Cation/anion ratio

Since hydrofluoride synthesis is based on thermal treatment at relatively high temperatures, the possibility of obtaining certain fluorotantalates can be predicted according to thermal stability of the compounds. In the case of compounds whose crystal structure is made up of an octahedral complex of ions, the most important parameter is the anion-cation ratio. Therefore, it is very important to take in to account the ionic radius of the second cation in relation to the ionic radius of tantalum. Large cations, are not included in the... 

Table 15 summarizes anion-cation ratio for different types of fluorotantalates. Thermally stable compounds that can be prepared using the hydrofluoride method are noted in bold. 

As solvents, ionic liquids can be uniquely tuned to a particular purpose by adjusting the anion/cation ratio. To decaffeinate coffee, for example, you could create an ionic liquid that would just dissolve caffeine and nothing else. Current research suggests that ionic liquids can be recovered from solution and reused. 

Uniformly, within this group of cations, perchlorate ion accompanying the transition-element cation is replaced by nitrate (7,31), thiocyanate (7,52), or halide (7,6). Nitrate is probably replaced by thiocyanate, but a secondary change takes place in many systems, which makes direct comparison difficult (see below). If one then makes the further reasonable assumption that solvent interference can be used as an inverse measure of tendency to bind to the central metal cations, thiocyanate, whose competition with alcohol is less efficient (52) than that of chloride (6), should be somewhat replaceable with chloride. Comparisons between chloride and thiocyanate in acetonitrile show also that the formation of a complex with a given anion/cation ratio takes place much more readily with chloride than with thiocyanate (55, 34). By the same criterion, from experiments in alcoholic solution (55), bromide should replace chloride, and an extrapolation of the behavior to iodide seems reasonable. 

Table 5. Calculated values of stoichiometric coefficients as dependent on the anion/cation ratio (C/S). Data were obtained from 15 pooled samples the last two lines show variability between two separate runs...

In all the systems, changes in the intensity of the CD band at a given wavelength depend on the molar anion/cation ratios indicate the formation of 1 1 associates. The 1 2 associate was detected for the A-[Co(sen)]3+...Sb2(Z-tart)2 system. The association constants of A-complexes with l-(Ki) and d-(Kd) chiral anions have been estimated (Table 30). 

Phases with an anion/cation ratio of 1.5 are mainly found in the case of A = K, Rb, and Cs, while those with an anion/cation ratio of 1.667 exist only for A = K and Rb. The crystal chemistry of all these A3RF6 and A2RF5 phases has been reviewed very recently by Greis (1982) and, therefore, only an overview will be given here. 

It is a general rule (O Keeffe Hyde, 1984) that increasing anion/cation ratio increases cation coordination number, but there are some exceptions readily explicable by the requirements of equal valences. Thus compare Ala Os (already discussed) with ivAiivpii04. Equally striking is the contrast in the compounds viNaiiiN iiOa and vNa3 N 04. 

If the chemical potential of the electronegative component of a (binary) ionic crystal (e.g. oxide) is raised (P=Po2), the concentrations increase (fall) of all those defects which considered individually would increase (reduce) the anion-cation ratio in addition, the concentration of oxydized states (holes) increases, that of the reduced states (conduction electrons) falls . 

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