In chemistry an activated complex is a transitional structure that results from an effective collision and that persists while old bonds are breaking and new bonds are forming. It is therefore a range of molecular geometries along the reaction coordinate:

When molecules collide, some of their kinetic energy is converted into potential energy within the colliding molecules. If enough energy is converted, the old bonds become sufficiently distorted for the colliding molecules to form an activated complex. New bonds can then begin to form. In this brief interval of bond breakage and bond formation, the collision complex is in a transitional state. Some sort of partial bonding exists in this transitional structure. The exact structure of this complex is often difficult to determine.

See also

• transition state