Search

The bis(diethyl ether) and 1,2-dimethoxyethane (dme) adducts of molybdenum(IV) chloride and tungsten(IV) chloride are valuable starting materials for a variety of synthetic inorganic and organometallic reactions. Despite the broad utility and extensive use of these 6-coordinate complexes, their syntheses remain unoptimized, and their characterization incomplete after more than three decades. While exploring the ligand exchange behaviour of

The valence electronic structures of two dinuclear alkyl compounds containing sigma(2)pi(4) triple bonds between group 6 metals, viz., M-2(CH2CMe3)(6) (M = Mo, W), have been investigated using a combination of molecular orbital theory and variable photon energy photoelectron spectroscopy (PES). Density functional theory (DFT) calculations using PBEO-dDsC functionals, which include dispersion forces, have been performed on the title compounds as well as several closely related M2X6 (M = Mo, W) compounds. The DFT calculations on the dinuclear neopentyl complexes are in excellent agreement with the solid-state structures, measured PES spectra, and ultraviolet-visible (UV-vis) spectra. The top nine filled orbitals in both cases are associated with M-M and M-C bonding. The orbital energy pattern conforms to that anticipated for a D-3d (staggered) M2C6 skeleton. For both Mo and W, the highest-energy pair of orbitals are of e(u) (pi) symmetry, followed by one of a(1g) (sigma) symmetry, and comprise the metal-metal triple bond. The orbital energies are higher for W than for Mo, and the separation between the pi and sigma orbitals is greater for W, reflecting a greater relativistic stabilization of the tungsten 6s orbital compared to that of the Mo 5s orbital. The spin-orbit splitting in the pi ionization of W-2(CH2CMe3)(6) has been resolved and successfully modeled. A graphical comparison of valence orbital energies for Mo2X6, where X = CH2CMe3, NMe2, and OCH2CMe3, shows how the Mo-Mo pi and sigma levels vary as a function of the ligand set.

"MoCl3(dme)" (dme = 1,2-dimethoxyethane) is an important precursor for midvalent molybdenum chemistry, particularly for triply Mo-Mo bonded compounds of the type Mo2X6 (X = bulky anionic ligand). However, its exact structural identity has been obscure for more than 50 years. In search of a convenient, large-scale synthesis, we have found that trans-MoCl4(Et2O)2 dissolved in dme can be cleanly reduced with dimethylphenylsilane, Me2PhSiH, to provide khaki Mo2Cl6(dme)2 in ∼90% yield. If the reduction is performed on a small scale, single crystals suitable for X-ray crystallography can be obtained. Two different crystal morphologies were identified, each belonging to the P21/n space group, but with slightly different unit cell constants. The refined structure of each form is an edge-shared bioctahedron with overall Ci symmetry and metal-metal separations on the order of 2.8 A. The bulk material is diamagnetic as determined by both the Gouy method and SQUID magnetometry. Density functional theory calculations suggest a σ2π2δ*2 ground state for the dimer with the diamagnetism arising from a singlet diradical "broken symmetry" electronic configuration. In addition to a definitive structural assignment for "MoCl3(dme)", this work highlights the utility of organosilanes as easy to handle, alternative reductants for inorganic synthesis.

The synthesis and investigation of multimetallic complexes bearing metal–metal bonds have been significantly advanced over the last 30 years with exciting applications across many fields. While the...

First reported in 1930, MoCl3O(Et2O)2 is a by-product of the reductive synthesis of MoCl4(OEt2)2 from MoCl5. We report herein the X-ray crystal structure and Hirshfeld surface characteristics of mer-MoCl3O(Et2O)2, or [MoCl3O(C4H10O)2]. The compound crystallizes in the orthorhombic space group P212121. The molybdenyl (Mo=O) bond length is 1.694 (3) Å and the cis- and trans-Mo—O distances are 2.157 (3) and 2.304 (3) Å, respectively. Intermolecular Mo=O...H bonding is present in the lattice, with the shortest distance being 2.572 Å.

Olefin metathesis is a broadly employed reaction with applications that range from fine chemicals to materials and petrochemicals. The design and investigation of olefin metathesis catalysts have b...

"MoCl

We present the full investigation of the atomic layer deposition (ALD) of a mixed rhenium–aluminum oxide, namely ReAl2O3CH3, a material with tunable resistance, comprising the building unit of cond...

The thermal behavior of Tc2O7 has been investigated by single-crystal X-ray diffraction of the solid state over a range of 80–280 K and by ab initio molecular dynamics (MD) simulations. The thermal expansion coefficient of the solid was experimentally determined to be 189 × 10–6 A3 K–1 at 280 K. The simulations accurately reproduce the experimentally determined crystal structures and thermal expansion within a few percent. The experimental melting point and vapor pressure for Tc2O7 are unusually high and low, respectively, in comparison to similar molecular solids. Through investigating the structure and the motion of the solid across a range of temperatures, we provide insights into the thermal behavior of Tc2O7.

