Substitution by Water, Aquation and Basic Hydrolysis

Reactions in which water replaces one of the ligands of a complex, for example, 

The rate constants for aquation of the polyamminedichloro complexes (min- 1, 25° C), when appropriately interpreted, also suggest aquation via dissociation. Six of these are listed  

Two trends are apparent. From the first three rate constants we see that increasing the crowding a little about the cobalt atom slows down aquation, whereas the final four constants demonstrate that beyond a certain degree of crowdedness, further crowding accelerates aquation strikingly. The second trend probably reflects steric assistance to breakage of a Co—Cl bond and suggests that, here at least, the dissociation mechanism operates. 

The rate trend in the first three complexes might, at first glance, be attributed to steric hindrance to attack on cobalt by the incoming water molecule, but we know from many eases observed in organic chemistry that if steric hindrance of this sort operates, the effects are generally 

The base-catalyzed hydrolyses of cobalt complexes are generally first order in hydroxide—that is, 

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