Activated complex

Unstable configurations of atoms formed while a chemical reaction progresses

In chemistry, an activated complex represents a collection of intermediate structures in a chemical reaction when bonds are breaking and forming. The activated complex is an arrangement of atoms in an arbitrary region near the saddle point of a potential energy surface. The region represents not one defined state, but a range of unstable configurations that a collection of atoms pass through between the reactants and products of a reaction.

From Wikipedia, under CC BY-SA. More on occurri.