Base-Catalyzed Reactions of Complexes

Although this reaction was discussed earlier, it is mentioned here because it is catalyzed by solid acids such as NH4SCN, which provide H+ ions that bond to the pairs of electrons after breaking them loose from the metal. Over a rather wide range of catalyst concentrations, the rate is linearly dependent on the amount of solid acid. Once the NH4+ ion donates a proton, NH3 is lost and the protonated ethyl-enediamine molecule is the acid that remains and continues to catalyze the reaction. While base catalyzed reactions of complexes may be better known, there are many acid catalyzed reactions as well. 

The mechanism that has been established for this reaction involves the reaction of OH with the complex to produce its conjugate base by removing a proton from a coordinated NH3 molecule in a rapid equilibrium. 

In this mechanism, the rate-determining step involves the dissociative reaction of the conjugate base. Because of this, the mechanism is known as the SN1CB mechanism, in which the substitution is 

The equilibrium constant for the formation of the conjugate base can be written as 

This expression shows that the rate is proportional to [OH ]. If the concentration of OH is held constant, the rate law can be written as 

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