Metal-ligand Bond Research Articles

Theoretical Evaluation of Metal-Ligand Bonding in Neptunium Compounds in Relation to 237Np Mössbauer Spectroscopy.

The 237Np Mössbauer isomer shift and quadrupole splitting (QS) are powerful probes for the metal-ligand bonding of neptunium, a 5f-element of vital importance in the nuclear fuel cycle. A large set of Np compounds with different oxidation states (III) to (VII) is studied to investigate, by first-principles calculations, isomer shifts and the QS trends in relation to the Np oxidation state. Natural Bond Orbital analysis reveals that in addition to donation bonding to the 5f shell, participation of the 6d and 7s neptunium shells in covalent (donation) bonding substantially impacts the isomer shifts. The isomer shift cannot be interpreted solely by the 5f shell electron count. The isomer shift for Np(II) compounds is estimated to be in the range of 31-34 mm/s, less positive than for Np(III) compounds. For the QS, density functional calculations fail to reproduce the quadrupole splitting for some Np(VI) ionic solids. A multiconfigurational wave function approach reproduces the observed QS trends. The calculations give a semiquantitative interpretation of the trends for Np oxidation states (V) to (VII). The contrasting QS for standard and "reverse" neptunyl(VI), at the opposite extremes of the observed QS scale, arises predominantly from the different crystal environments.

Significant π bonding in coinage metal complexes OCTMCCO- from infrared photodissociation spectroscopy and theoretical calculations.

A series of coinage metal complexes in the form of TMC(CO)n - (TM = Cu, Ag, Au; n = 0-3) were generated using a laser-ablation supersonic expansion ion source in the gas phase. Mass-selected infrared photodissociation spectroscopy in conjunction with quantum chemical calculations indicated that the TMC(CO)3 - complexes contain a linear OCTMCCO- core anion. Bonding analyses suggest that the linear OCTMCCO- anions are better described as the bonding interactions between a singlet ground state TM+ metal cation and the OC/CCO2- ligands in the singlet ground state. In addition to the strong ligands to metal σ donation bonding components, the π-bonding components also contribute significantly to the metal-ligand bonds due to the synergetic effects of the CO and CCO2- ligands. The strengths of the bonding of the three metals show a V-shaped trend in which the second-row transition metal Ag exhibits the weakest interactions whereas the third-row transition metal Au shows the strongest interactions due to relativistic effects.

From the Free Ligand to the Transition Metal Complex: FeEDTA- Formation Seen at Ligand K-Edges.

Chelating agents are an integral part of transition metal complex chemistry with broad biological and industrial relevance. The hexadentate chelating agent ethylenediaminetetraacetic acid (EDTA) has the capability to bind to metal ions at its two nitrogen and four of its carboxylate oxygen sites. We use resonant inelastic X-ray scattering at the 1s absorption edge of the aforementioned elements in EDTA and the iron(III)-EDTA complex to investigate the impact of the metal–ligand bond formation on the electronic structure of EDTA. Frontier orbital distortions, occupation changes, and energy shifts through metal–ligand bond formation are probed through distinct spectroscopic signatures.

One-Pot Synthesis and Characterization of Three Chromotropic Supramolecular Isomeric Cu(II) Coordination Polymers as 1-D Zigzag or Helical Chains with Homochiral or Heterochiral Cu(II) Coordination Centres

