Fluoride ions, charge distribution

The method has been confined to main-group compounds presumably because of irregularities expected with unsymmetrical charge distributions in transition metal ions. The noble gas compounds remain outside the scope of the method because of the way in which electronegativity is defined (atom compactness relative to interpolated noble atom compactness). The main weakness of the method when applied to fluorides is in the somewhat arbitrary choice of fluorine bond energies. 

A very effective means of increasing the nucleophilicity of naked fluorides is efficient charge distribution over a large organic cation, for example tetrakis(dialky-lamino)phosphonium or phosphazenium ions (Scheme 2.12). 

For fluorine there is little evidence for positive behavior, even formally, except perhaps in the ion FC12 in oxygen fluorides the F atom is probably somewhat negative with respect to oxygen, while, in C1F, evidence from chlorine nuclear quadrupole coupling shows that the actual charge distribution involves partial positive charge on Cl. 

On the other hand, when fluoride ion acts as a basic catalyst for the removal of a proton from an uncharged substrate, its catalytic activity is found to be very similar to that of a carboxylate anion of the same basic strength.In this situation the effect of charge delocalization is much less marked, since the effect of the proton upon the charge distribution in the catalyst anion is largely counterbalanced by the effect of the negative charge borne by the deprotonated substrate. ... 

The electron distributions in alkali ions and halide ions are shown in Figure 6-20. It is seen that these ions are closely similar to the corresponding argonons, which are shown, drawn to the same scale, in Figure 5-3. With increase in nuclear charge from - -9e for fluoride ion to - - e for sodium ion the electron shells are drawn closer to the nucleus, so that the sodium ion is about 30% smaller than the fluoride ion. The neon atom is intermediate in size. 

The solvation of X is of course an extremely important factor influencing the reactivity of X in solution. Edwards and Pearson (1962) have noted that the charge distribution as calculated by quantum mechanics of an anion F is almost the same as in HF. Thus they conclude that the basicity is determined largely by the initial charge distribution in the base. A fluoride ion is much more basic than the large iodide ion in the gas phase and in solution because in fluoride the same amount of charge is concentrated in a much smaller space. 

Phossy water, a waste product in the production of elemental phosphorus by the electric furnace process, contains from 1000 to 5000 mg/L suspended solids that include 400-2500 mg/L of elemental phosphorus, distributed as liquid colloidal particles. These particles are usually positively charged, although this varies depending on the operation of the electrostatic precipitators. Furthermore, the chemical equilibrium between the fluoride and flurosilicate ions introduces an important source of variation in suspended solids that is a pH function. Commonly... 

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