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The influence of the trans ligand on the electronic structure, the spectroscopic properties, and reactivity of the molybdenum and tungsten nitrido complexes is investigated by studying a series of molybdenum and tungsten nitrido and imido complexes with dppe and depe coligands and trans ligands of different sigma/pi-donor strengths. The results are interpreted by means of DFT calculations, and the general implications with respect to a catalytic ammonia synthesis on the basis of the Chatt cycle are discussed.

A new family of tetracarbonyl molybdenum(0) and tungsten(0) complexes based on new 6,6'-disubstituted-4,4'-bipyrimidine ligands was synthesized and characterized. The visible region of the absorption spectrum of each complex is dominated by a metal-to-ligand charge transfer band significantly lower in energy than the corresponding transition in 2,2'-bipyridine tetracarbonyl metal complexes. The 6,6'-substituents create a larger n-electronic system in the substituted bipyrimidines and are consequently better [pi] acceptors than even the parent 4,4'-bipyrimidine. The absorption bands are shifted bathochromically with a decrease in solvent polarity. Key words: molybdenum and tungsten, nitrogen ligands, tetracarbonyl metal complexes, absorption spectroscopy, electrochemistry. Resume : Une nouvelle famille de complexes de type tetracarbonyle de molybdbne(0) et tungstene(0), a base des 4,4'-bipyrimidine substituees dans les positions-6,6', a ete synthdtisde et caractdrisee. Le spectre d'absorption dans la region du visible de chaque compose indique un deplacement vers le rouge pour la bande de transfert de charge metal-ligand compare a celui du complexe tetracarbonyle de 2,2'-bipyridine. Ceci demontre une meilleure delocalisation du systeme [pi]-electronique darts la bipyrimidine substitude qui rend ces ligands des meilleurs accepteurs [pi]. Cette bande d'absorption est deplacee bathochromiquement avec la diminution de la polarite du solvant. Mots cles : molybdenum et tungsten, ligands azotes, complexes metal-tetracarbonyle, spectroscopie d'absorption, electrochimie.

X-ray absorption spectra (XAS) at the Cl and S K edge and molybdenum (Mo) L edge are calculated at the TDDFT level for a series of dioxomolybdenum complexes LMoO2X (L = hydrotris(3,5-dimethyl-1-pyrazolyl)borate, X = Cl, SCH2Ph, OPh), which play an important role in modeling the catalytic cycle of the sulfite oxidase enzyme.

X-ray absorption and near-edge structure (XANES) and X-ray photoelectron spectroscopy (XPS) were used to characterize molybdena-containing ultrastable (HY) zeolites that were prepared using several Mo precursors and activation procedures. Combined XANES Mo L-edge and XPS results revealed that the Mo 4d electronic level was not strongly influenced by the preparation procedures, but it was clearly modified by the presence of Pd.

A multinuclear MAS NMR study was conducted for examining the fresh and coked Mo/HMCM-22 catalysts for methane dehydroaromatization. Results suggest that the bad catalytic performance could be attributed to the destruction of the zeolite lattice.

Abstract -- The complexation of molybdenum(VI) with 2-hydroxy-5-tertbutylphenol-2'-methylazobenzene was studied by atomic absorption spectrometry and spectrophotometry. Optimum conditions for complex formation and extraction were found. The ratio of components in [...]

A combination of the x-ray photoelectron spectroscopy and x-ray photoelectron spectroscopy techniques is employed to explain the chemical interactions taking place between the [B.sub.4]C, B, and C and the Mo/Si layered structures. The stability of the compound thus formed is shown to be the main determinant of the diffusion barrier function of the [B.sub.4]C interlayers.

Ultrafast photoexcitation in the ultraviolet has cleaved the Mo-Mo bond and has helped in observing the subsequent geminate rebinding reaction by monitoring infrared bleach recoveries at frequencies corresponding to the CO stretches of the trans and gauche isomers. The rebinding reaction is obtained by mapping the full potential energy surface along the reaction coordinate by using electronic-structure methods.

Selenidobis(dithiolene)molybdenum(IV) and -tungsten(IV) complexes are prepared and characterized by using X-ray crystallographic analysis. [M.sup.IV]=Se complexes are formed as an intermediate in oxygen atom transfer from [Me.sub.3]NO to the [Mn.sup.IV]=Se centers.

The Fe valence state of [Sr.sub.2]FeMo[O.sub.6] samples is studied by using site specific x-ray absorption spectroscopy and x-ray magnetic circular dichroism. The chemical changes with time are important for potential applications of [Sr.sub.2]FeMo[O.sub.6] and related transition metal oxides.

The effect of Mo and W additions on the magnetostriction coefficient, elastic constants and the magnetomechanical coefficient of alpha-Fe is examined by using alpha-phase Fe-Mo and Fe-Mo alloy single crystals. The W additions to Fe have provided a larger increase in magnetostriction when compared to any other non-rare-earth addition that is examined with the exception of Ga.

Direct observation of different types of molybdenum species on fresh and working Mn/zeolite catalysts by using solid-state NMR is obtained on an ultrahigh field NMR spectrometer. The results have shown that the exchanged Mo species are the active centers for the methane dehydroaromatization (MDA) reaction.

The silica supported 2.5-dimethylpyrrolyl family of silica supported Mo imido alkylidine complexes constituted the most efficient d superscript o olefin metathesis which can be prepared in two steps from the readily available triflate derivatives. Preliminary account shows that having methyl substituents on the pyrrolyl ligand in position 2 and 5 improve the stability of the system and that tuning the imido substituents can increase the activity of the catalyst.

