ActivatedComplex&intermediate.

In the mechanism of a reaction, the reaction is broken down into one or more elementary steps(or simply "steps") in particle level.

Activated complex : According to the Transition State Theory, in every elementarystep, the reactant species must be raised to a state of high potential energyto form an activated complex (or known as a transition state) before the chemical change of the elementary step can take place.

Intermediate : One or more substances may be formed in early steps and then react. Such substances are called intermediate.

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Example :
The reaction mechanism of a proposed reaction
XY(g) + Z(g) →XZ (g) + Y(g)
is shown as follows :
Step 1 : XY(g) → X(g) + Y(g) (slow)
Step 2 : X(g) + Z(g) → XZ(g) (fast)

X(g) is formed in Step 1 and then reacts in Step 2. Therefore, X(g) is anintermediate.

There is an activated complex in each step.
In Step 1, XY(g) forms an activated complex X......Y before changing to X(g)and Y(g).
In Step 2, X(g) and Z(g) form an activated complex X......Z before changing toXZ(g).