The true identity of the diethyl ether adduct of tungsten(IV) chloride, WCl4(Et2O) x , has been in doubt since 1985. Initially postulated as the bis-adduct, WCl4(Et2O)2, questions arose when elemental analyses were more in line with a mono-ether adduct, viz. WCl4(Et2O). It was proposed that this was due to the thermal instability of the bis-adduct. Here, we report the room-temperature X-ray crystal structure and Hirshfeld surface characteristics of trans-tetrachloridobis(diethyl ether)tungsten(IV), trans-WCl4(Et2O)2 or trans-[WCl4(C4H10O)2]. The compound crystallizes, with half of the molecule in the asymmetric unit, in the centrosymmetric space group P21/n. The W—O distance is 2.070 (2) Å, while the W—Cl distances are 2.3586 (10) and 2.3554 (10) Å.

The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C-C and C-O bond cleavage, carbon-carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.

The oxidation of TcO2 with O2 in the presence of H2O at 250 °C was investigated; Tc2O7 and a red product were observed after the reaction. UV/Visible spectroscopy and a cerium titration on the “red” product are consistent with the presence of a reduced Tc species. It is proposed that the “red” product was formed by the hydrolysis of Tc2O7 to HTcO4 followed by the reduction of HTcO4. The reaction products of TcO2 with O2/H2O at 250 °C were also studied by mass spectrometry. The main dinuclear species were Tc2O7 and Tc2O5, while the main mononuclear species were TcO3 and HTcO4. The compound HTcO4 and associated hydrates were simulated to identify candidate structures for the “red” product; however, neither the acid nor any related hydrates exhibited the UV/Visible signature of the “red” product.

We show that a Tc2O5 molecular species is the likely identity of an unknown volatile oxide which has remained uncharacterized for 50+ years. Exploration of this molecule's absorption spectra and intermolecular self-interactions provides a close match to experimental data and an explanation for volatility and resistance to crystallization.

Technetium is the workhorse of the radiopharmaceutical imaging agents; it is also an important byproduct of the nuclear industry. Studies of the chemistry of Tc in strong acids are relevant to nuclear applications, environmental remediation and fundamental chemistry. Nitric acid is used at the industrial level for spent fuel reprocessing while H2SO4 and HClO4 are used at the laboratory scale for Tc separation from Mo or U. During reprocessing activities, radiolysis products from nitric acid (e.g., H2O2) and from organics extracting agents (e.g., alcohols) are formed and these might interact with Tc(VII). An understanding of Tc(VII) chemistry in the presence of H2O2 and/or organics in acidic solution is important to predict its behavior in separation processes. Concerning environmental remediation, sulfur has been proposed to immobilize “Tc2S7”. Technetium heptasulfide can be obtained from the reaction of Tc(VII) with H2S gas in acidic solutions. A better understanding of “Tc2S7” formation could give information to predict its formation and behavior in the environment. Under oxidizing conditions, the aqueous chemistry of Tc(VII) is dominated by TcO4−. In high acid concentrations, pertechnetic acid can be formed and control the reactivity of Tc. Speciation data on Tc(VII) in concentrated acids are still parse, and the structure and reactivity of pertechnetic acid are unknown. Here, the speciation of Tc(VII) in sulfuric, nitric and perchloric acids and its reactivity with H2O2, methanol and H2S is reviewed. Experimental results and density functional calculations show the formation of TcO3(OH)(H2O)2 in concentrated H2SO4 and HClO4. In 12–13 M H2SO4, Tc(V) species and Tc(IV) polymeric species were respectively detected in the presence of methanol and H2S. Finally, peroxo pertechnetic acid was identified in nitric and sulfuric acid in the presence of H2O2.

Quadruply bonded rhenium(III) dimers with the stoichiometry Re2L4F2 (1, L = hexahydro-2H-pyrimido[1,2a]pyrimidinate (hpp-); 2, L = diphenyl formamidinate (dpf-)) were prepared from the solid-state melt reactions (SSMRs) between (NH4)2[Re2F8]·2H2O and HL. Those compounds were characterized in the solid state by single-crystal X-ray diffraction and in solution by UV-visible spectroscopy and cyclic voltammetry. The compound [Re2(hpp)4F2]PF6 (3) was prepared from the one-electron oxidation of Re2(hpp)4F2 with [Cp2Fe]PF6. Compounds 1-3 are isostructural with the corresponding chloro derivatives. In solution, compound 1 undergoes two one-electron oxidations. Comparison with its higher halogen homologues reveals that Re2(hpp)4F2 (1) is more easily oxidized than its chloro and bromo analogues.

