Question

Why strength of π-bonds of
BF₃​​​​>BCl_{​​​​3}>BBr_{​​​​3}

Answer 1

Though Pi bonds are formed, they are polar in nature because of significant electronegativity difference between B and F.So the bond is strongest in BF3.When you go down the group in periodic table the electronegativity difference decreases, hence the bond polarity also decreases and bond strength. So the order is:
BF₃​​​​>BCl_{​​​​3}>BBr_{​​​​3}