Solvent exchange reactions experimental determination

Examinations of possible correlations between the volume of activation and the entropy of activation for series of similar reactions have been reported for reactions of transition metal coordination compounds, such as solvent exchange, ligand substitution, or isomerization.163 167 A limiting factor in a potential correlation may be the lack of precision that often attends experimental determination of the entropy of activation. Attention has been drawn specifically to the qualitative nature of the correlations between the two parameters for solvent exchange at some 3 + cations, and at square planar Pd2+ and Pt2+ ions.168... 

There is one special experimental indication of this mechanism. If the reaction is carried out in a deuterated solvent (D20 instead of H20) the rate of the reaction increases. This is a solvent isotope effect rather than a kinetic isotope effect and needs some explanation. If you examine the three examples of SAC in the previous pages you will see that they share these characteristics a fast proton exchange is followed by a rate-determining step that does not involve the making or breaking of any bonds to hydrogen. In general terms ... 

Deductions about the structure of the transition state are based on the measured values of and 02 and hence the kinetic study involves the determination of values for 0t and 02 and this can be achieved in three ways. If the kinetic data are sufficiently precise values of 0] and 02 may be obtained by fitting eqn. (127) to the experimental values of kn/k0 in different mixtures of H20 and D20 [47, 212(b)]. An alternative method involves measurement of the value of from the fraction of deuterium found in the product (product isotope effect). This method is applicable providing the product of reaction, and all intermediates along the reaction pathway after the transition state, do not exchange the proton at position 1 with solvent. If this is the case the fraction of D found in the product will be the same as the fraction of D in the transition state at position 1. This value of 4>i when combined with the rate ratio in H20 and D20 (128) will give a value for 02 [42, 47b, 122, 212(b)]. Thirdly a value for... 

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