The molecular and electronic structures of the group 7 heptoxides were investigated by computational methods as both isolated molecules and in the solid-state. The metal–oxygen–metal bending angle of the single molecule increased with increasing atomic number, with Re2O7 preferring a linear structure. Natural bond orbital and localized orbital bonding analyses indicate that there is a three-center covalent bond between the metal atoms and the bridging oxygen, and the increasing ionic character of the bonds favors larger bond angles. The calculations accurately reproduce the experimental crystal structures within a few percent. Analysis of the band structures and density of states shows similar bonding for all of the solid-state heptoxides, including the presence of the three-center covalent bond. DFT+U simulations show that PBE-D3 underpredicts the band gap by ∼0.2 eV due to an undercorrelation of the metal d conducting states. Homologue and compression studies show that Re2O7 adopts a polymeric structure...

The radioactive nature of technetium is discussed using a combination of introductory nuclear physics concepts and empirical trends observed in the chart of the nuclides and the periodic table of the elements. Trends such as the enhanced stability of nucleon pairs, magic numbers, and Mattauch’s rule are described. The concepts of nuclear binding energies and the nuclear shell model are introduced and used to explain the relative stability of radionuclides and, in particular, the isotopes of technetium.

The photochemical behavior of [Tc 2 Cl 8 ] 2− was investigated in acetonitrile. The speciation of Tc before and after irradiation at 254 nm was performed by UV–vis spectroscopy. Upon irradiation at 254 nm, [Tc 2 Cl 8 ] 2− was unstable, the scission of the Tc Tc unit occurred and the complex [TcCl 4 (CH 3 CN) 2 ] was identified. The disappearance rate of [M 2 Cl 8 ] 2− (M = Tc, Re) under irradiation has been measured and was ∼7.5 time faster for Tc than for Re.

The (NH4)2ReX6 (X = F, Cl, Br, I) salts have been investigated by x-ray absorption fine structure spectroscopy. The Re-F distance determined by EXAFS in (NH4)2ReF6 (i.e., 1.95 A) is in good agreement with the one determined by single crystal x-ray diffraction in A2ReF6 salts (A = K, Rb, Cs). The XANES studies of (NH4)2ReX6 (X = F, Cl, Br, I) indicates that the positions of the absorption edge and of the white line are shifted to higher energy when moving from I to F. These shifts have been explained in terms of the crystal field splitting parameter and covalent charge carried by the Re atoms. Calculations of the XANES spectra of the ReX62− (X = F, Cl, Br, I) anions at the Re-L3 edge have been performed and the calculated shifts and intensity of the white lines reproduce well the experimental observations.

The thermal behavior of Tc

The [NH4]4NpF8 salt was prepared from the solid-state reaction of NpO2 with NH4HF2 and characterized by powder X-ray diffraction and X-ray absorption fine structure spectroscopy. The diffraction results confirm the compound to be isostructural to [NH4]4UF8 with the following lattice parameter (a = 13.054(4) A, b = 6.681(2) A, c = 13.676(5) A, β = 121.14 A). For the first time, a neptunium fluoride complex has been characterized by XAFS spectroscopy. The energy position of the white line and inflection of the XANES spectra of [NH4]4NpF8 are consistent with the presence of Np(IV). Adjustment of the EXAFS spectra indicates that the coordination number (7.4 ± 1.5) and the average Np–F distance (2.26(1) A) are consistent with the presence of the NpF8 dodecahedron. The average Np–F distance is ∼0.02 A shorter than the U–F distance in [NH4]4UF8 and is a result of the actinide contraction.

Thin hafnium oxide films were prepared by atomic layer deposition using a carbon-free precursor, tetrakis(tetrahydroborato)hafnium [Hf(BH4)4], and H2O. Film growth was studied using an in situ quartz crystal microbalance and Fourier transform infrared spectroscopy measurements. Self-limiting growth was observed between 100 and 175 °C, but the thermal decomposition of the Hf precursor occurred at higher temperatures. The film properties were investigated using x-ray photoelectron spectroscopy, x-ray reflectivity, x-ray diffraction, ellipsometry, time-of-flight secondary ion mass spectrometry, and x-ray absorption spectroscopy. The as-deposited films were found to consist of an amorphous mixture of HfO2 and B2O3, and had a lower density and lower refractive index compared to pure HfO2 thin films. Annealing the films to >750 °C yielded crystalline monoclinic HfO2 with a density of 9 g/cm3 and a refractive index of 2.10.

