which showed that 4f-orbital bonding in CeLK4 has partial covalent character due to ligand-to-metal charge transfer (LMCT) in the ground state. The experimental results are presented in the context of earlier measurements on tetravalent cerium compounds, indicating that the amount of LMCT for CeLK4 is similar to that observed for [Et4N]2[CeCl6] and CeO2 and significantly less than that for the organometallic sandwich compound cerocene, (C8H8)2Ce. A simple model to rationalize changes in 4f orbital bonding for tri- and tetravalent lanthanide and actinide compounds is also provided.

Incorporation of Technetium into Spinel Ferrites.

Technetium (99Tc) is a problematic fission product for the long-term disposal of nuclear waste due to its long half-life, high fission yield, and to the environmental mobility of pertechnetate, the stable species in aerobic environments. One approach to preventing 99Tc contamination is using sufficiently durable waste forms. We report the incorporation of technetium into a family of synthetic spinel ferrites that have environmentally durable natural analogs. A combination of X-ray diffraction, X-ray absorption fine structure spectroscopy, and chemical analysis reveals that Tc(IV) replaces Fe(III) in octahedral sites and illustrates how the resulting charge mismatch is balanced. When a large excess of divalent metal ions is present, the charge is predominantly balanced by substitution of Fe(III) by M(II). When a large excess of divalent metal ions is absent, the charge is largely balanced by creation of vacancies among the Fe(III) sites (maghemitization). In most samples, Tc is present in Tc-rich regions rather than being homogeneously distributed.

The nature of chemical bonding in actinide and lanthanide ferrocyanides determined by X-ray absorption spectroscopy and density functional theory.

The electronic properties of actinide cations are of fundamental interest to describe intramolecular interactions and chemical bonding in the context of nuclear waste reprocessing or direct storage. The 5f and 6d orbitals are the first partially or totally vacant states in these elements, and the nature of the actinide ligand bonds is related to their ability to overlap with ligand orbitals. Because of its chemical and orbital selectivities, X-ray absorption spectroscopy (XAS) is an effective probe of actinide species frontier orbitals and for understanding actinide cation reactivity toward chelating ligands. The soft X-ray probes of the light elements provide better resolution than actinide L3-edges to obtain electronic information from the ligand. Thus coupling simulations to experimental soft X-ray spectral measurements and complementary quantum chemical calculations yields quantitative information on chemical bonding. In this study, soft X-ray XAS at the K-edges of C and N, and the L2,3-edges of Fe was used to investigate the electronic structures of the well-known ferrocyanide complexes K4Fe(II)(CN)6, thorium hexacyanoferrate Th(IV)Fe(II)(CN)6, and neodymium hexacyanoferrate KNd(III)Fe(II)(CN)6. The soft X-ray spectra were simulated based on quantum chemical calculations. Our results highlight the orbital overlapping effects and atomic effective charges in the Fe(II)(CN)6 building block. In addition to providing a detailed description of the electronic structure of the ferrocyanide complex (K4Fe(II)(CN)6), the results strongly contribute to confirming the actinide 5f and 6d orbital oddity in comparison to lanthanide 4f and 5d.

Dependence on Crystal Size of the Nanoscale Chemical Phase Distribution and Fracture in LixFePO4.

The performance of battery electrode materials is strongly affected by inefficiencies in utilization kinetics and cycle life as well as size effects. Observations of phase transformations in these materials with high chemical and spatial resolution can elucidate the relationship between chemical processes and mechanical degradation. Soft X-ray ptychographic microscopy combined with X-ray absorption spectroscopy and electron microscopy creates a powerful suite of tools that we use to assess the chemical and morphological changes in lithium iron phosphate (LiFePO4) micro- and nanocrystals that occur upon delithiation. All sizes of partly delithiated crystals were found to contain two phases with a complex correlation between crystallographic orientation and phase distribution. However, the lattice mismatch between LiFePO4 and FePO4 led to severe fracturing on microcrystals, whereas no mechanical damage was observed in nanoplates, indicating that mechanics are a principal driver in the outstanding electrode performance of LiFePO4 nanoparticles. These results demonstrate the importance of engineering the active electrode material in next generation electrical energy storage systems, which will achieve theoretical limits of energy density and extended stability. This work establishes soft X-ray ptychographic chemical imaging as an essential tool to build comprehensive relationships between mechanics and chemistry that guide this engineering design.

Theory and X-ray Absorption Spectroscopy for Aluminum Coordination Complexes ­ Al K-Edge Studies of Charge and Bonding in (BDI)Al, (BDI)AlR2, and (BDI)AlX2 Complexes.

