Polarisability versus Nucleophilicity

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In halides, iodide (I-) is the most polarizable anion, while fluoride (F-) is the least. This raises the question of whether I- should be the most nucleophilic in aprotic solvents due to its ability to distort its electron cloud easily. However, polarizability and nucleophilicity are not directly equivalent. While polarizability can correlate with nucleophilicity, especially for uncharged nucleophiles, negatively charged halides behave differently. In this case, nucleophilicity is influenced more by charge density, which inversely correlates with the size of the anion. Thus, despite I- being more polarizable, fluoride (F-) is considered a better nucleophile due to its higher charge density compared to iodide.
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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?
 
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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?
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).
 
Got it. Thanks
 
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