Correlations ligand exchange

Biirgi studied also a series of five coordinated cadmium complexes, 38, that contain three equatorial sulfur ligands, but in which the fourth and fifth, axial ligands, X and Y, are sometimes iodine, sometimes sulfur, and sometimes oxygen (84). The structural correlations have a clear interpretation in terms of the ligand exchange reaction and are reminiscent of the kind of process that is believed to occur in S 2-type nucleophilic substitution reactions ... 

Table 19 reports multicollisional dissociation thresholds of the selected complexes. A direct correlation exists between them and the proton affinity (PA) of the guest. However, although chiral specificity has been observed in similar systems using ligand-exchange reactions (see previous sections), the results of Table 19 show no such specificity. The differences in binding of the Ala and Phe enantiomers are evidently too small to be measured with this method. 

The correlation between rate of in-plane acetate ligand exchange [Eq. (1)] and catalytic activity for hydration of acetonitrile [Eq. (4)] has been studied for a series of Pt4 cluster complexes (42). The results show that there is a positive correlation. In general, as the exchange rate increases, the catalytic activity also increases. The correlation was not simply linear, however. Other factors, such as steric effects or electrostatic effects, also affect the catal3ftic activity. 

If concentrations of carbenium ions are too low to be observed directly, they must be detected indirectly in kinetic studies of the racemiza-tion of optically active dormant species, ligand exchange and/or detailed studies of the effect of substituents, solvent and salts. Some of the most convincing and elegant work in this area was presented in Chapter 2 using primarily benzhydryl derivatives. As discussed in the next section, correlations between ionization rates and equilibrium constants, rates of solvolysis and rate constants of electrophilic addition can be interpolated and in some cases extrapolated to cationic polymerizations of alkenes to evaluate the reactivities of various active species and the dynamics of their isomerization. 

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