Polarized aluminum K-edge X-ray absorption near edge structure (XANES) spectroscopy and first-principles calculations were used to probe electronic structure in a series of (BDI)Al, (BDI)AlX2, and (BDI)AlR2 coordination compounds (X = F, Cl, I; R = H, Me; BDI = 2,6-diisopropylphenyl-ß-diketiminate). Spectral interpretations were guided by examination of the calculated transition energies and polarization-dependent oscillator strengths, which agreed well with the XANES spectroscopy measurements. Pre-edge features were assigned to transitions associated with the Al 3p orbitals involved in metal-ligand bonding. Qualitative trends in Al 1s core energy and valence orbital occupation were established through a systematic comparison of excited states derived from Al 3p orbitals with similar symmetries in a molecular orbital framework. These trends suggested that the higher transition energies observed for (BDI)AlX2 systems with more electronegative X(1-) ligands could be ascribed to a decrease in electron density around the aluminum atom, which causes an increase in the attractive potential of the Al nucleus and concomitant increase in the binding energy of the Al 1s core orbitals. For (BDI)Al and (BDI)AlH2 the experimental Al K-edge XANES spectra and spectra calculated using the eXcited electron and Core-Hole (XCH) approach had nearly identical energies for transitions to final state orbitals of similar composition and symmetry. These results implied that the charge distributions about the aluminum atoms in (BDI)Al and (BDI)AlH2 are similar relative to the (BDI)AlX2 and (BDI)AlMe2 compounds, despite having different formal oxidation states of +1 and +3, respectively. However, (BDI)Al was unique in that it exhibited a low-energy feature that was attributed to transitions into a low-lying p-orbital of b1 symmetry that is localized on Al and orthogonal to the (BDI)Al plane. The presence of this low-energy unoccupied molecular orbital on electron-rich (BDI)Al distinguishes its valence electronic structure from that of the formally trivalent compounds (BDI)AlX2 and (BDI)AlR2. The work shows that Al K-edge XANES spectroscopy can be used to provide valuable insight into electronic structure and reactivity relationships for main-group coordination compounds.

Covalency in lanthanides. An X-ray absorption spectroscopy and density functional theory study of LnCl6(x-) (x = 3, 2).

Covalency in Ln-Cl bonds of Oh-LnCl6(x-) (x = 3 for Ln = Ce(III), Nd(III), Sm(III), Eu(III), Gd(III); x = 2 for Ln = Ce(IV)) anions has been investigated, primarily using Cl K-edge X-ray absorption spectroscopy (XAS) and time-dependent density functional theory (TDDFT); however, Ce L3,2-edge and M5,4-edge XAS were also used to characterize CeCl6(x-) (x = 2, 3). The M5,4-edge XAS spectra were modeled using configuration interaction calculations. The results were evaluated as a function of (1) the lanthanide (Ln) metal identity, which was varied across the series from Ce to Gd, and (2) the Ln oxidation state (when practical, i.e., formally Ce(III) and Ce(IV)). Pronounced mixing between the Cl 3p- and Ln 5d-orbitals (t2g* and eg*) was observed. Experimental results indicated that Ln 5d-orbital mixing decreased when moving across the lanthanide series. In contrast, oxidizing Ce(III) to Ce(IV) had little effect on Cl 3p and Ce 5d-orbital mixing. For LnCl6(3-) (formally Ln(III)), the 4f-orbitals participated only marginally in covalent bonding, which was consistent with historical descriptions. Surprisingly, there was a marked increase in Cl 3p- and Ce(IV) 4f-orbital mixing (t1u* + t2u*) in CeCl6(2-). This unexpected 4f- and 5d-orbital participation in covalent bonding is presented in the context of recent studies on both tetravalent transition metal and actinide hexahalides, MCl6(2-) (M = Ti, Zr, Hf, U).

Current-induced transition from particle-by-particle to concurrent intercalation in phase-separating battery electrodes.

Many battery electrodes contain ensembles of nanoparticles that phase-separate on (de)intercalation. In such electrodes, the fraction of actively intercalating particles directly impacts cycle life: a vanishing population concentrates the current in a small number of particles, leading to current hotspots. Reports of the active particle population in the phase-separating electrode lithium iron phosphate (LiFePO4; LFP) vary widely, ranging from near 0% (particle-by-particle) to 100% (concurrent intercalation). Using synchrotron-based X-ray microscopy, we probed the individual state-of-charge for over 3,000 LFP particles. We observed that the active population depends strongly on the cycling current, exhibiting particle-by-particle-like behaviour at low rates and increasingly concurrent behaviour at high rates, consistent with our phase-field porous electrode simulations. Contrary to intuition, the current density, or current per active internal surface area, is nearly invariant with the global electrode cycling rate. Rather, the electrode accommodates higher current by increasing the active particle population. This behaviour results from thermodynamic transformation barriers in LFP, and such a phenomenon probably extends to other phase-separating battery materials. We propose that modifying the transformation barrier and exchange current density can increase the active population and thus the current homogeneity. This could introduce new paradigms to enhance the cycle life of phase-separating battery electrodes.

