Let’s take the example of an electron-rich, electronegative species with a negative charge like F- . Yes, fluoride has a full octet, and yes, the fluorine nucleus is highly electronegative. However, having a negative charge (i.e. high charge density) is somewhat destabilizing.
If the fluoride ion were to combine with H+ [for example] yes fluoride would go from “owning” a pair of electrons to “sharing” a pair with hydrogen, but this is not really a “compromise”. This is like a happy marriage where the partners are better off together than they were apart! The new H-F bond is worth about 136 kcal/mol and the resulting species is neutral.
What’s happening here is that in the process of creating the bond, that pair of electrons has gone from being localized around the fluorine nucleus to being in a larger volume (the H-F sigma bond) which significantly lowers the overall energy.

In practice there are no point charges in nature so a real-life example would be NaF plus HCl [so we’d also obtain NaCl in the bargain.] Still a highly exothermic process.

It’s only [neutral] neon and its relatives who can obtain a full octet of electrons and not have to “compromise” through bonding.
Thanks for the question, Mallika!

The question I have is very basic (the problem is, over time, I keep forgetting or getting confused with the basic facts that help understand/figure out why reactions happen the way they do). My question is: If we have an electronegative atom, rich in electron density, perhaps with a negative charge on it, why would it attack another atom that is slightly deficient in electrons (like in the examples above). Don’t electronegative atoms like to “keep” their negative charges? I know that electrons flow from higher density to lower density, and it makes sense. But I can never shake off the feeling that the electronegative atom is “compromising”.

Thank you!

You’re right, he has based an entire research program out of that! Scott Denmark gives great talks. I love it when you can come away from a lecture not only having a good idea of what the researcher does, but also having learned something: he really dug into the physical organic chemistry aspect of it. I remember vividly him asking the audience (via a story about Albert Eschenmoser) “where are the electrons on the borohydride ion”, and when you think about it, the boron actually has a partial positive charge. Then he extended it to silicon. Great stuff.

http://www.scs.illinois.edu/denmark/research/index.html

The idea is that for certain acid-base pairs, coordination increases the partial charge on the coordinating atoms, in contrast to the intuitive “acid + base = neutralization” idea. Frustrated Lewis acid/base pairs are kind of the same idea, taken to an extreme:

http://en.wikipedia.org/wiki/Frustrated_Lewis_pair

Great post! Using electronegativity to think about formal charge is GREAT advice.

Thanks for the spot. Fixed!!!