Ekkehard Diemann

Polysulfide complexes of metals

Publisher Summary Sulfide minerals were formed mostly hydrothermally from postmagnetic fluids; it appears remarkable that the formation of many ore deposits cannot be conclusively explained because of the very low solubility of the corresponding sulfide. Polysulfido complexes are interesting because of their structures and reactivity, and also because of their possible applications. They can—for example, be used to prepare sulfur rings of predetermined size and they are suspected to play a role in catalysis, particularly in hydrodesulfurization. Free polysulfide ions consist of sulfur chains. Complexes with S22– ligands show a remarkable variety of structures that result from the extension of the fundamental structural type Ia (side-on coordination) as well as IIa and IIb by using the remaining lone pairs of electrons (on sulfur) to coordinate additional metal atoms. Such coordination of additional metal atoms occurs for all three fundamental structural types (Ia, IIa, and IIb). Complexes with type II structures are presented in table along with their S–S distances and S–S vibrational frequencies. There are several routes for the synthesis of polysulfido complexes. Oxidative addition of elemental sulfur to a coordinatively unsaturated electron-rich metal is a convenient method for preparing Sn2– complexes.

Thermal decomposition of (NH4)2MoO2S2, (NH4)2MoS4, (NH4)2WO2S2 and (NH4)2WS4

Abstract The thermal decomposition of (NH4)2MoO2S2,(NH4)2MoS4, (NH4)2WO2S2 and (NH4)2WS4 has been studied by DTA and TGA methods. All compounds decompose in two steps. The tetrathio compounds decompose as represented by the equations, (NH4)2MeS4 → 2NH3 + H2O + MeS3 MeS3 → MeS2 + S (Me  Mo, W). The thermal decomposition of the dithio salts is very complex. It is presumed that mixtures of oxysulphides and sulphides are formed as intermediate compounds. The final product of decomposition of the dithio-molybdate is very close to MoS2, the final product of the decomposition of the dithio-tungstate yielding a mixture of WS2 and WO2 of the average composition WO0·5S1·75. The formal energies of activation which qualitatively describe the thermal properties of the compounds and heats of decomposition for the first reaction step were calculated from the thermal decomposition curves.

Structure of a cavity-encapsulated nanodrop of water

A (H2O)(100) nanodrop of water was found in the cavity of a spherical polyoxomolybdate cluster of the type {(Mo)Mo-5}(12) (spacer)(30) and structurally characterised. The molecules within the water cluster are arranged in three concentric shells with radii of 3.84-4.04, 6.51-6.83 and 7.56-7.88 Angstrom spanned by 20, 20, and 60 molecules, respectively. The first shell dodecahedron is formed exclusively by 20 hydrogen bonds (O...O 2.77-2.92 Angstrom) in the centre and 12 (H2O)(5) pentagons (O...O = 2.66-2.81 Angstrom) formed by hydrogen bonds at the periphery, i.e., the third shell. Each of the H2O molecules of the second (dodecahedral) shell connects the molecules from three different pentagons of the third shell with one molecule of the central dodecahedron

Selective removal of molybdenum traces from growth media of N2-fixing bacteria.

A new method for the selective removal of traces of molybdenum from growth media of N2-fixing bacteria (Rhodobacter capsulatus and Klebsiella pneumoniae) was developed. This method is based on the filtration of nutrient solutions through a layer of activated carbon (pulverized charcoal). The adsorption of Mo (molybdate) to activated carbon was optimal if a charcoal suspension (50 g/liter) was degassed by boiling before use and if the pH of the solutions, which had to be purified, was adjusted to values between 1.5 and 4. In this pH region no or only negligible amounts of other metal ions were adsorbed. The activated carbon method was compared with other Mo-eliminating procedures, including 8-hydroxyquinoline/dichloromethane extraction, Chelex 100 chromatography, and treatment with Mo-starved Azotobacter vinelandii cells. The activated carbon filtration appeared to be the most effective, specific, and rapid method. Whereas the untreated Rhodobacter growth medium was contaminated with 1.2 ppb Mo, as analyzed by inductively coupled plasma mass spectrometry (ICP-MS), the activated carbon-treated medium was below the ICP-MS detection limit (less than 0.05 ppb). A similarly effective removal of Mo impurities was obtained by the Azotobacter treatment. Even at low optical densities (2-5 at 436 nm) Mo traces were removed very rapidly within 10-15 min. However, because the Mo uptake/Mo adsorption capacity of A. vinelandii depended on freshly cultivated cells and on the growth phase at which the cells were harvested, this microbiological method was generally more time-consuming and less reproducible than the activated carbon method.(ABSTRACT TRUNCATED AT 250 WORDS)

The He(I) photoelectron spectra of OsO4, and RuO4

Abstract The photoelectron spectra of ruthenium tetroxide and osmium tetroxide excited by He(I) radiation are reported. From their interpretation it follows that the first two strong low energy transitions in the electronic absorption spectra of OsO4, RuO4, TcO−4, ReO−4, MoO2−4 and WO2−4 can be assigned to t1 → 2e and 3t2 → 2e respectively.

