Substitution in trans complexes

Beyond the possibility of the conjngate base mechanism, substitution of Y for X in tra i-[M(LL)2BX] (LL = a bidentate ligand) can proceed by three dissociative pathways. If dissociation of X from the reactant leaves a square-pyramidal intermediate that adds the new ligand to the vacant site, the result is retention of configuration, and the product, like the 

FIGURE 12.8 Mechanisms of Base Hydrolysis of A-c/ s-lCofenljClj] . (a) Retention of configuration in dilute hydroxide, (b) Inversion of configuration in concentrated hydroxide. 

FIGURE 12.9 Dissociation Mechanism and Stereochemical Changes for frans-lMfLLIjBX]. 

X dissociation), (c) Less likely trigonal-bipyramidal intermediate (two possible products). 

Another factor that must be considered to rationalize the product stereochemistry is the isomerization rates of the starting complexes (as well as those of the products). Table 12.11 lists data for [Co(en)2(H20)X] isomerization and water exchange reactions. 

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