[spirochete]

BF₃ is considered a hard acid.

BBr₃ is a borderline hard acid.

BH₃ is a soft acid.

This trend apparently reflects how much electron density is being donated into boron's empty P orbital.  Fluorine donates the most (somewhat surprisingly based on its electronegativity) because it has the best energy overlap with the boron P orbital.  Hydrogen donates the least electron density because it only has a spherical S orbital so there must be little or no overlap.

I'm not sure how to explain this in terms of HSABT.  I know a hard acid is going to be less polarizable and have more ionic interactions, but I can't see how that relates to the trend.