Stuart H. Laurie

Thiomolybdates — Simple but Very Versatile Reagents

A large number of thiometallate anions, MOxS4−xn−, and some of the seleno analogues, are known in which M is a transition metal in a high oxidation state. They are characterised by their strong colours arising from low energy S(Se)M charge transfer transitions, by unusual redox properties and by their ability to act as ligands to produce a wide range of heterometal complexes. This review outlines some of these properties associated with the thiometallate anions of molybdenum(VI). These anions are prepared by passing H2S through an alkaline molybdate solution, giving a sequential replacement of O atoms by S and ultimately MoS42− (TTM) as product. The thiomolybdates, particularly TTM, have been the most intensively studied because of their ease of preparation, their relatively greater thermal and hydrolytic stabilities and their biological roles. Unusual redox changes which involve internal electron transfer have been shown by reaction with organic disulfides and with CuII. In the latter case insoluble polymeric products are readily formed which, from EPR and EXAFS studies, involve S-bridged reduced Cu and Mo centres. Reaction of TTM with CuI, on the other hand, produces a range of complexes in which from 1 to 6 Cu ions add across the tetrahedral faces of TTM with no redox changes. Further CuI ions can be added to the [Cu6(MoS4)] moiety to produce cluster ions. This illustrates the ability of the thiometallates to act as building blocks for a very wide range of heterometal complexes ranging from simple linear ions to complex clusters, a prime example of the latter being the FeMo cofactor of the enzyme nitrogenase. TTM was identified sometime ago as the reactive intermediate in the Mo-induced Cu-deficiency that afflicts ruminants with serious consequences. Subsequently TTM, as its ammonium salt, has been successfully used in removing the excess Cu and then controlling the Cu levels of patients of Wilson’s disease. It appears to act by blocking the intestinal absorption of Cu and by converting Cu into a metabolically inert form, and in these respects is different to the mode of action of chelating agents which can be used for treating this disease.

The structures of bis-maltolato-zinc(II) and of bis-3-hydroxy-1,2-dimethyl-4-pyridinonato–zinc(II) and –lead(II)

Abstract The structures of bis-(3-hydroxy-4-pyronato)zinc(II) {bis-maltolato-zinc(II), Zn(malt)2·1.5H2O}, bis-(1,2-dimethyl-3-hydroxy-4-pyridinonato)zinc(II) {Zn(L1)2·7H2O}, and bis-(1,2-dimethyl-3-hydroxy-4-pyridinonato)lead(II) {Pb(L1)2·7H2O}, determined by X-ray diffraction techniques, are reported. The zinc in {Zn(L1)2·7H2O} is five coordinated; in Zn(malt)2·1.5H2O zinc occurs in both five- and six-coordination. Pb(L1)2·7H2O is dimeric; each lead has five oxygens, two of them bridging, in its coordination shell. All three compounds contain water molecules coordinated to the metal; both L1 complexes also contain water of crystallisation in the form of extended chains. All three compounds are extensively hydrogen bonded.

INFLUENCE OF PRESSURE ON CHEMICAL EQUILIBRIA IN AQUEOUS SYSTEMS : WITH PARTICULAR REFERENCE TO SEAWATER

Synopsis: The influence of pressure on chemical equilibria in aqueous solutions and the parameters that influence such equilibria are reviewed in this work. Particular emphasis is given to equilibria in seawater, including mineral dissolution, weak acid dissociation and metal ion complexation. Intended perspectives include the capabilities of predictive models to describe chemical equilibria in low temperature geochemical systems and the extent to which predictive models are constrained by direct observations. In context of the importance of pressure as a key variable controlling the pathways and fate of environmental chemicals, a need for further direct observations of chemical equilibria at high pressure, under conditions which are often challenging to the experimentalist, is evident.

Nuclear magnetic resonance and mass spectra of organomercury hydrides and deuterides, part II

Abstract Further mass spectroscopic and NMR evidence is given for the existence of several organomercury hydrides and deuterides.

Diaquabis(3,6-dioxaheptanoato)copper(II): crystal structure and EPR characteristics

Abstract The structure of the title compound has been determined by X-ray diffraction at 190 K. The complex has an all trans configuration with an elongated tetragonally distorted octahedral CuO 6 chromophore. The elongated axis corresponding to the trans -Cu–O(ether) bonds. The ligand molecules are bidentate via the carboxyl and the 3-ether O atoms; the 6-ether O atoms are not coordinated and are remote from the Cu centres. The bond lengths to the Cu centres are Cu–O(ether) 2.355 A, Cu–O(Carboxyl) 1.933 A and Cu–OH 2 1.995 A. The EPR spectrum of both the powder and frozen solution forms is typical of a rhombic system with a d x 2 − y 2 1 electronic configuration. There were no significant differences in spectra recorded over the temperature range 77 K to room temperature. These results are discussed in relation to earlier published results on closely related oxa-carboxyl complexes.

Polymeric ternary metal thiols I. Products from reaction of Cu(II) with MoS42

Abstract The black, polymeric, product isolated from the reaction of Cu(II) salts with MoS42− has been shown to have some unexpected and unusual properties, i.e. a stoichiometry of Cu1.6MOS4Xy (X = Cl−, Br−, y ≈ 1; X = SO42−, y ≈ 0.5), reduction of the metal centres to Cu(I) and Mo(V), the presence of S radical centres, and the absorption of ca. 2 mol O2. EXAFS spectra have also been recorded. The W anion WS42− behaves identically.

Extent and rate of solubilization of tin by iodomethane–water mixtures

The dissolution of tin metal by iodomethane-water mixtures has been studied and the influence of several parameters on both the extent and rate of reaction investigated. The rate-determining step shows a pseudo-first-order dependence on iodomethane (CH 3 I). The reaction is strongly dependent upon both water and oxygen. The activation energy of only 16kJ mol -1 is interpreted in terms of a facile bond-breaking and making process at the metal surface, generating methyltin species. Analysis by hydride generation and GC-AA confirmed the formation of methylated species ranging from monomethyl- to tetramethyl-tin; inorganic tin (from hydrolysis) was the major form, however. These findings are similar to those made earlier by us on the solubilization of arsenic from GaAs by alkyl halide-water mixtures, and a similar multi-step scheme is proposed.

Use of cadmium to determine complexing capacities and conditional stability constants of other metal-ligand systems via anodic stripping voltammetry

Abstract Quantification of the binding of metal ions which strongly complex with the organic components in natural waters is very difficult or impossible to measure directly. This paper shows that using competition via Cd 2+ and the differential pulse stripping voltammetric technique provides an indirect route to quantifying organic ligand complexation by e.g. lanthanide metal ions.

EPR STUDIES ON IRRADIATED GROUP VI TETRACHALCONIDE IONS

Abstract EPR measurements have been made at X-band frequency and at 77 K on γ-irradiated glassy solutions of salts of the d 0 chalconide anions MX 4 2− (M = Cr, X = O; M = Mo, X = O or S; M = W, X = O or S). Annealing of the irradiated solutions revealed broad high-field spectral features attributable to one-electron addition to the anions. The exception being WS 4 2− , for which no EPR signal was detectable at 77 K or above. The results point to the added electron being predominantly located on the metal centre ( d 1 ). The g-shifts are interpreted in terms of a distorted T d geometry with a d x 2 - y 2 configuration.