The relative Lewis acid character of boron trihalides is found to follow the following order,

but the expected order on the basis of electro ‐ negativity of the halogens (electronegativity of halogens decreases from to I) should be,

This anomaly is explained on the basis ofthe relative tendency of the halogen atom to back donate its unutilised electrons to vacant ‐ orbital of boron atom. In , boron has a vacant ‐ orbital and each fluorine has fully filled unutilised

‐ orbitals. Fluorine transfers two electrons to vacant ‐ orbital of boron, thus forming 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 to I as energy level diff erence between and halogen atom increases from to I. So, the order is