Explanation of Inert versus Labile Complexes

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. 

A reaction profile a plot of potential energy versus reaction pathway. Reactants must acquire the energy of activation, E in order to achieve the transition state or activated complex before they can be trarrsfbrmed into products. The higher the ertergy of activation, the slower the reaction. 

So now that we have a fair understanding of why the second- and third-row metals are inert, we can turn to a discussion of the relative labilities of the first-row ions. Recall that most of these are labile except for Co and Cr, which are inert. 

What is the consequence of a gain or loss of CFSE on going from the octahedral reactant to the square pyramidal transition state It makes sense that if there is additional CFSE in the transition state, then its formation is favored and the ratedetermining step is faster. On the other hand, if there is less CFSE in the transition state than in the reactants, this would make it less stable (higher in energy) and more difficult to achieve. Therefore, the reaction would be slower. 

To see the effect of the change of crystal field stabilization energy (ACFSE) more clearly, consider the general substitution reaction shown in Equation (5.36)  

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