The Ion-Cavity Concept

Examination of the diameters of ligand cavities and of the diameters of alkali and alkaline earth ions as given in Table 9 clearly shows that either the metal ion is too small to fill the cavity, or too large to fit in it, or it just meets the cavity size. The 

Both Rb and Cs are too large to be accommodated into the 18-crown-6 cavity. Therefore, they occupy a site somewhat distant from the plane of the ether oxygens. Rb is situated by 1.19 AandCs even by 1.44 A above this plane, which leads to a less favorable interaction with the ligand donor atoms, thus explaining the diminished stabilities of the [18]crown-6 complexes with these ions. The crown ether conformation does not alter in comparison to the complex. Coordination of the metal ion is completed by contacts with the bridging SCN ions, thus resulting in the formation of a 2 2 dimeric structure (Fig. 28) 

On the other hand, Na is too small to completely fill the ligand cavity. To render a sufficient interaction with all the possible ether oxygens, the li nd wraps around the sodium ion, one of the donor atoms thus occupying an apical position of the coordination sphere while the remaining five oxygens lie approximately in a plane. The metal ion is also coordinated to a water molecule. The SCN anion 

Spatial structure of the 2 2 complex between [18]crown-6 and CsNCS 

3 Effects of Anion and Cation Type on Complex Stnicture 

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