Dissociative substitution reactions octahedral compounds

Throughout our discussion, we still have not answered the fundamental question What is the preferred mechanism of substitution reactions of octahedral coordination compounds The answer is that dissociative mechanisms are preferred. In this section, we investigate some of the evidence that supports such a conclusion. We cite three different types of reactions (1) the rates of exchange of water molecules, (2) anation reactions, and (3) aquation reactions. 

To conclude this section, we have seen thatvarious pieces of data from (1) rates of water exchange, (2) anations, and (3) various aquation reactions all seem to generally favor a dissociative mechanism for substitution reactions of octahedral compounds. 

Now that we have fairly well established that the dissociative mechanism generally applies for the substitution reactions of octahedral complexes, we are in a good position to begin to answer some of our earlier (p. 100) critical questions about inert versus labile complexes. As defined earlier,and inert-xro, kinetic terms describing the rates of reactions of coordination compounds. As you should recall from earlier courses, rates depend on the magnitude of the energy of activation, of the ratedetermining step. 

It had been hoped that the reaction of [MoOaCdien)] with oxine, to yield the known compound [Mo02(OH)2(oxine)], would be amenable to kinetic study, to provide rate and mechanism data pertaining to substitution at octahedral molybdenum(vi). Sadly the proposed investigation proved impossible, as [MoOaCdien)] was found to be extensively hydrolysed in aqueous solution. An understanding of multidentate ligand-replacement reactions is helped by knowledge of dissociation mechanisms for these chelates. Recent studies resulting in kinetic or mechanistic information on the dissociation of amino-carb-... 

Turning from the commonly studied tetrahedral compounds of tin to octahedral complexes, the kinetics of substitution at SnCl4py2 in nitrobenzene have been investigated. The activation parameters are reported to an astonishing precision the mechanism for solvolysis, as for chloride exchange, is said to be dissociative despite the very different rates reported for the two reactions. 

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