The reaction of 1,10-phenanthroline (Phen) and 4′,4′''-(perfluoropropane-2,2-diyl) bis(([1,1′-biphenyl]-4-carboxylic acid)) (H2L2) with Cu(CH3COO)2⋅H2O in DMF/H2O at 50 ∼ 90°C in capped glass vials afforded three products with distinctively different colours and shapes. Single crystal X-ray diffraction analysis revealed that the three products, namely {[Cu(phen)(L2)] ⋅ 1.25H2O} (1) (teal acicula, KPI = 60.6%), {[Cu(phen)(L2)]⋅ H2O⋅DMF}n(2) (sapphire blue plate, KPI = 65.6%) and {[Cu(phen)(L2)]⋅0.31H2O}n (3) (cool grey plate, KPI = 66.8%) are 1-D supramolecular Cu(II) coordination polymers. Both 1 and 2 are composed of 1-D zigzag chains with 1 contains heterochiral Cu(II) (Λ- and Δ- alternatively) centres in the same chain and 2 contains homochiral Cu(II) (Λ- or Δ-) centres in the same chain, while 3, containing homochiral Cu(II) (Λ- or Δ-) centres, has a helical structure along a axis, with a rectangular channel of a size 19.61 × 16.61 Å2. Each helical turn spans 10.74 Å, composed of two [Cu(L2)(Phen)] units. With the largest KPI (∼66.3%), the highest thermal stability, highest formation energy from DFT calculations and lowest hygroscopicity, 3 can be obtained as the only product at higher temperature (95°C) and/or higher ratio of water (DMF/water (v/v) = 20:6), which implies that it is the thermodynamic product. 2 can be obtained at mediate temperature (60 ∼ 80°C) as the only product or co-exist with other products, while 1 only appeared at low temperature (60 and 70 °C) in the solvents of DMF/water (v/v) = 20:2 or 20:3 at a very small amount. The chromotropism of the three isomers was examined using UV-Vis spectroscopic technique, and the coordination geometry of Cu(II) was examined with Hathaway's tetragonality analysis and Continuous Shape Measures Calculation using SHAPE 2.1 programme. Linear correlations between the solid-state UV-Vis absorption maxima of the three isomers against the tetragonality (T) or the distortion percentage of the coordination geometry of Cu(II) from a perfect octahedral (Doct) were observed. The three complexes were also analyzed by FT IR, TGA and DFT formation energy calculations. The thermodynamic stability and the thermal stability of the three isomer increases in order of 1 < 2 < 3, supported by the analysis of the metal-ligand coordination bonds and the intermolecular interactions, the study of chromotropism and crystal fields, the effects of solvent polarity and temperature to the formation of isomers, the close-packing theory (KPI calculations), the DFT calculations and the TGA analysis.

Self-healable recyclable thermoplastic polyurethane elastomers: Enabled by metal–ligand bonds between the cerium(III) triflate and phloretin

This study initially developed highly efficient self-healing materials with cerium(III) triflate [Ce(SO3CF3)3] as the metal center and phloretin as the ligand to synthesize a self-healable polyurethane elastomer with excellent self-healable and mechanical properties. Nuclear magnetic resonance data from experimental research on the self-healing mechanism indicated that Ce(SO3CF3)3 and phloretin could effectively form coordination bonds. Then, the hydroxyl group (−OH) on the para position of the phloretin reacted with the isocyanate group in the polyurethane prepolymer to form coordination-bond-facilitated self-healable polyurethane elastomers. An optimal sample (PUPC-2) with high tensile strength (5.17 Mpa) and high stretchability (1,415.7%), ideal toughness (35.08 MJ m−3), and remarkable healing efficiency (near to 100% healed after 48 h at room temperature) was obtained. In addition, the PUPC sample also exhibited good thermoplasticity, which means that the self-healable polyurethane elastomer can be recycled. These excellent performances ensure that the self-healable thermoplastic polyurethane elastomers have a wide range of practical applications. Therefore, a flexible conductive electrode with a self-healable capacity was obtained by compositing the PUPC-2 sample with flexible graphene-based conductive paper, which demonstrated good potential for application in the field of wearable devices and flexible electronics.