Femtosecond pump-probe spectroscopy is used for measuring the propagation of sound in a randomly formed [Mo.sub.6][S.sub.3][I.sub.6] nanowire network. Different probe laser wavelengths are used in combination with a measurement of the refractive index in order to determine the sound velocity.

An excess of BuLi in the presence of N,N,N',N'-tetramethylethylenediamine at slightly elevated temperatures is used to dilithiate the homoleptic sandwich complex bis(benzene)molybdenum. The results reveal that isolation of an elusive ansa-bridged derivative is accomplished along with the characterization of a molybdenum-containing paracyclophane complex.

The temperature-programmed reaction spectroscopy and reflection-absorption infrared spectroscopy are used to study the rearrangement reactions of [C.sub.4]-alkoxides on O-covered Mo(110). The reactions of cyclo-propylmethyl, cyclobutyl, and 3-buten-1-yl bromides and alcohols have disclosed that near O-covered Mo(110), cyclopropylmethyl groups can rearrange to cyclobutyl groups and vice versa.

X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) are used to investigate the Co chemisorption on the metallic molybdenum nanoparticles supported on the thin alumina film. The formation of the [Mo.sub.x][(CO).sub.y] species has implied that the property of the metallic molybdenum nanoparticles on the thin alumina film is much different from that of the bulk molybdenum, thereby implying a significant nanometer size effect.

A study used site directed mutagenesis to construct variants of quinaldine 4-oxidase (Qox) with replacement of positions C40, C45, C48, C60, C120, and C154 of [Qox.sub.s] by serine to assess which iron ions of FeSI and FeSII are likely to be involved in intramolecular electron transfer between the molybdenum center and flavin. The properties of the Qox variants when taken collectively reveal that Fe1 of both FeSI and FeSII are the reducible iron sties whereas the Fe2 ions remain in the ferric state.

X-ray absorption spectroscopy (XAS) and electrical conductivity measurements are used to study the mechanism of the oxidation-reduction of the MoVSbNbO catalyst in dynamic conditions. The results have proved that different types of oxygen must be involved in the catalytic process although their relative contributions are different and it is shown that the electrical conductance of the M1 phase correlates with oxidation states of Sb and V and the concentration of oxygen vacancies.

Photoemission spectra of single MoS2 nanotubes deposited on a Si surface are recorded in order to explain their electronic structure. MoS2 nanotubes are found to be semiconducting with a band gap of about 1 eV.

Synthesis, spectroscopic studies and structures of molybdenum(VI) complexes with homocitrate complexes with homocitrate are reported. Results show that the homocitrate ligand trapped by tetranuclear molybdate coordinates to the molybdenum(VI) atom through alpha-alkoxy and alpha-, beta-carboxy groups.

The first information about vibrational modes of the Fe-S clusters in nitrogenase, using the technique of nuclear resonance vibrational spectroscopy (NRVS) is reported. The vibrational spectra derived from NRVS experiments support the presence of an interstitial atom in both isolated FeMoCo and in the Azotobacter vinelandii protein (Avl)-bound FeMo-cofactor.

An ab initio multiconfigurational approach is used to calculate the ligand-field spectrum and magnetic properties of the title cyano-bridged dinuclear molybdenum complex. The rather large magnetic coupling parameter J for a single cyano bridge, as derived experimentally for this complex by susceptibility measurements, is confirmed to a high degree of accuracy by multiconfigurational second-order perturbation method calculations.

The separately purified human MOCS3-RLD domain is analyzed in molecular detail by site-directed mutagenesis and mass spectrometry. MOCS3 is proposed to catalyze both the adenylation and the subsequent generation of a thiocarboxylate group at the C-terminus of the smaller subunit of molybdopterin synthase during Moco biosynthesis in humans.

A study investigates the solubility of Mo(sub 6)S(sub 4.5)I(sub 4.5) nanowire soot in a range of common solvents by performing sedimentation studies and microscopic and spectroscopic characterization. The findings of the study shows that in all solvents, Mo(sub 6)S(sub 4.5)I(sub 4.5) nanowires soot contains three phases, two of which are insoluble with one stable phase.

A study of the molecular, electronic, and vibrational characteristics of the molybdenum-containing enzyme complex xanthium oxidase with violapterin is carried out using density functional theory calculations and resonance Raman spectroscopy. A normal mode analysis is undertaken that has led to specific assignments of vibrational modes seen in the rR spectra of the complex of reduced XO with product violapterin.

The interactions between the p orbitals of dithiolate ligands and b orbitals of titanium in bent titanocene complexes as minimum molecular models of active site features of pyranopterin Mo/W enzymes are investigated using gas-phase photoelectron spectroscopy and density functional theory. A 'dithiolate-folding-effect' involving an interaction of metal and sulfur is demonstrated in complexes with arene- and enedithiolates.

The bovine serum albumin (BSA) adsorption processes on two different metals, chromium (Cr) and molybdenum (Mo) in two solutions, pure water and water with a phosphate buffer (pH7) is compared. The results suggest that the interactions between the negatively charged carboxylate groups or the positive side groups of the BSA and the passivated metal surfaces may be different on Cr and Mo.

The structure and density of Mo species in Mo/H-ZSM5 during catalytic CH4 reactions is investigated by using in-situ X-ray absorption spectroscopy (XAS). Results show that CH4 reactions cause exchange in Mo(sub 2)O(sub 5)(super 2+) dimers, formed from physical mixtures of MoO3 and H-ZSM5.

A molybdenum K-edge X-ray absorption spectroscopy (XAS) study is conducted to analyze the ligand environment of the Mo bound on the as purified NifEN. It is strongly proposed that the Mo is a part of a cluster whose first two coordination spheres represent a [Mo[Fe.sub.3][S.sub.4]] cubane fragment as part (or whole ) of some cluster on NifEN.