Significance & applications of polarizability for nucleophiles

Question

One of the reasons given for the greater nucleophilicity of iodine ion compared to fluorine ion is the greater polarizability of the large electron cloud of iodine. The other reason given is that it is less crowded by polar protic solvent molecules.

However, when the solvent is changed to a polar aprotic solvent the nucleophilicity order is reversed.

If polarizability had any significant role to play, the order would've not been completely reversed. It seems to me that the only reason iodine ion is more nucleophilic than the fluoride ion is because it is less crowded by the solvent.

Now, this leads me to believe that polarizability is not a very important factor and should only be used when comparing nucleophiles of the same basicity, but of different size (and hence polarizability) in a polar aprotic spent.

But why then, for uncharged nucleophiles does polarizability dominate over basicity in determining the order of nucleophilicity (regardless of the solvent used)?

Answer 1

The examples the site you mentioned gave of uncharged nucleophiles were: the hydrides of Group V and VI. They say that the bigger atoms can have a more effective overlap of atomic orbitals and thus, the bigger the uncharged nucleophile, the stronger it is.

I would like to dispute this reasoning and answer your question regarding this trend.

How we should see a nucleophile should always primarily be in terms of electron donation because a nucleophile donates electron density to electrophiles to form bonds. This is a very important consideration.

For bigger atoms, the valence electron pair, which is being donated to the electrophile, is further away from the nucleus and is more shielded from the nuclear charge. Thus, it is easier for donation to take place. This should be the reason why uncharged nucleophiles with bigger atoms are more nucleophilic; that is, they are more capable of donating the electron pair of theirs. My explanation of the trend has nothing to do with polarisability.

The reasoning stated is incorrect because although bond formation is easier for bigger atoms, it does not mean that the overlap of atomic orbitals is more effective. In fact, it is likely less effective because the atomic orbitals used are more diffused (Recall that as the principle quantum number n increases, the orbitals become more diffuse in terms of electron density). Thus, their reasoning is nonsensical.

Hope I have clarified everything you were not sure of.

Answer 2

It seems to me that both trends end up pointing the same best uncharged nucleophiles, that means, polarizability increases down the periodic table (bigger electron clouds have a better overlap in the Sn2 transition state) while basicity decreases (the electron density is more disperse around bigger atoms). So polarizability does not dominate over basicity anyway, because it would be unfair to relate a property which is considerably increasing with another which is decreasing down the PT, it is just that polarizability has a more pronounced effect then basicity in this situation. If you compare analogous alcohols and thiols you can see this trend.