Constraints on the formation environment of two chondrule-like igneous particles from Comet 81P/Wild 2.

Using chemical and petrologic evidence and modeling, we deduce that two chondrule-like particles named Iris and Callie, from Stardust cometary track C2052,12,74, formed in an environment very similar to that seen for type II chondrules in meteorites. Iris was heated near liquidus, equilibrated, and cooled at ≤ 100 °C/hr and within ≈ 2 log units of the IW buffer with a high partial pressure of Na such as would be present with dust enrichments of ≈ 103. There was no detectable metamorphic, nebular or aqueous alteration. In previous work Ogliore et al. (2012) reported that Iris formed late, > 3 Myr after CAIs, assuming 26Al was homogenously distributed, and was rich in heavy oxygen. Iris may be similar to assemblages found only in interplanetary dust particles and Stardust cometary samples called Kool particles. Callie is chemically and isotopically very similar but not identical to Iris.

Influence of pyrazolate vs N-heterocyclic carbene ligands on the slow magnetic relaxation of homoleptic trischelate lanthanide(III) and uranium(III) complexes.

Two isostructural series of trigonal prismatic complexes, M(Bp(Me))3 and M(Bc(Me))3 (M = Y, Tb, Dy, Ho, Er, U; [Bp(Me)](-) = dihydrobis(methypyrazolyl)borate; [Bc(Me)](-) = dihydrobis(methylimidazolyl)borate) are synthesized and fully characterized to examine the influence of ligand donor strength on slow magnetic relaxation. Investigation of the dynamic magnetic properties reveals that the oblate electron density distributions of the Tb(3+), Dy(3+), and U(3+) metal ions within the axial ligand field lead to slow relaxation upon application of a small dc magnetic field. Significantly, the magnetization relaxation is orders of magnitude slower for the N-heterocyclic carbene complexes, M(Bc(Me))3, than for the isomeric pyrazolate complexes, M(Bp(Me))3. Further, investigation of magnetically dilute samples containing 11-14 mol % of Tb(3+), Dy(3+), or U(3+) within the corresponding Y(3+) complex matrix reveals thermally activated relaxation is favored for the M(Bc(Me))3 complexes, even when dipolar interactions are largely absent. Notably, the dilute species U(Bc(Me))3 exhibits Ueff ≈ 33 cm(-1), representing the highest barrier yet observed for a U(3+) molecule demonstrating slow relaxation. Additional analysis through lanthanide XANES, X-band EPR, and (1)H NMR spectroscopies provides evidence that the origin of the slower relaxation derives from the greater magnetic anisotropy enforced within the strongly donating N-heterocyclic carbene coordination sphere. These results show that, like molecular symmetry, ligand-donating ability is a variable that can be controlled to the advantage of the synthetic chemist in the design of single-molecule magnets with enhanced relaxation barriers.

Synthesis and characterization of eight compounds of the MU8Q17 family: ScU8S17, CoU8S17, NiU8S17, TiU8Se17, VU8Se17, CrU8Se17, CoU8Se17, and NiU8Se17.

The solid-state MU8Q17 compounds ScU8S17, CoU8S17, NiU8S17, TiU8Se17, VU8Se17, CrU8Se17, CoU8Se17, and NiU8Se17 were synthesized from the reactions of the elements at 1173 or 1123 K. These isostructural compounds crystallize in space group C2h3 - C2/m of the monoclinic system in the CrU8S17 structure type. X-ray absorption near-edge structure spectroscopic studies of ScU8S17 indicate that it contains Sc3+, and hence charge balance is achieved with a composition that includes U3+ as well as U4+. The other compounds charge balance with M2+ and U4+. Magnetic susceptibility measurements on ScU8S17 indicate antiferromagnetic couplings and a highly reduced effective magnetic moment. Ab Initio calculations find the compound to be metallic. Surprisingly, the Sc­S distances are actually longer than all the other M­S interactions, even though the ionic radii of Sc3+, low-spin Cr2+, and Ni2+ are similar.

