What determines strength of nucleophile--Polarisability or Electron density

My understanding is that nucleophiles are lewis bases. Apparently nucleophile and base strength are not the same though. I thought Nu strength/Base strength is determined by how well it can donate an e- pair which is thus affected by the electron charge density on the e- donating atom.


However I've come across weak bases but strong nucleophiles Eg: Br-. It's a weak base cause it is large and has low electron density. However Br- is a strong nucleophile and this was attributed to its polarisability.


However I thought the issue of polarisability was more of deciding the hardness of the lewis acids/bases and am thus very confused as to what really determines the strength of a nucleophile/base and how polarisability comes into play.




The most correct answer would be that the bromide ion is both a strong nucleophile (since the electrophiles in reactions involving bromide are usually soft) and both a weak base in aqueous solution(since now the electrophile is the proton, which is too hard to be friends with the soft bromide).


(For the definition of "soft" and "hard" see e.g. [this Wikipedia page](https://en.wikipedia.org/wiki/HSAB_theory))


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Nucleophilicity is a kinetic concept, and differs from basicity which is a thermodynamic concept.


There are some molecules which are both strong bases, like the ethoxide anion, and hence some factors, like having a negative or delta negative charge will contribute to a molecule being a strong base and nucleophile.


However, due to the mechanism of nucleophilic subsitiution via an SN2 reaction, sterically hindered molecules will not be strong nucleophiles. A good example is the text-butoxide anion- strong sterically hindered base, but poor nucleophile.


Likewise, some factors, such as conjugate acid stability will be more important for promoting basicity than nucleophilicity.


Back to your example, Bromide ions are a weak base, because although they have a negative charge, they are fairly stable, but they are strong nucleophiles due to the lack of steric hindrance.


A chloride ion, for example would be both a stronger base and nucleophile. A stronger base because the Kpa of HCl is lower than that of HBr and so the Cl- ion is stable than the Br- ion, and a stronger nucleophile as the Cl- ion is slightly smaller and so less sterically hindered.


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