Activated Complex

The activated complex is not an intermediate. It sits at a saddle point on the potential energy surface rather than in a well, has no finite lifetime, and cannot be isolated or spectroscopically characterised by conventional means; a genuine intermediate, by contrast, occupies a local minimum and can in principle be trapped. Transition state theory builds the rate constant from the assumption of a quasi-equilibrium between reactants and this configuration, giving the Eyring equation, in which the enthalpy and entropy of activation replace the empirical activation energy of the Arrhenius expression. The entropy term carries mechanistic information: a strongly negative value indicates that two species must associate into an ordered arrangement, while a positive value points to a dissociative step.