Oxyanions stoichiometry

Adsorption of anions on oxides is usually accompanied by the uptake of protons (or the release of hydroxyl ions). The ratio between the number of protons that are coadsorbed and the level of anion adsorption is not usually stoichiometric. Studies of adsorption of oxyanions on goethite as a function of pH appear to indicate that, provided only one adsorbed species is present, the proton anion ratio is related to the mode of adsorption (Rietra et al., 1999). It follows that oxyanions which adsorb with a similar proton/anion stoichiometry, have a similar adsorption mechanism. 

The oxyanions as ligands may be classified according to (a) the structural type of the oxyanion (X02, X03, X04 or X06) (b) the coordination number of the oxyanion (1-18), i.e. the number of metal atoms to which a single oxyanion may be coordinated (c) the mode of coordination of the oxyanion, i.e. monodentate, bidentate, tridentate, etc. and (d) the number of oxyanions per metal atom, the stoichiometry p, from one to six, i.e. [M(XO ) ]. Table 1 lists the oxyanions that will be considered in this section according to their structural types, with their approximate stereochemistry and point group symmetry. The carbon-containing oxyanions will be described in Chapter 15.6, and the cyanates in Chapter 13.5, For reasons of space this review will be primarily restricted to mononuclear oxyanions. Figures 2-5 illustrate the mode of coordination of the oxyanions as a function of their coordination number 1-18. 

Within each coordination number the oxyanion may function as a monodentate, a bidentate or, very occasionally, a tridentate ligand to an individual metal cation. With the higher anion coordination numbers the M O distances to different metal atoms may not be identical. In practice, coordination numbers of 1 and 2 predominate and are independent of the three main structural types X02, X03 and X04. The coordination number 3 is essentially confined to tetrahedral X04 anions, and the much less numerous coordination numbers of 4-12 also involve mainly tetrahedral X04 type anions, particularly in their anhydrous oxyacid salts M(XO )9 or double salts M M(XO )(f+,. The stoichiometry number, p, is very often a funciton of the oxyanion/metal ratio of the preparative conditions. The higher p, the lower the coordination number of the oxyanion and the more the bonding is likely to involve a monodentate rather than a bidentate function. Nevertheless, the latter is very little influenced by the stoichiometry p. This is illustrated for the bidentate nitrate ion in the six structures (l)-(6),29,31-34 in which p increases from one to six and the bonding role of the nitrato group is essentially unchanged. 

The above analysis demonstrates that the coordination of oxyanions varies by the anion species. And it accords well with the stoichiometry of oxyanion / Fe at the adsorption saturations. The coordination structure of en is still unclear and the data analysis is in progress in order to clarify the origin of the small coordination numbers after the adsorption of anions. 

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