Lanthanides water exchange mechanisms

The kinetic effect on (1/72—1/7)) is proportional to (Amagnetic field as shown in Fig. 7.23. From the temperature and pressure dependence, the kinetic parameters presented in Table 7.13 were obtained. The two activation parameters, namely the entropy and the volumes of activation, are negative and also are of the same magnitude for all the lanthanide ions. These activation parameters imply a common water exchange mechanism for all the lanthanides studied and possibly an associative activation path of exchange. The activation volume, AV of —6.0 cm3 mol-1 probably reflects the difference between a large negative contribution due to the transfer of a water molecule electrostricted in the second coordination sphere to the first coordination sphere and a positive contribution due to the difference in partial molar volumes of N + 1 coordinated transition state and N coordinated aquo lanthanide ion. It should be noted that the latter difference (in partial molar volumes of Fn(H20)w+i and Fn(H20)jv is due to the increase in Fn-O bond distance (Fig. 7.16). 

EXAFS study on Eu2+ and Sr2+ in both solid state and aqueous solution gave coordination numbers of 8.0 for strontium(II) and 7.2 for europium(II) (228). The water exchange rate measured on the divalent europium aqua ion is the fastest ever measured by 170 NMR (Table XVI) (2). The activation volume is much more negative (—11.7 cm3 mol-1) than those determined on trivalent lanthanide aqua ions clearly indicating an a-activation mechanism which is most probably a limiting... 

Helm and Merbach have summarised efforts aimed at calculating exchange mechanisms for water exchange on first row, second and third row transition metal cations and on lanthanide ions. [26] The limitations within the calculations and where there is consistency with parameters obtained from ambient and high pressure kinetics results have been pointed out. An example of a particular theoretical investigation will be cited below. 

The differences in solvent exchange in HjO and DMF are also reflected in the exchange mechanisms. The activation volumes for water exchange are negative and, within experimental error, essentially constant for the different lanthanides this suggests a concerted associative (I ) mechanism. By contrast, the data for DMF exchange are more consistent with a gradual transition from an to a D mechanism between Tb " and Yb ". ... 

In conclusion, many questions remain about the mechanism of water exchange. While activation volume data support an association mechanism (I ), the ultrasound and kinetic data are interpreted in terms of an overall dissociative mechanism. Kinetic results show a discontinuity in plots of log k values versus lanthanide radius in the region of Sm -Tb which would seem to be good evidence for a change in hydration number in this region. 

Similarity of 34 values for complexation between Mg2+ and SO2-, Cr02 and S2C) and solvent exchange rates of paramagnetic ions to the 34 values obtained by the present ultrasonic studies give evidence for a dissociative type of mechanism. The removal of a water molecule from the solvated lanthanide in step 3 is probably the rate-determining step. We may then write... 

The residence time of the coordinated water tm the mechanism of IS relaxation is based on an exchange between bnlk water molecules surrounding the complex and the water molecule(s) coordinated to the lanthanide. Consequently, the exchange rate ( ex = 1 /tm) is an essential parameter for transmitting the relaxing effect to the solvent. Its measurement is based on the works of Swift and Connick for diluted paramagnetic solutions and consists of an analysis of the transverse relaxation rate as a function of temperature. 

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