Ligand-exchange mechanism, inner-sphere surface complex formation

Strongly binding anions, such as molybdate, sorb on oxides by a ligand-exchange mechanism that involves the exchange of an oxide-surface hydroxyl group (SOH) for an aqueous anion (A ) (Stumm, Kummert, and Sigg, 1980 Sposito, 1984 Stumm, 1992). This reaction results in the formation of an inner sphere complex and is illustrated as follows ... 

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. 

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