 Solution

A transition structure is a saddle point on the potential-energy surface: motion along the reaction coordinate lowers energy in either direction. It has one unstable vibrational direction and no intermediate residence basin.

An intermediate is a local minimum, separated from neighboring states by barriers, with a finite lifetime. It may be short-lived, but that does not make it a transition structure. The elementary SN2 reaction step has a transition structure and no distinct bonded intermediate.