Investigations of solvothermal reactions with pertechnetate, [TcO4]−, have produced four new low-valent Tc extended metal atom chain (EMAC) complexes with multiple metal-metal bonds. The thermal reaction of potassium pertechnetate with glacial acetic acid, under in-situ hydrogen production from the decomposition of sodium borohydride, yields purple elongated crystals determined to be tetraacetato ditechnetium (II,III) acetate (1), [Tc2(O2CCH3)4(μ-O2CCH3]n. Compound 1 was characterized by single crystal X-ray diffraction and its magnetic properties recorded between 10 and 300 K. Single crystal X-ray diffraction analysis of 1 indicates the Tc2+5 tetraacetato unit is linked by the carboxylate oxygens of the bridging acetate, similar to known chloride and bromide linkage polymers of composition [Tc2(μ-O2CCH3)4Cl]n, and [Tc2(μ-O2CCH3)4Br]n. Using the same synthetic method three similar compounds have been synthesized from propionic, benzoic and pivalic acid.

Dimeric and monomeric supported single-site Fe(II) pre-catalysts on SiO2 have been prepared via organometallic grafting and characterized with advanced spectroscopic techniques. Manipulation of the surface hydroxyl concentration on the support influences monomer/dimer formation. While both pre-catalysts are highly active in liquid-phase hydrogenation, the dimeric pre-catalyst is ∼3× faster than the monomer. Preliminary XAS experiments on the H2-activated samples suggest the active species are isolated Fe(II) sites.

Vitrification of sodium pertechnetate into borosilicate glass was performed in air at 1100 °C. A glass with a composition similar to the one developed for vitrification of the low activity waste at the Hanford site was used. A red volatile species was observed above 600 °C. The extended X-ray absorption fine structure results indicate the environment of the absorbing Tc atom consists of 2.9(6) O atoms at 1.73(2) A, 2.2(4) O atoms at 2.02(2) A, and 0.8(2) O atoms at 2.18(2) A. The results are consistent with the presence of a mononuclear species with a structure closely related to TcO3(OH)(H2O)2.

Two new technetium iodide compounds, Tc 2 (O 2 CCH 3 ) 4 I and K[Tc 8 (μ-I) 8 I 4 ]I, were synthesized in an autoclave from the reaction of KTcO 4 in glacial acetic acid with hydroiodic acid and/or alkali metal iodide salts at 210 °C under 60–70 atm hydrogen pressure. The structures of Tc 2 (O 2 CCH 3 ) 4 I and K[Tc 8 (μ-I) 8 I 4 ]I were solved by single-crystal X-ray diffraction. The compound Tc 2 (O 2 CCH 3 ) 4 I crystallizes in the monoclinic space group C2/m with a = 7.1194(6) A, b = 14.5851(13) A, c = 7.1586(6) A, and β = 110.9540(10)°. The structure of Tc 2 (O 2 CCH 3 ) 4 I consists of infinite chains of Tc 2 (O 2 CCH 3 ) 4 + units linked by bridging iodides, an arrangement similar to the one found in Tc 2 (O 2 CCH 3 ) 4 X (X = Cl, Br). The Tc Tc separation in Tc 2 (O 2 CCH 3 ) 4 I (i.e., 2.1146(4) A) is consistent with the presence of a Tc Tc bond of order 3.5. Magnetic susceptibility measurements reveal Tc 2 (O 2 CCH 3 ) 4 I to be paramagnetic ( μ eff = 1.84 B.M.) and support the electronic configuration σ 2 π 2 δ 2 δ ∗1 for the Tc 2 5+ unit in the compound. The compound K[Tc 8 (μ-I) 8 I 4 ]I crystallizes in the monoclinic space group P2 1 /n with a = 8.0018(5) A, b = 14.5125(10) A, c = 13.1948(9) A, and β = 102.3090(10)°, and is the first octanuclear technetium iodide cluster to be reported. The Tc Tc separations in the [Tc 8 (μ-I) 8 I 4 ] cluster (i.e., 2.164(3) A, 2.5308(8) A and 2.72(3) A) suggest the presence of Tc Tc triple bonds, Tc Tc double bonds and Tc Tc single bonds.

The molecular and electronic structures of the group 7 heptoxides were investigated by computational methods as both isolated molecules and in the solid-state. The metal-oxygen-metal bending angle of the single molecule increased with increasing atomic number, with Re

The reaction of Tc(+7) with H2O2 has been studied in H2SO4 and the speciation of technetium performed by UV–visible and 99-Tc NMR spectroscopy. UV–visible measurements show that for H2SO4 ≥ 9 M and H2O2 = 0.17 M, TcO3(OH)(H2O)2 reacts immediately and blue solutions are obtained, while no reaction occurs for H2SO4

