Fluorinated ionic interactions

An additional observation made by Sauer, Wang and Hinds [83] was the Stark effect which enabled them to determine the electric dipole moment to be 3.91 D. This value taken with the relatively small fluorine hyperfine interaction suggests that YbF has a largely ionic structure. 

Analysis of weight loss isotherms displayed in Fig. 8 shows that the first step in the interaction between Nb02F and carbonates of other alkali metals is similar to the interaction described by Equation (11). However, compounds of the M(Nb04F form, where M = Na, K, Rb, Cs, were not found [85]. The instability of such compounds is related to the ionic radii of the alkali metals, which are greater than that of Nb5+, thus the ions are too large to occupy the octahedral cavities formed by the oxygen and fluorine ions. 

A slight but systematic decrease in the wave number of the complexes bond vibrations, observed when moving from sodium to cesium, corresponds to the increase in the covalency of the inner-sphere bonds. Taking into account that the ionic radii of rubidium and cesium are greater than that of fluorine, it can be assumed that the covalent bond share results not only from the polarization of the complex ion but from that of the outer-sphere cation as well. This mechanism could explain the main differences between fluoride ions and oxides. For instance, melts of alkali metal nitrates display a similar influence of the alkali metal on the vibration frequency, but covalent interactions are affected mostly by the polarization of nitrate ions in the field of the outer-sphere alkali metal cations [359]. 

SbF5, BF3, NbFj, and TaF5 led likewise to the conclusion that, in the solid state, these compounds are best formulated as predominantly ionic, although the ions interact rather strongly by fluorine bridging. This bridging apparently persists in the molten state and to some extent in solution in nitrobenzene (25, 82). 

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