What Factors Affect the Neucleophilicity and Basicity Order of I- Br- Cl- F-?

[cupid.callin]

I always have been confused about neucleophilicity and Basicity order of I^- Br^- Cl^- F^-

whenever i think of this ... 2 things come in my mind:

1. As F has smallest size so it will hold e^- pair strolgly and thus will provide a very strong center for attack by some + charge ... so it must be a strong base
At same time (i read somewhere that Neucleophilicity depends on polarisibility i.e. how much an ion can deform to form bond ... please correct me if I'm wrong) it must be a bad Neucleophile ue to less polarisibility.
now I is big this e^- pair is delocalized and thus will be a werk weak - spot for attack. now due to large polarisibility it must be a good Neucleophilicity.

2. As F is small so it will hold the e_ pair strongly and thus will not allow it to be shared by atoms easily. thus it must be a weak base

I read a page on wiki and understood this concept clearly but forgot what i understood over time
i tried ... a lot ... i can't find that page again ...

please help me
also please tell me on what things Neucleophilicity and Basicity depends ...

Thanks !

 

[Nemus]

You are fishing in deep water indeed! You have probably hit on one of the areas where textbooks are least trustworthy. I summary, there are no such things as hard and soft acids, bases ornucleophiles. But, if you assume there are, you often end up with the correct answer and that's why the myth lives on.
In a vaccum, F- is the strongest nucleophile in the series, like you would expect. Poarizability has nothing to do with this. However, in a polar solvent F- is most strongly solvated, screened from interaction and therefore the weakest nucleophile.
The acidity/basicity order is as expected apart from HF being much weaker than expected. But, it isn't really, it just looks like it. Free HF is just a bit weaker than hydrochloric acid but most of the HF in aqueous solution is strongly hydrogen bonded to water or even forms something like HO...HFH. This masks the strenght of HF and makes it in effect a very weak acid.