Rates of water exchange in octahedral aqua complexes

Rates of water exchange in octahedral aqua complexes 

The smaller the ions, the lower are the rates of water replacements. This suggests that the rates are controlled by the elefctric charge of the ion, i.e., by The same holds for group 2 (IIA). An exception is Be , which is probably caused by the complexity of the mechanism of water replacement. There is a large difference in replacement rates of water coordinated to Mg + and Ca, a fact important in enzymatic reactions. 

A further observation is that for M and ions of similar size the ions with smaller charge exchange water faster. Since the bond strength M-OH2 

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Rates of water exchange in octahedral aqua complexes can vary up to 20 orders of magnitude. Modem techniques enable determination of rate constants in the mentioned rate span. There are several reasons for these rate differences. Thus, the replacement rates for univalent cations increase in the order Li" " < Na" " < K" < Rb" < Cs" ". Figure 2.6.1 shows that the ionic radii increase in the same manner. 

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