Sn1CB Mechanism
The rate for the hydrolysis of cobalt(III) ammine halide complexes are deceptive, appearing to be associative but proceeding by an alternative pathway. The hydrolysis of 2+ follows second order kinetics: the rate increases linearly with concentration of hydroxide as well as the starting complex. Based on this information, the reactions would appear to proceed via nucleophilic attack of hydroxide at cobalt. Studies show, however, that in the hydroxide deprotonates one NH3 ligand to give the conjugate base of the starting complex, i.e., +. In this monocation, the chloride spontaneously dissociates. This pathway is called the Sn1CB mechanism.
Read more about this topic: Associative Substitution
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