With respect to bimolecular substitution and bimolecular elimination reactions, both electron delocalization and polarizability have an analogous effect on a species, making them good leaving groups.
For example I- is a good leaving group since its negative charge is spread out over a large area.
Also, the tosylate ion is a good leaving group because of electron delocalization where its electrons are spread out over a large area due to resonance stabilization.
However, the analogy seems to break down when considering their nucleophilicity in aprotic media, I- is a good nucleophile and tosylate is not.
Is there a fundamental problem of the analogy between polarizability and electron delocalization?