Rate-controlling step, chelate formation

In additions of hydride donors to a-chiral carbonyl compounds, whether Cram or anti-Cram selectivity, or Felkin-Anh or Cram chelate selectivity occurs is the result of kinetic control. The rate-determining step in either of these additions is the formation of a tetrahedral intermediate. It takes place irreversibly. The tetrahedral intermediate that is accessible via the most stable transition state is produced most rapidly. However, in contrast to what is found in many other considerations in this book, this intermediate does not represent a good transition state model for its formation reaction. The reason for this deviation is that it is produced in an... 

Most group A metal ions have the electronic configuration of the rare gases, and the interaction of the metal with a ligand is primarily electrostatic in nature. Because the charge density (oc charge radius) is small, the water molecules are weakly held and the rate of their loss is comparable with the diffusion-controlled value of around 10 —10 s. The complex formation rate-constant for an ion in this group is therefore approximately equal to the maximum possible value, unless chelation steps are important, and the stability of the complex is reflected in the dissociation rate constant. 

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