When we begin to move the reacting molecules toward each other along their collision path, a new source of potential energy is introduced, the repulsion between the electron clouds of the two reacting species. This is shown in the figure below, which shows the approach of the reactant molecules and the corresponding increase in the potential energy of the system. At the transition state , the potential energy has reached a maximum. The relative positions of the reacting species is that of the activated complex . This is not an intermediate in the reaction mechanism.
Intermediates are molecular species that are stable enough to be isolated.
The activated complex is completely unstable and can not be isolated. It is simply a transition between the reactants and products. It resembles both the reactants and the products. These bonds break because the increased potential energy of the system is localized in vibrational modes of the molecules. When the energy stored in one of these bonds exceeds a threshold value, the bond breaks.