Dynamics of rotation in two‐coordinate thiazolyl copper(I) carbazolyl complexes

Two‐coordinate carbene Cu(Ι) amide complexes with sterically bulky groups such as the diisopropyl phenyl (dipp) on the carbenes have been shown to have comparable performance to the phosphorescent emitters bearing heavy atoms such as iridium and platinum. These bulky groups enforce a coplanar molecular structure and suppress the nonradiative decay rates. Here, three different two‐coordinate Cu(Ι) complexes were investigated that bear a common thiazole carbene, 3‐(2,6‐diisopropylphenyl)‐4,5‐dimethylthiazol‐2‐ylidene, with only a single dipp group, and carbazolyl ligands with substituents of varying steric bulk ortho to N. These substituents have a negligible impact on luminescence energies of the complexes but serve to modulate the rotation barriers along the metal–ligand coordinate bond. The geometric arrangement of ligands (syn‐ or anti‐conformer) in complexes with alkyl substituents were found to differ, being syn in the solid state versus anti in solution as revealed by crystallographic analysis and nuclear magnetic resonance spectroscopy. In addition, calculations were performed to determine potential energy surfaces for different conformations of the three complexes to provide a theoretical evaluation of rotation barriers around the metal–ligand bond axis. The relationship between rotation barriers and photophysical properties demonstrate that rates for nonradiative decay decrease with increasing bulk of the substituents on the carbazolyl ligand.

Metal-Ligand Bonds in Rare Earth Metal-Biphenyl Complexes.

A series of theoretical methods, including density functional theory, multiconfiguration molecular orbital theory, and ab initio valence bond theory, are devoted to understanding the metal-ligand bonds in M-BP (BP = biphenyl; M = Sc, Y, or La) complexes. Different from most transition metal-BP complexes, the most stable metal-biphenyl conformers are not half-sandwich but clamshell. Energy decomposition analysis results reveal that the M-BP bonds in the clamshell conformers possess extra-large orbital relaxation. According to the wave function analysis, 2-fold donations and 2-fold back-donations exist in the clamshell M-BP bonds. The back-donations from M to BP are quite strong, while donations from BP to M are quite weak. Our work improves our understanding of the metal-ligand bonds, which can be considered as the "reversed" Dewar-Chatt-Duncanson model.

Ligand Additivity and Divergent Trends in Two Types of Delocalization Errors from Approximate Density Functional Theory.

The predictive accuracy of density functional theory (DFT) is hampered by delocalization errors, especially for correlated systems such as transition-metal complexes. Two complementary strategies have been developed to reduce delocalization error: eliminating the global curvature with change in charge, and applying a linear response Hubbard U as a measure of local curvature at a metal center at fixed charge in a DFT+U framework. We investigate the relationship between the two delocalization error measures as the ligand field strength is varied with the number of strong-field ligands in a series of heteroleptic complexes or by geometrically constraining the metal-ligand bond length in homoleptic octahedral complexes. We show that across these sets of complexes an inverse relationship generally exists between global and local curvatures. We find that effects of ligand substitution on both measures of delocalization are typically additive, but the quantities seldom coincide.

Gelatin from leather waste to tough biodegradable packaging film: One valuable recycling solution for waste gelatin from leather industry

Large amount of gelatin can be extracted from the solid waste in leather industry. The advanced application of such gelatin is always desired by the leather industry, but remains challenging. Considering the urgent requirement of biodegradable plastic film, in this study, the gelatin extracted from waste skin scrap in the leather industry was used to fabricate a waste gelatin-based film with a high gelatin content, excellent mechanical performance, and autonomous biodegradability in natural soil. The film was prepared by introducing covalent bonds and metal–ligand bonds to the gelatin matrix. These covalent bonds, metal–ligand bonds, and inherent hydrogen bonds in the gelatin matrix serve as multiple sacrificial bonds for effective energy dissipation giving the waste gelatin-based film excellent mechanical parameters with the highest fracture stress of ≈ 32 MPa, maximum fracture strain of ≈1.25 mm/mm, and a high Young’s modulus of ≈ 471 MPa, which are significantly higher than those of the original gelatin film (fracture stress ≈ 4 MPa, fracture strain ≈ 0.70 mm/mm, and Young’s modulus ≈ 22 MPa). Owing to the water resistance of covalent bonds and metal–ligand bonds existed in gelatin matrix, the gelatin film possesses good water resistance. Additionally, after use, the fabricated film can completely biodegrade in natural soil in approximately 7 weeks. This strategy not only provides a valuable recycling solution for the gelatin from the unwelcome solid waste of the leather industry, but it also broadens the range of ecofriendly and cost effective biodegradable films available.