The molecular and electronic structure, as well as the electronic absorption spectrum of Tc2(O2CCH3)2Cl4 were studied by multiconfigurational quantum chemical methods. The computed ground state geometry is in excellent agreement with the experimental structure determined by single crystal X-ray diffraction (SCXRD). The total bond order (i.e., 3.20) is consistent with the presence of a moderately strong quadruple Tc–Tc bond and is the largest bond order reported so far for a multiple Tc–Tc bonded complex. Effective bond order analysis indicates stronger p and d bonds for Tc2(O2CCH3)2Cl4 (i.e., 1.71 for p and 0.59 for d) than for Tc2Cl8 2� (i.e., 1.68 for p and 0.47 for d). The electronic absorption spectrum was recorded in

Technetium tetrachloride and β-TcCl3, synthesized from the reaction of Tc metal and Cl2(g) in sealed tubes, were characterized by X-ray absorption fine structure spectroscopy. Extended X-ray absorption fine structure (EXAFS) spectroscopy measurements are in good agreement with the X-ray diffraction structures of the two compounds. For TcCl4, the absorbing Tc atom is surrounded by Cl atoms at 2.34(2) A and Tc atoms at 3.66(4) A. For β-TcCl3, the absorbing Tc atom is surrounded by Cl atoms at 2.40(2) A and Tc atoms at 2.81(3), 3.66(4) and 5.71(6) A. EXAFS spectroscopy indicates that the TcCl4 and β-TcCl3 samples obtained by sealed tube reactions are single phase. The X-ray absorption near edge structure spectra of TcCl4 and β-TcCl3 were recorded; the positions of the Tc K-edges of β-TcCl3 (21,050.5 eV) and TcCl4 (21,053.0 eV) are compared to the ones measured for α-TcCl3 (21,051.0 eV) and TcCl2 (21,048.8 eV). A correlation between the positions of the Tc K-edges and the oxidation state of the Tc atom in technetium binary chlorides was determined.

The reaction of Tc(+7) with H2O2 has been studied in HNO3 and the speciation of technetium was performed by UV/Vis spectroscopic analysis. UV/Vis measurements show that for HNO3 concentrations of more than 7 M and H2O2 concentrations of 4.25 M, Tc(+7) reacts immediately and red solutions are obtained, whereas no reaction occurs at H2O2 concentrations of 0.1 M. The UV/Vis spectra of the red solutions exhibit a single band centered at 500 nm that is attributed to a Tc(+7) peroxo species. The nature of the Tc(+7) peroxo species was investigated by computational methods. The structures and electronic spectra of the putative species Tc(O2)4–, TcO(O2)3–, TcO3(O2)–, TcO(O2)2(H2O)(OH), and TcO2(O2)(H2O)2(OH) were studied by first principles methods. Calculations show these species to be stable; their calculated spectra were compared to the experimental spectrum and the results are consistent with the presence of diperoxo pertechnetic acid, i.e., TcO(O2)2(H2O)(OH).

Tc2(μ-O2CCH3)4Cl2 is a key starting material for further explorations of dinuclear technetium(III) chemistry and is obtained in 70% yield from readily available starting materials via hydrothermal techniques. Its single crystal X-ray structure reveals the familiar paddle-wheel motif of four bridging acetate groups spanning a short Tc–Tc bond (2.1758(3) A), augmented by axial chlorides at a Tc–Cl separation of 2.5078(4) A. The Tc–Tc quadruple bond length is slightly shorter than the one found in the pivalate derivative, Tc2(O2CCMe3)4Cl2 (2.192(1) A), and slightly longer than found in [Tc2(O2CCH3)4](TcO4)2 (2.149(1) A), the only other structurally characterized members of the small family of Tc2(O2CR)4X2 dimers.

The speciation of Tc(+7) was performed in HClO4 and HNO3 by 99-Tc NMR, UV–Vis and XAFS spectroscopy. The speciation of Tc(+7) depends on the concentration and strength of the acid. Pertechnetic acid, HTcO4, forms above 8 M HClO4 while in concentrated HNO3, [TcO4]− is still the predominant species. EXAFS spectroscopy shows that the structure of HTcO4 in HClO4 is similar to the one in H2SO4. The reactivity of Tc(+7) was analyzed in the frame of the partial charge model. The partial charge calculated on the Tc atoms (ΔTc) indicates that HTcO4 (ΔTc = +057) is more electrophilic than [TcO4]− (ΔTc = +0.52). The difference in the oxidizing properties between [TcO4]− and HTcO4 is given from the reaction of these species with 12 M HCl(aq). In 13 M sulfuric acid HTcO4 is reduced to Tc(+5) while [TcO4]− is not reduced in 6 M H2SO4.