New method for the preparation of hydrodesulphurization catalysts: Use of the molybdenum sulphur cluster compound (NH4)2[Mo3IVS13]

An other method which appears very promising and general for the preparation of sulphide catalysts is described in the present paper. This method consists in the direct use of metal sulphur cluster (NH 4 ) 2 Mo 3 (S)(S 2 ) 6 ] as precursor for the preparation of molybdenum sulphide based catalysts

A cyclic voltammetric study of mangano(II)undecatungstosilicate: an illustrative example of the reduction, protonation and disproportionation pathways of transition metal substituted heteropolytungstates in aqueous solution

Mangano(II)undecatungstosilicate [(H2O)MnSiW11O39]6− was studied by cyclic voltammetry in aqueous solution at various pH values. The pH-dependent wave patterns are influenced by additional processes and were further investigated by variation of scan rates, substance concentration and buffer concentration, focussing on the first 2-electron reduction. Polarographic data and results obtained from [(H2O)CoSiW11O39]6− and [(H2O)2Mn4(PW9O34)]10− were included for comparison. A suitable pH range of 2.5–9 was determined from monitoring the decay of [(H2O)MnSiW11O39]6− by electronic spectroscopy over a period of several weeks. Around pH 5, the cyclic voltammetric wave patterns appear more distorted than at higher and lower pH values. Conditions were specified under which the reoxidation scans exhibit two species with different degrees of protonation. The results were interpreted in terms of protonation and successive disproportionation of reduced species and suggest that a complex network of reversible reaction pathways takes place during the initial reduction processes of [(H2O)MnSiW11O39]6− and related compounds in aqueous solution.

A new highly active hydrotreating catalyst prepared by the decomposition of thiotungstatonickelate and characterized by high resolution electron microscopy

Abstract The bisthiometalato complex [Ni(WS 4 ) 2 ] 2− was utilized as a precursor for the preparation of the unsupported mixed nickel tungsten sulphide catalyst. Catalytic activities in hydrotreating model reactions are better for the decomposed complex than for catalysts prepared by conventional methods. HREM and AEM studies show that the structure of the catalyst is highly disordered, and that nickel is well distributed in all particles. The activity loss observed after a catalytic test at 673 k is related to a partial nickel segregation. Better crystallized particles are formed and the position of nickel atoms is mainly restricted to edge positions of the WS 2 crystallites. The decomposed complex is considered as a model for mixed hydrotreating catalysts.

Electronic structure and spectra of oxothiometalates MO4—nSn2− (M = Mo, W;n = 0–4)

Abstract SCCC-EH MO calculations on MoO4—nSn2− (n = 0–4) and WX42− (X = O, S) have been carried out. On the basis of the calculations, experimental, and empirical observations, the electronic spectra of the oxothiometalates MO4—nSn2− (M = Mo, W; n = 0–4) can be assigned almost completely in terms of different types of L → M charge transfer transitions. The calculations explain the bond strength relations n(MO) > n(MS) and n(WX) > n(MoX) and also other chemical and physical properties of these species (stronger σ-bonding for MW, stronger π-bonding for XO) as well as the extraordinary electronic properties of the species MO4—nSn2− (n = 2, 3, 4) which are strong electron acceptor ligands towards additional metal centres. A new parameter set for SCCC-EH calculations on 5d transition metal complexes is given. The assignment for MoS 42− is supported by an analysis of the spectrum of [NCCu(MoS4)]2−.

The assignments of electron transfer bands of molybdates (VI), tungstates (VI) and rhenates (VII) of the type MXY3(C3V) (X,YO,S,Se)

Abstract The electronic spectra of the ions MoSSe 2− 3 , MoS 2 Se 2− 2 , MoS 3 Se 2− and WSSe 2− 3 are reported. An assignment of the charge transfer bands of these ions and of thio-oxo- and seleno-oxoanions of Mo, W and Re with C 3V symmetry is proposed and discussed.