There is inter molecular hydrogen bonding in HF and because of this it is weakly dissociated. So AlF 3 is not soluble in anhydrous HF. On the other hand KF is ionic compound and thus it is highly dissociated giving a high concentration of F – ion which leads to the formation of a colourless soluble complex,

AlF 3 + KF ⎯→ K 3 [AlF 6 ].

BF 3 is more acidic than AlF 3 because of the small size of B than that of Al. Thus BF 3 pulls out F – from [AlF 6 ] 3– forming [BF 4 ] – and AlF 3 . Hence AlF 3 is precipitated on adding BF 3 to [AlF 6 ] 3– .

K 3 [AlF 6 ] + 3BF 3 ⎯→ 3K[BF 4 ] + AlF 3 ↓ .