Dipotassium pentachloridooxidotechnetate, K2[TcOCl5], has been isolated as green single crystals by the dissolution of (NH4)TcO4 in 12 M HCl at 0 °C and careful precipitation with KCl. The structure of this compound was determined by single-crystal X-ray diffraction analysis [a = 13.0815(9), b = 9.8982(6), c = 6.7623(4) A; V = 875.605 A3, orthorhombic, Pnma, Z = 4] and compared with the corresponding molybdenum and rhenium analogues. The structure of K2[TcOCl5] was also investigated by density functional theory, and the results are in agreement with the crystallographic data. The oscillator strengths of the electronic transitions in the C4v complex anions [TcOCl5]2– and [TcOCl4]– were also calculated by using time-dependent density functional theory and compared with the experimental UV/Vis spectra.

Technetium is intimately linked with nuclear reactions. The ultraminute natural levels in the environment are due to the spontaneous fission of uranium isotopes. The discovery of technetium was born from accelerator reactions, and its use and presence in the modern world are directly due to nuclear reactors. While occupying a central location in the periodic table, the chemistry of technetium is poorly explored, especially when compared to its neighboring elements, i.e., molybdenum, ruthenium, and rhenium. This state of affairs, which is tied to the small number of laboratories equipped to work with the long-lived (99)Tc isotope, provides a remarkable opportunity to combine basic studies with applications for the nuclear fuel cycle. An example is given through examination of the technetium halide compounds. Binary metal halides represent some of the most fundamental of inorganic compounds. The synthesis of new technetium halides demonstrates trends with structure, coordination number, and speciation that can be utilized in the nuclear fuel cycle. Examples are provided for technetium-zirconium alloys as waste forms and the formation of reduced technetium species in separations.

The dinuclear rhenium(II) complex Re2Br4(PMe3)4 was prepared from the reduction of [Re2Br8](2-) with (n-Bu4N)BH4 in the presence of PMe3 in propanol. The complex was characterized by single-crystal X-ray diffraction (SCXRD) and UV-visible spectroscopy. It crystallizes in the monoclinic C2/c space group and is isostructural with its molybdenum and technetium analogues. The Re-Re distance (2.2521(3) Å) is slightly longer than the one in Re2Cl4(PMe3)4 (2.247(1) Å). The molecular and electronic structure of Re2X4(PMe3)4 (X = Cl, Br) were studied by multiconfigurational quantum chemical methods. The computed ground-state geometry is in excellent agreement with the experimental structure determined by SCXRD. The calculated total bond order (2.75) is consistent with the presence of an electron-rich triple bond and is similar to the one found for Re2Cl4(PMe3)4. The electronic absorption spectrum of Re2Br4(PMe3)4 was recorded in benzene and shows a series of low-intensity bands in the range 10 000-26 000 cm(-1). The absorption bands were assigned based on calculations of the excitation energies with the multireference wave functions followed by second-order perturbation theory using the CASSCF/CASPT2 method. Calculations predict that the lowest energy band corresponds to the δ* → σ* transition, while the next higher energy bands were attributed to the δ* → π*, δ → σ*, and δ → π* transitions.

A simple method for the high-yield preparation of (NH4)2[Re2F8]·2H2O has been developed that involves the reaction of (n-Bu4N)2[Re2Cl8] with molten ammonium bifluoride (NH4HF2). Using this method, the new salt [NH4]2[Re2F8]·2H2O was prepared in ∼90% yield. The product was characterized in solution by ultraviolet-visible light (UV-vis) and (19)F nuclear magnetic resonance ((19)F NMR) spectroscopies and in the solid-state by elemental analysis, powder X-ray diffraction (XRD), and infrared (IR) spectroscopy. Multiconfigurational CASSCF/CASPT2 quantum chemical calculations were performed to investigate the molecular and electronic structure, as well as the electronic absorption spectrum of the [Re2F8](2-) anion. The metal-metal bonding in the Re2(6+) unit was quantified in terms of effective bond order (EBO) and compared to that of its [Re2Cl8](2-) and [Re2Br8](2-) analogues.

The cesium salts of [Tc(2)X(8)](3-) (X = Cl, Br), the reduction product of (n-Bu(4)N)[TcOCl(4)] with (n-Bu(4)N)BH(4) in THF, and the product obtained from reaction of Tc(2)(O(2)CCH(3))(4)Cl(2) with HCl(g) at 300 °C have been characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy. For the [Tc(2)X(8)](3-) anions, the Tc-Tc separations found by EXAFS spectroscopy (2.12(2) Å for both X = Cl and Br) are in excellent agreement with those found by single-crystal X-ray diffraction (SCXRD) measurements (2.117[4] Å for X = Cl and 2.1265(1) Å for X = Br). The Tc-Tc separation found by EXAFS in these anions is slightly shorter than those found in the [Tc(2)X(8)](2-) anions (2.16(2) Å for X = Cl and Br). Spectroscopic and SCXRD characterization of the reduction product of (n-Bu(4)N)[TcOCl(4)] with (n-Bu(4)N)BH(4) are consistent with the presence of dinuclear species that are related to the [Tc(2)Cl(8)](n-) (n = 2, 3) anions. From these results, a new preparation of (n-Bu(4)N)(2)[Tc(2)Cl(8)] was developed. Finally, EXAFS characterization of the product obtained from reaction of Tc(2)(O(2)CCH(3))(4)Cl(2) with HCl(g) at 300 °C indicates the presence of amorphous α-TcCl(3). The Tc-Tc separation (i.e., 2.46(2) Å) measured in this compound is consistent with the presence of Tc═Tc double bonds in the [Tc(3)](9+) core.

