The term activated complex refers to the molecular compound or compounds that exist in the highest energy state, or activated stage, during a chemical reaction. An activated complex acts as an intermediary between the reactants and the products of the reaction.
A chemical reaction is the reorganization of atoms of chemically compatible and chemically reactive molecular compounds, called reactants. A chemical reaction goes through three stages, the initial stage consisting of the reactants, the transition stage of the activated complex, and the final stage, in which the products are formed.
For example, consider the chemical reaction
where A and B are reactants, A-B is the activated complex, and C and D are the products. For a chemical reaction to occur, the reactant molecules should collide. Collisions between molecules are facilitated by an increase in the concentration of reactant, an increase in temperature, or the presence of a catalyst. Not every collision is successful, that is, produces a chemical reaction. For a successful collision to occur, reactants require a minimum amount of energy, called the activation energy. Once the reactant reaches the energy level, it enters the transition stage and forms the activated complex.
The energy of the activated complex is higher than that of reactants or the products, and the state is temporary. If there is not sufficient energy to sustain the chemical reaction, the activated complex can reform into the reactants in a backward reaction. With proper energy, though, the activated complex forms the products in a forward reaction.