Thermo-Induced Fluorochromism in Two AIE Zinc Complexes: A Deep Insight into the Structure-Property Relationship.

Solid-state emitters exhibiting mechano-fluorochromic or thermo-fluorochromic responses represent the foundation of smart tools for novel technological applications. Among fluorochromic (FC) materials, solid-state emissive coordination complexes offer a variety of fluorescence responses related to the dynamic of noncovalent metal-ligand coordination bonds. Relevant FC behaviour can result from the targeted choice of metal cation and ligands. Herein, we report the synthesis and characterization of two different colour emitters consisting of zinc complexes obtained from N,O bidentate ligands with different electron-withdrawing substituents. The two complexes are blue and orange solid-state fluorophores, respectively, highly responsive to thermal and mechanical stress. These emitters show a very different photoluminescent (PL) pattern as recorded before and after the annealing treatment. Through X-ray structural analysis combined with thermal analysis, infrared (IR) spectroscopy, PL, and DFT simulation we provide a comprehensive analysis of the structural feature involved in the fluorochromic response. Notably, we were able to correlate the on-off thermo-fluorochromism of the complexes with the structural rearrangement at the zinc coordination core.

X-ray Electron Density Study of the Chemical Bonding Origin of Glass Formation in Metal-Organic Frameworks.

Glass‐forming metal–organic frameworks (MOFs) have novel applications, but the origin of their peculiar melting behavior is unclear. Here, we report synchrotron X‐ray diffraction electron densities of two zeolitic imidazolate frameworks (ZIFs), the glass‐forming Zn‐ZIF‐zni and the isostructural thermally decomposing Co‐ZIF‐zni. Electron density analysis shows that the Zn−N bonds are more ionic than the Co−N bonds, which have distinct covalent features. Variable‐temperature Raman spectra reveal the onset of significant imidazolate bond weakening in Co‐ZIF‐zni above 673 K. Melting can be controlled by tuning the metal–ligand and imidazole bonding strength as shown from thermal analysis of nine solid‐solution Co x Zn1−x ‐ZIF‐zni (x=0.3 to 0.003) MOFs, and a mere 4 % Co‐doping into Zn‐ZIF‐zni results in thermal decomposition instead of melting. The present findings demonstrate the key role of the metal–ligand bonds and imidazolate bonds in controlling the delicate balance between melting and decomposition processes in this class of ZIF compounds.

X‐ray Electron Density Study of the Chemical Bonding Origin of Glass Formation in Metal–Organic Frameworks**

AbstractGlass‐forming metal–organic frameworks (MOFs) have novel applications, but the origin of their peculiar melting behavior is unclear. Here, we report synchrotron X‐ray diffraction electron densities of two zeolitic imidazolate frameworks (ZIFs), the glass‐forming Zn‐ZIF‐zni and the isostructural thermally decomposing Co‐ZIF‐zni. Electron density analysis shows that the Zn−N bonds are more ionic than the Co−N bonds, which have distinct covalent features. Variable‐temperature Raman spectra reveal the onset of significant imidazolate bond weakening in Co‐ZIF‐zni above 673 K. Melting can be controlled by tuning the metal–ligand and imidazole bonding strength as shown from thermal analysis of nine solid‐solution CoxZn1−x‐ZIF‐zni (x=0.3 to 0.003) MOFs, and a mere 4 % Co‐doping into Zn‐ZIF‐zni results in thermal decomposition instead of melting. The present findings demonstrate the key role of the metal–ligand bonds and imidazolate bonds in controlling the delicate balance between melting and decomposition processes in this class of ZIF compounds.

Chiral Metal-Organic Frameworks.