Toward equatorial planarity about uranyl: synthesis and structure of tridentate nitrogen-donor {UO2}2+ complexes.

The reaction of UO2Cl2·3THF with the tridentate nitrogen donor ligand 2,6-bis(2-benzimidazolyl)pyridine (H2BBP) in pyridine leads to the formation of three different complexes: [(UO2)(H2BBP)Cl2] (1), [(UO)2(HBBP)(Py)Cl] (2), and [(UO2)(BBP)(Py)2] (3) after successive deprotonation of H2BBP with a strong base. Crystallographic determination of 1-3 reveals that increased charge through ligand deprotonation and displacement of chloride leads to equatorial planarity about uranyl as well as a more compact overall coordination geometry. Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectra of 1-3 at the U-4d edges have been recorded using a soft X-ray Scanning Transmission X-ray Microscope (STXM) and reveal the uranium 4d5/2 and 4d3/2 transitions at energies associated with uranium in the hexavalent oxidation state. First-principles Density Functional Theory (DFT) electronic structure calculations for the complexes have been performed to determine and validate the coordination characteristics, which correspond well to the experimental results.

Nanoscale chemical imaging of a working catalyst by scanning transmission X-ray microscopy.

The modern chemical industry uses heterogeneous catalysts in almost every production process. They commonly consist of nanometre-size active components (typically metals or metal oxides) dispersed on a high-surface-area solid support, with performance depending on the catalysts' nanometre-size features and on interactions involving the active components, the support and the reactant and product molecules. To gain insight into the mechanisms of heterogeneous catalysts, which could guide the design of improved or novel catalysts, it is thus necessary to have a detailed characterization of the physicochemical composition of heterogeneous catalysts in their working state at the nanometre scale. Scanning probe microscopy methods have been used to study inorganic catalyst phases at subnanometre resolution, but detailed chemical information of the materials in their working state is often difficult to obtain. By contrast, optical microspectroscopic approaches offer much flexibility for in situ chemical characterization; however, this comes at the expense of limited spatial resolution. A recent development promising high spatial resolution and chemical characterization capabilities is scanning transmission X-ray microscopy, which has been used in a proof-of-principle study to characterize a solid catalyst. Here we show that when adapting a nanoreactor specially designed for high-resolution electron microscopy, scanning transmission X-ray microscopy can be used at atmospheric pressure and up to 350 degrees C to monitor in situ phase changes in a complex iron-based Fisher-Tropsch catalyst and the nature and location of carbon species produced. We expect that our system, which is capable of operating up to 500 degrees C, will open new opportunities for nanometre-resolution imaging of a range of important chemical processes taking place on solids in gaseous or liquid environments.

Comparison of x-ray absorption spectra between water and ice: new ice data with low pre-edge absorption cross-section.

The effect of crystal growth conditions on the O K-edge x-ray absorption spectra of ice is investigated through detailed analysis of the spectral features. The amount of ice defects is found to be minimized on hydrophobic surfaces, such as BaF2(111), with low concentration of nucleation centers. This is manifested through a reduction of the absorption cross-section at 535 eV, which is associated with distorted hydrogen bonds. Furthermore, a connection is made between the observed increase in spectral intensity between 544 and 548 eV and high-symmetry points in the electronic band structure, suggesting a more extended hydrogen-bond network as compared to ices prepared differently. The spectral differences for various ice preparations are compared to the temperature dependence of spectra of liquid water upon supercooling. A double-peak feature in the absorption cross-section between 540 and 543 eV is identified as a characteristic of the crystalline phase. The connection to the interpretation of the liquid phase O K-edge x-ray absorption spectrum is extensively discussed.

Intercalation pathway in many-particle LiFePO4 electrode revealed by nanoscale state-of-charge mapping.

The intercalation pathway of lithium iron phosphate (LFP) in the positive electrode of a lithium-ion battery was probed at the ∼40 nm length scale using oxidation-state-sensitive X-ray microscopy. Combined with morphological observations of the same exact locations using transmission electron microscopy, we quantified the local state-of-charge of approximately 450 individual LFP particles over nearly the entire thickness of the porous electrode. With the electrode charged to 50% state-of-charge in 0.5 h, we observed that the overwhelming majority of particles were either almost completely delithiated or lithiated. Specifically, only ∼2% of individual particles were at an intermediate state-of-charge. From this small fraction of particles that were actively undergoing delithiation, we conclude that the time needed to charge a particle is ∼1/50 the time needed to charge the entire particle ensemble. Surprisingly, we observed a very weak correlation between the sequence of delithiation and the particle size, contrary to the common expectation that smaller particles delithiate before larger ones. Our quantitative results unambiguously confirm the mosaic (particle-by-particle) pathway of intercalation and suggest that the rate-limiting process of charging is initiating the phase transformation by, for example, a nucleation-like event. Therefore, strategies for further enhancing the performance of LFP electrodes should not focus on increasing the phase-boundary velocity but on the rate of phase-transformation initiation.

