Polarizability correlates reasonably well with nucleophilicity, but really only when the nucleophile is uncharged. So H2S is a better nucleophile than H2O. But in the case of negatively charged halides, the nucleophilicity is driven more by the charge density, which correlates inversely with the size of the anion (that is, iodide has a lower charge density than fluoride, so fluoride is a better nucleophile).Half Infinity said:In halides, I- is the most polarisable and F- is the least. So shouldn't I- be most nucleophilic ( in aprotic solvent) as its cloud should most easily be distorted and transition state with electrophile should be achieved most easily? Or is polarisability very different from nucleophilicity?