Diebler-Eigen mechanism

The values for the rate constant 0 are nearly the same for the mixed complexes when L = picolinate, sulfosalicylate and 8-oxyquinoline-5-sulfonate indicating and hence it appears that they fit Model 1 (i.e.) Diebler-Eigen mechanism involving a dissociative pathway. This is true in the case of lighter lanthanides. In the case of heavier lanthanides,... 

The values of log/Cj and 0gk2 are 8.85 and 7.70, respectively. Similar conclusions were reached for the tartrate (Yun and Bear 1976), anthranilate (Silber et al. 1969) and picolinate (Erikson et al. 1987) complexation and for mixed-complex formation involving Ln (EDTA)(X) where X is a picolinate (Ekstrom et al. 1980) or a 5-sulfosalicylate anion (Ekstrom et al. 1981). The kinetic parameters for the overall reactions of these systems (Erikson et al. 1987) were consistent with the Diebler - Eigen mechanism (diebler and Eigen 1959, Purdie and Farrow 1973),... 

The complexation reactions usually proceed by the Eigen mechanism (Diebler and Eigen 1959, Eigen 1963, Eigen and Tamm 1962). This mechanism involves two steps, the rapid formation of an outer-sphere association complex (i.e., an ion pair) and the subsequent rate-determining step in which the ligand displaces one or more water molecules. 

Relaxation of complicated ligands may occur as a step in both pathways. Diebler and Eigen 461 indicated the ways in which such mechanisms could be analysed using fast reaction methods. Several studies of Ln(III) complex formation and of the formation of Ln(III) mixed complexes have been analysed. Generally the dissociative mechanism is considered to dominate and we are then concerned with the water exchange rate. Several studies have shown that the rate decreases from La(III) to Lu(III) but there seems to be a minimum rate around Tm(III). This is also seen in the rate of rotation of ligands on the surface of the ions, Fig. 7. There may be a small crystal field term, or another contribution to a tetrad -like effect from the 4f electron core. However in the hydrate the precise relationship between the inner and outer sphere water may also be important as we saw when we discussed the heat and entropy of complex ion formation. 

Diebler, H., Eigen, M., Ilgenfritz, G., Maas, G. Winkler, R. (1969). Kinetics and mechanism of reactions of main group metal ions with biological carriers. Pure and Applied Chemistry, 20,93-115. 

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See also in sourсe #XX -- [ ]

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