In the past two decades, metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) assembled from metal ions or clusters and organic linkers via metal-ligand coordination bonds have captivated significant scientific interest on account of their high crystallinity, exceptional porosity, and tunable pore size, high modularity, and diverse functionality. The opportunity to achieve functional porous materials by design with promising properties, unattainable for solid-state materials in general, distinguishes MOFs from other classes of materials, in particular, traditional porous materials such as activated carbon, silica, and zeolites, thereby leading to complementary properties. Scientists have conducted intense research in the production of chiral MOF (CMOF) materials for specific applications including but not limited to chiral recognition, separation, and catalysis since the discovery of the first functional CMOF (i.e., d- or l-POST-1). At present, CMOFs have become interdisciplinary between chirality chemistry, coordination chemistry, and material chemistry, which involve in many subjects including chemistry, physics, optics, medicine, pharmacology, biology, crystal engineering, environmental science, etc. In this review, we will systematically summarize the recent progress of CMOFs regarding design strategies, synthetic approaches, and cutting-edge applications. In particular, we will highlight the successful implementation of CMOFs in asymmetric catalysis, enantioselective separation, enantioselective recognition, and sensing. We envision that this review will provide readers a good understanding of CMOF chemistry and, more importantly, facilitate research endeavors for the rational design of multifunctional CMOFs and their industrial implementation.

Optical and spectroscopic behavior of Eu3+ doped heavy metal phosphate glasses

Europium (Eu3+)-doped heavy metal phosphate glass systems have been synthesized by melt-quench technique. Optical as well as spectroscopic characteristics are analyzed in detail. X-ray diffraction (XRD) pattern of synthesized samples confirms their glassy nature. Optical band-gap (Eg) for the synthesized samples has been calculated using different methods. Refractive index and other parameters such as dielectric constant, molar refraction, reflection losses, and polarizability have also been determined for prepared glass samples. Photoluminescence properties are examined from the excitation as well as emission spectra and results revealed prominent emissions in red region. The optical band gap for the prepared samples has been evaluated using three different methods viz. Tauc's method, absorption spectrum fitting (ASF) method and differential absorption spectrum fitting (DASF) method. Similar results have been obtained by all the three methods. Judd-Ofelt (J-O) theory has been employed to emission bands of Eu3+ ions for determination of phenomenological intensity parameters i.e. Ω2,Ω4 and Ω6, radiative rate, branching ratio and radiative lifetime. J-O analysis revealed that the large value of Ω2 parameter indicated high covalency between metal-ligand bonds along with the distortion among symmetric sites of Eu3+. Branching ratio (βR) is greater than 0.5 for 5D0→7F2 (transition) indicating their prospective for usefulness in laser-based applications. High branching ratio values and color purity suggests the potential use of glass samples for red laser applications.

Synthesis, characterization, spectral studies and evaluation of noncovalent interactions in co-crystal of μ-oxobridged polymeric copper(II) complex derived from pyrazolone by theoretical studies

The new co-crystal of μ-oxobridged polymeric copper(II) complex {[0.714(9) CuII2(HL-CH2OH)2].[0.286(9) CuII2(HL-CHO2H)2]}n with Schiff base ligand derived from pyrazolone, (HL = 4-[(2-hydroxy-3-methoxybenzylidene)amino-1,5-dimethyl-2-phenyl-1H-pyrazol-3-one), was synthesized and characterized by spectral and analytical techniques. The optimal and unexpected quality structures of the copper (II) polymer complex were determined using X-ray. Crystals are monoclinic, a = 7.8472(2), b = 12.5860(3), c = 17.0842(3) Å, β = 97.140(2)° and space group P21/c. The complexes are 4 + 1 coordinated, with four standard coordination distances in a square-planar geometry and a fifth, longer, secondary coordination distance at the square pyramid's apex. The basic complexes have a Ci-symmetrical Cu2L2 composition and extend into polymeric chains along the x crystallographic axis due to the longer contact. The electrochemical properties of complex in DMSO displayed the electronic effects on the groups on redox potential. The density functional theory by B3LYP method with (6-311 G) basis set was used to optimize the molecular geometry of the copper complex. Following the optimized DFT, FT-IR spectroscopy was used to determine geometric characteristics such as bond length and bond angle, which were then compared to experimental data. The intermolecular interactions were characterized using X-ray crystal structure analysis, HS (Hirshfeld surface analysis) and FPs (fingerprint diagrams). In addition, QTAIM (Quantum Theory of Atoms in Molecules) analysis was studied to determine the type of bonds and the energies of the metal-ligand bond. The adsorption process of complexes (1) and (2) in the aqueous phase on graphene oxide (GO) was calculated by the Monte Carlo adsorption locator. The results showed complex (2) had a high binding efficiency on GO compared with complex (1). The molecular docking study was also carried out for the copper complex to find its binding affinity to protein B-cell lymphorna (BCL-2, PDB ID: 4LXD).

