Why is HF Not a Lewis Base and MgBr2 a Lewis Acid?

[asdf1]

why's HF not a lewis base? doesn't F- have lone pairs which could be used to donate electrons?
why's MgBr2 a Lewis acid and not a lewis base?

 

[Ester]

well to start with, since F has three lone pairs, it would seem to be a good explanation, except that it is so electronegative that most of the time, electron of hydrogen are with F instead, giving it a dipole moment. A Lewis acid is when it can accept electrons or donate protons, in this case it could do that because hydrogen is just hanging there barely (F is so negative that the hydrogen bond can easily break).

 

[GCT]

Depends on what context you're referring to. HF can act as a lewis base, when it participates in intermolecular hydrogen bonding (and a lewis acid in this case). The LUMO on the hydrogen will interact with the HOMO on the Flourine.

In aqueous solutions, and in the case of its reaction with water, it's an acid, that is the stronger acid.

In either case you've got to consider what the compound is reacting with.

 

[asdf1]

thank you very much! ^^