Selenium compounds and complexes

Nickel-selenium compoimds have recently been reviewed by the NEA TDB review team on Se, (see [2005OLI/NOL] for the corresponding NEA TDB critical review). 

It is reasonable to assume that the solubility product values for these solid phases determined at 16°C and 30°C apply to 25°C because 1) the experimental temperatures (ranging from 16 to 30°C) differ only slightly from 25°C, 2) the equilibrium constants for the formation of ThSO and Th(S04)2(aq) reported at 10°C, 25°C, and 30°C differ insignificantly from each other, and 3) the solubility products calculated for Th(S04)2-9H20(cr) at 16°C and 25°C are essentially identical as listed in this table. 

No experimental information is available on aqueous thiosulphate complexes of thorium. 

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This chapter presents the chemical thermodynamic data set for selenium species which has been selected in this review. Table lll-l contains the reeommended thermodynamic data of the selenium species, Table III-2 the recommended thermodynamic data of chemical equilibrium reactions by which the selenium compounds and complexes are formed, and Table III-3 the temperature coefficients of the heat capacity data of Table lll-lwhere available (see Appendix E for additional selenium data, cf. Section 11.7). 

Table 111-1 Selected thermodynamic data for selenium compounds and complexes. All ionic species listed in this table are aqueous species. Unless noted otherwise, all data refer to the reference temperature of 298.15 K and to the standard state, i.e., a pressure of 0.1 MPa and, for aqueous species, infinite dilution (/ = 0). The uncertainties listed below each value represent total uncertainties and correspond in principle to the statistically defined 95% confidence interval. Values obtained from internal calculation, cf. footnotes (a) and (b), are rounded at the third digit after the decimal point and may therefore not be exactly identical to those given in Part V. Systematically, all the values are presented with three digits after the decimal point, regardless of the significance of these digits. The data presented in this table are available on computer media from the OECD Nuclear Energy Agency.

IX.2 Selenium compounds and complexes IX.2.1 Solid and gaseous thorium selenides IX.2.1.1 Phase diagram and crystal structures... 

Chapter III contains a table of selected thermodynamic data for individual compounds and complexes of selenium (Table III-l), a table of selected reaction data (Table III-2) for reactions concerning selenium species and a table containing selected thermal functions of the heat capacities of individual species of selenium (Table III-3). The selection of these data is discussed in Chapter V. 

Chapter IV contains, for auxiliary compounds and complexes that do not contain selenium, a table of the thermodynamic data for individual species (Table IV-1) and a table of reaction data (Table IV-2). Most of these values are the CODATA Key Values [89COX/WAG]. The selection of the remaining auxiliary data is discussed in [92GRE/FUG], and other preceding volumes of this series. As just mentioned, Appendix E contains a table. Table E-1, of the adopted thermodynamic data for individual selenium compounds calculated with auxiliary data not contained in Chapter IV. The non-TDB auxiliary data, usually from [82WAG/EVA], are presented in Table E-2 and the evaluations are discussed in Chapter V and Appendix A. 

This chapter presents the chemical thermodynamic data for auxiliary compounds and complexes which are used within the NEA s TDB project. Most of these auxiliary species are used in the evaluation of the recommended selenium data in Table lll-l, Table II1-2, and Table III-3. It is therefore essential to always use these auxiliary data in conjunction with the selected data for selenium. The use of other auxiliary data can lead to inconsistencies and erroneous results. Additional auxiliary data used in this review are found in Appendix E, Table E-2 cf. Section II.7). 

Various arrangements of the presentation of the material in Chapter V have been considered. A sub-division according to type of compound such as selenide, selenite etc. would lead to the best overview of a particular type of species. This arrangement was tried but abandoned and the system used in previous reviews was adhered to, i.e. the compounds and complexes formed by selenium with a certain element are presented together. 

Copper interacts with numerous compounds normally found in natural waters. The amounts of the various copper compounds and complexes present in solution depend on water pH, temperature, and alkalinity and on the concentrations of bicarbonate, sulfide, and organic ligands. In animals, copper interacts with essential trace elements such as iron, zinc, molybdenum, manganese, nickel, and selenium and also with nonessential elements including silver, cadmium, mercury, and lead interactions may be either beneficial or harmful to the organism. The patterns of copper accumulation, metabolism, and toxicity from these interactions frequently differ from those produced by copper alone. Acknowledgment of these interactions is essential for understanding copper toxicokinetics. 

The first step consists in the attack of a proton on the W-H bond to yield a labile dihydrogen intermediate (Eq. (3)) that rapidly releases H2 to form a coordi-natively unsaturated complex (Eq. (4)). This complex adds water in the next step to form an aqua complex (Eq. (5)) that completes the reaction by substituting the coordinated water by the X anion (Eq. (6)). Steps (3)-(6) are repeated for each W-H bond and the factor of 3 in the rate constants appears as a consequence of the statistical kinetics at the three metal centers. The rate constants for both the initial attack by the acid (ki) and water attack to the coordinatively unsaturated intermediate (k2) are faster in the sulfur complex, whereas the substitution of coordinated water (k3) is faster for the selenium compound. 

The methods available for synthesis have advanced dramatically in the past half-century. Improvements have been made in selectivity of conditions, versatility of transformations, stereochemical control, and the efficiency of synthetic processes. The range of available reagents has expanded. Many reactions involve compounds of boron, silicon, sulfur, selenium, phosphorus, and tin. Catalysis, particularly by transition metal complexes, has also become a key part of organic synthesis. The mechanisms of catalytic reactions are characterized by catalytic cycles and require an understanding not only of the ultimate bond-forming and bond-breaking steps, but also of the mechanism for regeneration of the active catalytic species and the effect of products, by-products, and other reaction components in the catalytic cycle. 

A number of zinc selenium complexes have now been characterized, with particular interest in the formation of zinc selenide semiconductors and quantum dots. In many cases analogous structures to those observed with thiol or thiolates are recorded. 77Se NMR is frequently used in characterization, and comparison with the sulfur equivalent is relevant. Zinc selenium compounds are of particular interest as precursors for metal/selenide materials and their relevance as models for selenocysteine-containing metalloproteins. 

Urea complex with hydrogen peroxide (UHP) seems to be a very useful and convenient oxidation system in nonaqueous medium. The UHP reaction proceeds in methanol and is catalyzed by Mo (VI), W (VI) salts, or SeC>2 (83). Catalytic oxidation in a water-free medium can be carried out with alkylhydroxyperoxides, the catalysts being titanium alkoxides (84) or selenium compounds (85). The reaction appears to proceed quickly and with good selectivity. 

The comparison of the Mo-Mo distances in the sulfur and selenium complexes shows that the selenium analogues have longer (ca. 0.04 A) distances. This can be explained by the larger intracluster matrix effect of selenium. This contrasts with the shorter average Mo-Mo distances for the selenium compounds in the solid-state Chevrel phases (52). 

Complexes of both selenium(IV) and tellurium(IV) with sulfur-containing ligands are known although the structural properties appear to have been determined only for the tellurium(IV) species. However, due to the smaller size of selenium(IV) and its poorer acceptor properties, as compared with tellurium(IV), its maximum coordination number appears to be six whilst it has been found to be eight for tellurium.78 Furthermore, the greater tendency of tellurium to form secondary bonds may cause the structures of similar selenium(IV) and tellurium(IV) compounds to be different. 9 It is also pertinent to note that the reduction of tellurium(IV) to... 

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