Group water exchange rate constants

Fig. 26.1 Water exchange rate constants and average residence times for water molecules in the first coordination sphere of aquated metal ions at 298 K. Group 1, 2 and 13 metal ions are shown in pink, and i/-block metal ions in blue. [Based on S.F. Lincoln (2005) Helv. Chim. Acta, vol. 88, p. 523 (Figure 1).]...

There are only few main group metal ion hydrates open to detailed mechanistic study of water exchange by NMR Be , Mg , Al" , Ga" and to a less extent, In" . They provide the opportunity to study the influence of size and charge on exchange rate constant and mechanism without the complicating effects of the variation of the electronic occupancy of the d-orbitals. All of the alkali ions as well as Ca , Sr, and Ba are very labile as a consequence of their relatively low surface charge density. However, indications on water exchange on Sr " can be obtained from... 

Genereux, D.P., Hemond, H.F., 199Z Determination of gas exchange rate constants for a first order stream. Walker Branch Watershed, Tennessee. Water Resour. Res. 28 (9), 2365-2374. Goldhaber, M.B., Aller, R.C., Cochran, J.K., Rosenfeld, J.K., Martens, C.S., Berner, R.A., 1977. Sulfate reduction, diffusion, and bioturbation in Long Island Sound sediments report of the FOAM group. Am. J. Sd. 277,193-237. 

In the case of sulfate and dihydrogen phosphate where the ion pair association constants have been clearly identified, the anation rates are known for the 1 1 outer sphere complex. These rate values vary somewhat, and this perhaps indicates participation of the entering group. But there may be another way to interpret what is going on. These two rates of anion entry are smaller than the rate of water exchange of the aquopentammine complex. In fact, the univalent anion enters the complex from the outer sphere at approximately one-eighth of the rate of water exchange, and the divalent anion enters the complex from the outer sphere about twice as fast. 

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