Evidence for Participation of 4f and 5d Orbitals in Lanthanide Metal–Ligand Bonding and That Y(III) Has Less of This Complex-Stabilizing Ability. A Thermodynamic, Spectroscopic, and DFT Study of Their Complexation by the Nitrogen Donor Ligand TPEN

The formation constants (log K1) of lanthanide(III) (Ln) ions [except for Pm(III)] and the Y(III) cation have been measured with the ligand TPEN (N,N,N',N'-tetra-2-picolylethylenediamine). These log K1 values show a typical variation with ionic radius, with a local maximum at Sm(III) and a local minimum at Gd(III), with an overall increase in log K1 from La(III) to Lu(III) as the ionic radius decreases. The log K1 for the Y(III)/TPEN complex is much lower than expected from its ionic radius, while the literature log K1 for Am(III) is much higher. The latter effect is thought to be due to greater covalence in the M-L (metal-ligand) bond than for Ln(III) ions: the low log K1 for Y(III) is interpreted as being due to lower covalence. The f → f transitions in the Nd(III) and Pr(III) complexes were examined for effects that might indicate the participation of f orbitals in M-L bonding. The intensity of the f → f transitions in the Nd(III)/TPEN complex was greatly increased compared to that of the Nd3+ aqua ion, which appeared to be due to additional sharp peaks, possibly parity forbidden transitions where parity rules were broken by covalence in the M-L bond. The Pr(III)/TPEN complex showed that all of the f → f transitions shifted to longer wavelengths by some 5 nm, with modest increases in intensity. The effects seen in the f → f transitions of Nd(III) and Pr(III) with TPEN with its six nitrogen donors were present to a much smaller extent in the EDTA and other complexes with fewer nitrogen donors. The changes in the f → f transitions of the TPEN complexes of Er(III) and Ho(III) were small, suggesting a smaller contribution of f orbitals to M-L bonding in these heavier Ln(III) ions. The intense Laporte allowed f → d transitions in Ce(III) complexes show large shifts to longer wavelengths as complexes of, for example, EDTA with increasing numbers of nitrogen donors, suggesting the participation of both f and d orbitals, or either, in M-L bonding. The nature of M-L bonding in M(III)/TPEN complexes was further investigated via density functional theory calculations.

Analysis of the Geometric and Electronic Structure of Spin-Coupled Iron-Sulfur Dimers with Broken-Symmetry DFT: Implications for FeMoco.

