The relative Lewis acid character of boron trihalides is found to follow the following order.
BBr3>BCl3>BF3 but the expected order on the basis of electronegativity of the halogens (electronegativity of halogens decreases from
F to
I ) should be,
BF2>BCl2>BBr2 This anomaly is explained on the basis of the relative tendency of the halogen atom to back donate its unutilised electrons to vacant
p -orbital of boron atom. In
BF3, boron has a vacant
2p -orbital and each fluorine has fully filled unutilised
2p -orbitals. Fluorine transfers two electrons to vacant
2p -orbital of boron, thus forming
pπ−pπ bond.
This type of bond has some double bond character and is known as dative or back bonding. All the three bond lengths are same. It is possible when double bond is delocalized. The delocalization may be represented as :
The tendency to back donate decreases from F to I as energy level difference between
B and halogen atom increases from
F to
I. So, the order is
BF3<BCl3<BBr3