Question

The relative strength of Lewis acidity of boron trihalides is found to follow the following trend. BI3>BBr3>BCl3>BF3

Answer 1

Lewis acidic strength is said to be high if a substance is a good electron pair acceptor.

But in Boron trihalides, the lewis acidic strength of boron decreases as, when we see intramolecularly the halide donate an electron pair towards boron, thus boron will not be able to accept anymore from outside.
So, the lewis acidic strength of boron trihalides decreases.

but this decrease is very high in case of boron trifluoride as fluorine is a better donater to boron due to similar  size.
Hence BF3 will have the least Lewis acidity.

Thus the order is BI3>BBr3>BCl3>BF3