Free Energy of Activation and Mechanism

Using the activation parameters outlined in the previous sections, we probe their value in assessing mechanism. 

The actual value of a rate constant for a reaction only infrequently gives a clue to its mechanism. Assessment of values within a reaction series may be more revealing, while comparisons of free energies of activation AG with free energies for the reactions AG, leading to the linear free-energy relationships (LFER), can be very useful in diagnosing mechanism. 

The existence of similar rate constants for a series of reactions in which there is only a change in the central metal or in the ligand in one of the reactants, suggests that a common mechanism is operative for the whole reaction series. The rate constants for aquation of a series of complexes 

A subtle example of steric acceleration associated with a dissociative mechanism is shown in the reaction 

As the size of R3P increases so does the value of k. A popular method of assessing the size of R3P is by using Tolman s cone angle. This is the apex angle of the cone centered on P which just encloses the van der Waals radii of the outermost atoms of R3P. The data are shown in Table 2.3 for (2.134) in CO-saturated C2CI4 at 70°C (Ref. 124) 

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