Selectivity of Cation Complexation

For the seemingly simplest case of spherical metal ion complexation, the hole-size fit often, but not necessarily, holds. Fig. 2.5 illustrates the classical case where the cavity diameter of an ionophore determines the selectivity of cation complexation according to its radius [31]. As long as sufficient contact between the metal ion and the donor atom of the hgand is possible, the complexation free energy will be just a linear function of the number of such interactions and their donor quality... 

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