= Solution
A \b[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 \b[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.
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