Diniobium inverted sandwich complexes with µ-η6:η6-arene ligands: synthesis, kinetics of formation, and electronic structure.

Monometallic niobium arene complexes [Nb(BDI)(N(t)Bu)(R-C(6)H(5))] (2a: R = H and 2b: R = Me, BDI = N,N'-diisopropylbenzene-ß-diketiminate) were synthesized and found to undergo slow conversion into the diniobium inverted arene sandwich complexes [[(BDI)Nb(N(t)Bu)](2)(µ-RC(6)H(5))] (7a: R = H and 7b: R = Me) in solution. The kinetics of this reaction were followed by (1)H NMR spectroscopy and are in agreement with a dissociative mechanism. Compounds 7a-b showed a lack of reactivity toward small molecules, even at elevated temperatures, which is unusual in the chemistry of inverted sandwich complexes. However, protonation of the BDI ligands occurred readily on treatment with [H(OEt(2))][B(C(6)F(5))(4)], resulting in the monoprotonated cationic inverted sandwich complex 8 [[(BDI(#))Nb(N(t)Bu)][(BDI)Nb(N(t)Bu)](µ-C(6)H(5))][B(C(6)F(5))(4)] and the dicationic complex 9 [[(BDI(#))Nb(N(t)Bu)](2)(µ-RC(6)H(5))][B(C(6)F(5))(4)](2) (BDI(#) = (ArNC(Me))(2)CH(2)). NMR, UV-vis, and X-ray absorption near-edge structure (XANES) spectroscopies were used to characterize this unique series of diamagnetic molecules as a means of determining how best to describe the Nb-arene interactions. The X-ray crystal structures, UV-vis spectra, arene (1)H NMR chemical shifts, and large J(CH) coupling constants provide evidence for donation of electron density from the Nb d-orbitals into the antibonding π system of the arene ligands. However, Nb L(3,2)-edge XANES spectra and the lack of sp(3) hybridization of the arene carbons indicate that the Nb → arene donation is not accompanied by an increase in Nb formal oxidation state and suggests that 4d(2) electronic configurations are appropriate to describe the Nb atoms in all four complexes.

Carbon K-edge X-ray absorption spectroscopy and time-dependent density functional theory examination of metal-carbon bonding in metallocene dichlorides.

Metal-carbon covalence in (C5H5)2MCl2 (M = Ti, Zr, Hf) has been evaluated using carbon K-edge X-ray absorption spectroscopy (XAS) as well as ground-state and time-dependent hybrid density functional theory (DFT and TDDFT). Differences in orbital mixing were determined experimentally using transmission XAS of thin crystalline material with a scanning transmission X-ray microscope (STXM). Moving down the periodic table (Ti to Hf) has a marked effect on the experimental transition intensities associated with the low-lying antibonding 1a1* and 1b2* orbitals. The peak intensities, which are directly related to the M-(C5H5) orbital mixing coefficients, increase from 0.08(1) and 0.26(3) for (C5H5)2TiCl2 to 0.31(3) and 0.75(8) for (C5H5)2ZrCl2, and finally to 0.54(5) and 0.83(8) for (C5H5)2HfCl2. The experimental trend toward increased peak intensity for transitions associated with 1a1* and 1b2* orbitals agrees with the calculated TDDFT oscillator strengths [0.10 and 0.21, (C5H5)2TiCl2; 0.21 and 0.73, (C5H5)2ZrCl2; 0.35 and 0.69, (C5H5)2HfCl2] and with the amount of C 2p character obtained from the Mulliken populations for the antibonding 1a1* and 1b2* orbitals [8.2 and 23.4%, (C5H5)2TiCl2; 15.3 and 39.7%, (C5H5)2ZrCl2; 20.1 and 50.9%, (C5H5)2HfCl2]. The excellent agreement between experiment, theory, and recent Cl K-edge XAS and DFT measurements shows that C 2p orbital mixing is enhanced for the diffuse Hf (5d) and Zr (4d) atomic orbitals in relation to the more localized Ti (3d) orbitals. These results provide insight into how changes in M-Cl orbital mixing within the metallocene wedge are correlated with periodic trends in covalent bonding between the metal and the cyclopentadienide ancillary ligands.