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I have a rather quick question on the definition of Nucleophilicity. To get straight to the point, am I right in saying that the most appropriate definition of Nucleophilicity, regardless of whether the reaction takes place in solvent or in the gas phase, is the following; Nucleophilicity is a measure of how close in energy the HOMO of a Lewis Base is to the LUMO of a Lewis Acid?

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As far as I remember from my lectures nucleophilicity has the same definition problems as electronegativity. So unlike e.g. ionisation energy you cannot define an absolute scale of ordering which you can verify experimentally and just as in the case of electronegativity there are several possible definitions (compare https://en.wikipedia.org/wiki/Nucleophile). Similar to electronegativity the definitions arrive on quite similar results. ($\ce{Br^-}$ is a good nucleophile in polar, protic solvents...) Other formulations should be "qualitatively consistent" with previous ones.

Your alternative formulation has two big problems:

  1. There is no solvent dependency.
  2. You completely omit steric effects and nucleophilicity is also a kinetic phenomenon.
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The ability for an atom or ion or molecule to give away extra electron and form bond with nucleus or some other is known as nucleophilicity

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