A low-temperature fluoride route was utilized to synthesize neptunium mononitride, NpN. Through the development of this process, two new neptunium nitride species, NpN(2) and Np(2)N(3), were identified. The NpN(2) and Np(2)N(3) have crystal structures isomorphous to those of UN(2) and U(2)N(3), respectively. NpN(2) crystallizes in a face-centered cubic CaF(2)-type structure with a space group of Fm3m and a refined lattice parameter of 5.3236(1) Å. The Np(2)N(3) adopts the body-centered cubic Mn(2)O(3)-type structure with a space group of Ia3. Its refined lattice parameter is 10.6513(4) Å. The NpN synthesis at temperatures ≤900 °C using the fluoride route discussed here was also demonstrated. Previous computational studies of the neptunium nitride system have focused exclusively on the NpN phase because no evidence was reported experimentally on the presence of NpN(x) systems. Here, the crystal structures of NpN(2) and Np(2)N(3) are discussed for the first time, confirming the experimental results by density functional calculations (DFT). These DFT calculations were performed within the local-density approximation (LDA+U) and the generalized-gradient approximation (GGA+U) corrected with an effective Hubbard parameter to account for the strong on-site Coulomb repulsion between Np 5f electrons. The effects of the spin-orbit coupling in the GGA+U calculations have also been investigated for NpN(2) and NpN.

We are living in an era of unprecedented change in academic, industrial, and government-based research worldwide, and navigating these rough waters requires “all hands on deck”. Toward this end, Organometallics has assembled a panel of seventeen experts who share their thoughts on a variety of matters of importance to our field. In constituting this panel, an attempt was made to secure representation from a number of countries and career stages, as well as from industry. We were fortunate that so many busy experts could take the time to spend with us. The following pages constitute an edited transcript of the panel discussion held on August 29, 2011, which was structured around the 10 questions summarized in the side bar and repeated below.

Uranium mononitride (UN), sesquinitride (U2N3) and dinitride (UN2) were characterized by extended X-Ray absorption fine structure spectroscopy. Analysis on UN indicate the presence of three uranium shells at distances of 3.46(3), 4.89(5) and 6.01(6) A and a nitrogen shell at a distance of 2.46(2) A. For U2N3, two absorbing uranium atoms at different crystallographic positions are present in the structure. One of the uranium atoms is surrounded by nitrogen atoms at 2.28(2) A and by uranium atoms at 3.66(4) and 3.95(4) A. The second type of uranium atom is surrounded by nitrogen atoms at 2.33(2) and 2.64(3) A and by uranium atoms at 3.66(4), 3.95(4) and 5.31(5) A. Results on UN2 indicate two uranium shells at 3.71(4) and 5.32(5) A and two nitrogen shells at 2.28(2) and 4.34(4) A. The lattice parameters of UN, U2N3 and UN2 unit cells were respectively determined to be 4.89(5), 10.62(10) and 5.32(5) A. Those results are well in agreement with those obtained by X-Ray diffraction analysis.

The compounds Tc(2)Cl(4)(PMe(3))(4) and Tc(2)Br(4)(PMe(3))(4) were formed from the reaction between (n-Bu(4)N)(2)Tc(2)X(8) (X = Cl, Br) and trimethylphosphine. The Tc(II) dinuclear species were characterized by single-crystal XRD, UV-visible spectroscopy, and cyclic voltammetry techniques, and the results are compared to those obtained from density functional theory and multiconfigurational (CASSCF/CASPT2) quantum chemical studies. The compound Tc(2)Cl(4)(PMe(3))(4) crystallizes in the monoclinic space group C2/c [a = 17.9995(9) A, b = 9.1821(5) A, c = 17.0090(9) A, beta = 115.4530(10) degrees ] and is isostructural to M(2)Cl(4)(PMe(3))(4) (M = Re, Mo, W) and to Tc(2)Br(4)(PMe(3))(4). The metal-metal distance (2.1318(2) A) is similar to the one found in Tc(2)Br(4)(PMe(3))(4) (2.1316(5) A). The calculated molecular structures of the ground states are in excellent agreement with the structures determined experimentally. Calculations of effective bond orders for Tc(2)X(8)(2-) and Tc(2)X(4)(PMe(3))(4) (X = Cl, Br) indicate stronger pi bonds in the Tc(2)(4+) core than in Tc(2)(6+) core. The electronic spectra were recorded in benzene and show a series of low intensity bands in the range 10 000-26 000 cm(-1). Assignment of the bands as well as computing their excitation energies and intensities were performed at both TD-DFT and CASSCF/CASPT2 levels of theory. Calculations predict that the lowest energy band corresponds to the delta* --sigma* transition, the difference between calculated and experimental values being 228 cm(-1) for X = Cl and 866 cm(-1) for X = Br. The next bands are attributed to delta* --pi*, delta --sigma*, and delta --pi* transitions. The cyclic voltammograms exhibit two reversible waves and indicate that Tc(2)Br(4)(PMe(3))(4) exhibits more positive oxidation potentials than Tc(2)Cl(4)(PMe(3))(4.) This phenomenon is discussed and ascribed to stronger metal (d) to halide (d) back bonding in the bromo complex. Further analysis indicates that Tc(II) dinuclear species containing pi-acidic phosphines are more difficult to oxidize, and a correlation between oxidation potential and phosphine acidity was established.

