Anions mutual arrangements

Careful purification of solutions of fluorosilane and fluorosilicate anions and a decrease in concentration and temperature result in a fine structure of F NMR spectra due to a non-equivalent arrangement of ligands (Table 16). For example, in the F NMR spectra of octahedral complexes of the type RSiFj" a doublet (equatorial atoms) and a quintuplet are observed whose mutual arrangement depends on the nature of the ligand In fluorine derivatives of penta- or hexa-coordinate silicon with slow exchange, the Hgip values (if they can be determined) are greater for the fluorine atoms with chemical shifts in low field. 

Fig. 5. Structure of the phosphaalkyne [(dme)jLi] [IMi -S] 30 % probability measurement at -100 3 °C the reproduction of cation and anion reflects their mutual arrangement in the solid.

A trigonal symmetry of host molecules provides a typical template for obtaining cavities or channels. In the crystal structure of a-TMA (TMA = trimeric acid, the parent acid of the BTC anion), mutual triple catenation of hydrogen-bonded TMA was found. However, selfpenetration is preventable, if the TMA was crystallized with template molecules or is functionalized with bulky substituents to obtain a rigid arrangement. 

To achieve the state of lowest energy, the cations and anions will tend to maximize attractions and minimize repulsions. Attractions are maximized when each cation surrounds itself with as many anions as possible, with the proviso that neither the cations nor the anions touch. To illustrate, consider the four anions surrounding cations of increasing radii as shown in Fig. 3.2. The atomic arrangement in Fig. 3.2a is not stable because of the obvious anion-anion repulsions. Figure 3.2c, however, is stabilized by the mutual attraction of the cation and the anions. When the anions are just touching (Fig. Z.lb), the configuration is termed critically stable and is used to calculate the critical radii at which one structure becomes unstable with respect to another (Worked Example 3.1). 

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