activated complex
Think of two dancers about to swap partners. For one tense instant, all four hands are linked — the old hold not quite let go, the new hold not quite taken. Nobody can stay frozen like that; the moment the music moves, the grips resolve into the new pairing. The activated complex is the chemical version of that tangled, all-hands-joined arrangement of atoms.
More precisely, the activated complex is the actual cluster of atoms as they exist at the top of the energy barrier, with partially broken old bonds and partially formed new ones. In practice the term is used almost interchangeably with transition state, with a subtle difference of emphasis: 'activated complex' names the physical grouping of atoms, while 'transition state' names the point and condition at the summit of the energy path. Both describe the same fleeting, high-energy configuration that decides whether a collision succeeds.
Why it matters: this idea is the heart of transition-state theory, which calculates reaction rates by treating the activated complex as a quasi-species in equilibrium with the reactants. The honest caveat is the same as for the transition state: the activated complex cannot be isolated or stored, because it sits at an energy maximum and falls apart within a single vibration. It is a powerful conceptual tool, not a bottleable chemical.
When a hydrogen atom transfers between H2 and another H, the brief three-atom cluster H···H···H — one bond half-broken, one half-formed, stretched in a line — is the activated complex. It exists for less than a trillionth of a second before settling into the new H2.
The all-hands-joined cluster at the peak — too short-lived to ever isolate.
Treat 'activated complex' and 'transition state' as the same thing for most purposes. The dotted-bond notation (A···B···C) is the usual way to draw one.