The open-shell electronic structure of iron–sulfur clusters presents considerable challenges to quantum chemistry, with the complex iron–molybdenum cofactor (FeMoco) of nitrogenase representing perhaps the ultimate challenge for either wavefunction or density functional theory. While broken-symmetry density functional theory has seen some success in describing the electronic structure of such cofactors, there is a large exchange–correlation functional dependence in calculations that is not fully understood. In this work, we present a geometric benchmarking test set, FeMoD11, of synthetic spin-coupled Fe–Fe and Mo–Fe dimers, with relevance to the molecular and electronic structure of the Mo-nitrogenase FeMo cofactor. The reference data consists of high-resolution crystal structures of metal dimer compounds in different oxidation states. Multiple density functionals are tested on their ability to reproduce the local geometry, specifically the Fe–Fe/Mo–Fe distance, for both antiferromagnetically coupled and ferromagnetically coupled dimers via the broken-symmetry approach. The metal–metal distance is revealed not only to be highly sensitive to the amount of exact exchange in the functional but also to the specific exchange and correlation functionals. For the antiferromagnetically coupled dimers, the calculated metal–metal distance correlates well with the covalency of the bridging metal–ligand bonds, as revealed via the corresponding orbital analysis, Hirshfeld S/Fe charges, and Fe–S Mayer bond order. Superexchange via bridging ligands is expected to be the dominant interaction in these dimers, and our results suggest that functionals that predict accurate Fe–Fe and Mo–Fe distances describe the overall metal–ligand covalency more accurately and in turn the superexchange of these systems. The best performing density functionals of the 16 tested for the FeMoD11 test set are revealed to be either the nonhybrid functionals r2SCAN and B97-D3 or hybrid functionals with 10–15% exact exchange: TPSSh and B3LYP*. These same four functionals are furthermore found to reproduce the high-resolution X-ray structure of FeMoco well according to quantum mechanics/molecular mechanics (QM/MM) calculations. Almost all nonhybrid functionals systematically underestimate Fe–Fe and Mo–Fe distances (with r2SCAN and B97-D3 being the sole exceptions), while hybrid functionals with >15% exact exchange (including range-separated hybrid functionals) overestimate them. The results overall suggest r2SCAN, B97-D3, TPSSh, and B3LYP* as accurate density functionals for describing the electronic structure of iron–sulfur clusters in general, including the complex FeMoco cluster of nitrogenase.

A Strong and Rigid Coordination Adaptable Network that Can Be Reprocessed and Recycled at Mild Conditions

A Strong and Rigid Coordination Adaptable Network that Can Be Reprocessed and Recycled at Mild Conditions

A terminal neptunium(V)-mono(oxo) complex.

Neptunium was the first actinide element to be artificially synthesized, yet, compared with its more famous neighbours uranium and plutonium, is less conspicuously studied. Most neptunium chemistry involves the neptunyl di(oxo)-motif, and transuranic compounds with one metal-ligand multiple bond are rare, being found only in extended-structure oxide, fluoride or oxyhalide materials. These combinations stabilize the required high oxidation states, which are otherwise challenging to realize for transuranic ions. Here we report the synthesis, isolation and characterization of a stable molecular neptunium(V)-mono(oxo) triamidoamine complex. We describe a strong Np≡O triple bond with dominant 5f-orbital contributions and σu > πu energy ordering, akin to terminal uranium-nitrides and di(oxo)-actinyls, but not the uranium-mono(oxo) triple bonds or other actinide multiple bonds reported so far. This work demonstrates that molecular high-oxidation-state transuranic complexes with a single metal-ligand bond can be stabilized and studied in isolation.

Revisiting the Fundamental Nature of Metal-Ligand Bonding: An Impartial and Automated Fitting Procedure for Angular Overlap Model Parameters.

The properties and reactivities of transition metal complexes are often discussed in terms of Ligand Field Theory (LFT), and with ab initio LFT a direct connection to quantum chemical wavefunctions was recently established. The Angular Overlap Model (AOM) is a widely used, ligand‐specific parameterization scheme of the ligand field splitting that has, however, been restricted by the availability and resolution of experimental data. Using ab initio LFT, we present here a generalised, symmetry‐independent and automated fitting procedure for AOM parameters that is even applicable to formally underdetermined or experimentally inaccessible systems. This method allows quantitative evaluations of assumptions commonly made in AOM applications, for example, transferability or the relative magnitudes of AOM parameters, and the response of the ligand field to structural or electronic changes. A two‐dimensional spectrochemical series of tetrahedral halido metalates ([MIIX4]2−, M=Mn−Cu) served as a case study. A previously unknown linear relationship between the halide ligands’ chemical hardness and their AOM parameters was found. The impartial and automated procedure for identifying AOM parameters introduced here can be used to systematically improve our understanding of ligand–metal interactions in coordination complexes.