The Kinetic Chelate Effect

The thermodynamic chelate effect, which causes polydentate complexes to be thermodynamically more stable than their monodentate counterparts, was described in Section 10-1-1. The difference in the attachment and dissociation of the second (and third or higher numbered) point of attachment for the ligand is also observed kineticaUy. 

Substitution for a chelated ligand is generally a slower reaction than that for a similar monodentate ligand. Explanations for this effect center on two factors, the increased energy needed to remove the first bound atom and the probability of a reversal of this first step.  

The reaction must have two dissociation steps for a bidentate ligand, one for each bound atom (the addition of water in Steps 2 and 4 is likely to be fast, because of its high concentration)  

The first dissociation (1) is expected to be slower than a similar dissociation of ammonia because the ligand must bend and rotate to move the free amine group away from the metal. The second dissociation (3) is likely to be slow because the concentration of the intermediate is low and because the first dissociation can readily reverse. The 

The chelate effect (Section 10.1.1) causes polydentate complexes to be thermodynamically more stable than their monodentate counterparts. Substitution for a chelated ligand is generally a slower reaction than that for a similar monodentate ligand. Explanations for this effect center on two factors. First, the AH associated with removal of the first bound atom is larger than for a related monodentate ligand. If this atom does separate from the metal center, its kinetic barrier for subsequent reattachment is lower than for a related monodentate ligand since the former remains in close proximity to the metal center. Consider the general scheme below  

---