The covalent attachment of tris(allyl)iridium to partially dehydroxylated gamma-alumina is found to proceed via surface hydroxyl group protonation of one allyl ligand to form an immobilized bis(allyl)iridium moiety, (=AlO)Ir(allyl)(2), as characterized by CP-MAS (13)C NMR, inductively coupled plasma-mass spectrometry, and Ir L(3) edge X-ray absorption spectroscopy. Extended X-ray absorption fine-structure (EXAFS) measurements taken on unsupported Ir(allyl)(3) and several associated tertiary phosphine addition complexes suggest that the eta(3)-allyl ligands generally account for an Ir-C coordination number of 2 rather than 3, with an average Ir-C distance of 2.16 A. Using this knowledge, combined EXAFS and X-ray absorption near-edge structure studies reveal that a small amount of Ir(0) is also formed upon reaction of Ir(allyl)(3) with the surface. It was found that the addition of either 2,6-dimethylphenyl isocyanide or carbon monoxide to the supported complex allows spectroscopic identification of the supported bis(allyl)iridium complexes, (=AlO)Ir(allyl)(2)(CNAr) [Ar = 2,6-(CH(3))(2)C(6)H(4)] and (=AlO)Ir(allyl)(2)(CO)(2), respectively. Although samples of the supported bis(allyl)iridium complex are active for the dehydrogenation of cyclohexane to benzene at temperatures between 180 and 220 degrees C, in situ temperature-programmed reaction XAFS and continuous-flow reactor studies suggest that Ir(0) nanoparticles, rather than a well-defined Ir(3+) complex, are responsible for the observed activity.

The chemistry of technetium is being explored at the University of Nevada Las Vegas. Our goal is to investigate both the applied and fundamental aspects of technetium chemistry, with a special emphasis on synthesis, separations, and materials science. The synthetic chemistry focuses on metal–metal multiple bonding, oxides and halides. Synthesis and characterizations of (n-Bu4N)2Tc2X8, Tc2(O2CCH3)4X2 (X = Cl, Br), TcO2, Bi2Tc2O7, Bi3TcO8, TcBr3 and TcBr4 have been performed. The applied chemistry is related to the behavior of Tc in the UREX process. Separation of U/Tc has been conducted using anion exchange resin and metallic Tc waste form synthesized and characterized.

We report the crystal structures of the novel technetium tetrahalides TcX(4) [X = F, I], as predicted from first-principles calculations. Isomorphous with TcCl(4) and TcBr(4) crystals, TcF(4) is orthorhombic with the centro-symmetric space group Pbca, while TcI(4) crystallizes in the monoclinic space group P2(1)/c. The structures, [TcX(2)(mu-X)(4/2)](infinity), consist of distorted edge-sharing octahedral groups of composition TcX(6) linked into endless cis chains. A possible explanation for the differences between these structures is offered in terms of varying degrees of bonding within the chains.

Technetium dimers Tc2(O2CCH3)4X2 (X = Cl, Br) were synthesized and studied by X-ray Absorption Fine Structure spectroscopy (XAFS). EXAFS analysis gave for Tc2(O2CCH3)4Cl2: d Tc-Tc = 2.18(1) A, d Tc–Cl = 2.43(1) A and for Tc2(O2CCH3)4Br2: d Tc–Tc = 2.19(1) A, d Tc-Br = 2.63(1) A. The Tc Tc separations are in agreement with Raman studies while the Tc–X distances are somewhat larger. Comparison with other Tc(III) quadruply bonded dimers indicates that the carboxylate compounds exhibit larger Tc–Tc separations. The effect of the terminal ligand (nature and position) on the Tc–Tc separation is discussed.