Se-O systems

The number of known polycationic compounds of main group elements is far less than that of polyanionic compounds. Examples include the chalcogen cations S]+, S]+, Se]o and Teg that are obtained when the elements react with Lewis acids under oxidizing conditions. The ions S]1, Sc]1 and Te] have a square structure that can be assumed to have a 6n electron system. 

Systems with one extra N atom and one extra other heteroatom, 1 1 (Section 12.04.3). The other heteroatoms are O, S, and Se. No system with heteroatoms different from these has been reported in the reviewed period. 

In addition to sulphate, selenate (J. M. Bigham, unpubl.) and chromate (S. Regen-spurg unpubl.) can also be incorporated in the tunnels of synthetic schwertmannite. Whether or not two different Se-O distances (based on EXAFS) attributable to surface and tunnel selenate, respectively, exist in the Se-form is still under discussion (Waychunas et al., 1995, 1995 a). The Cr form has the bulk composition Fei6Oi6(OH)i0.23(CrO4)2.gg. In fact, synthetic schwertmannite formed in the sul-phate/arsenate system tolerates arsenate only up to a As/(As-rS) mole ratio of ca. 0.5, and it is likely that most of this arsenate is surface-bound. Above this ratio, a new, very poorly ordered Fe-hydroxy arsenate with two broad XRD peaks at ca. 0.31 and 0.16 nm and BhfS at 4.2K and ca. 1.5 K of 41.6 and 47.3T, respectively, forms (Carlson et al. 2002). From this one may conclude that, whereas the tetrahedral oxyanions with hexavalent central cations (S Se Cr) can be accomodated in the tunnel positions, the pentavalent cations can not, or not as easily. Schwertmannite from acid mine water contained between 6 and 70 g kg As (Carlson et al. 2002). 

Heteroaromatic cations containing oxygen, sulfur, or selenium ring heteroatoms are far more susceptible to pseudobase formation than the corresponding AT-methyl cations. A comparison of data for structurally related cations is given in Table IV. It is clear that in most cases pKR + values fall in the order O < Se < S NMe, and a linear relationship of unit slope exists between pKk+ values for at least two series (20 and 21) of cations (Fig. 2). The pKR + values reported81 for the linear tricyclic system 22 are unusual in that they fall in the order Se < O < S and furthermore suggest that in these cases 22 are less susceptible to pseudobase formation than their isomers 21, which is unexpected in view of the relative pKR,... 

Oppermann et al. studied some of the equilibria in the system Se-O-Br by total pressure measurements. Thermodynamic quantities were derived from the temperature dependence of the equilibrium constant. The extrapolations of the results to standard conditions do not use experimental thermal functions but employ various estimates. 

FIGURE 12.12 Eh-pH stability diagram for Se—O—H system. The activity of dissolved Se = 10 (Modified from Brookins, D. G., Eh-pH Diagrams for Geochemistry, Spring-Verlag, Heidelberg, 1988.)... 

The molecules in crystaUine phosphorous acid, H3PO3, are linked together by a three-dimensional system of H bonds formed from HPO(OH)2 units. The H atom directly linked to P does not form any H bonds. H-bonds of type -S-O-H-O-P-O-H-O-S- are found in crystalline CS3 (HS04)2(H2P04) [15,16] and related salts (Chapter 5.3). In some of these salts the H bond systems appear to be deficient in H atoms moreover there is a random distribution of SO4 and PO4 tetrahedra. Continuous chains of -Se-O-H-O-P-O-H-O-Se- linkages are found in crystalline Cs5(HSe04)3(H2P04)2 [17,18]. 

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