Question

BH3 is soft acid while BF3 is hard acid. Give reasons.

Answer 1

According to SHAB(Soft Hard acid base ) theory,Soft acids and bases are larger and more polarizable , whereas hard acids and bases are small and non-polarizable.In BH₃ , it is a larger compound where electrons are loosely packed,they also have low change and easily polarisable.So it a soft acid.In BF₃ , it is a smaller compound where electrons are tightly packed,they are not easily polarised.So it is a hard acid.
In otherhand,due to hydrogen bonding in BH₃,it becomes soft while due to presence of p-p back bonding in BF₃ , it becomes harder.