Compounds with beryllium or boron as the central atom are often electron deficient, with fewer than eight electrons around the Be or B atom.
Why don’t nonbonding pairs on the halogen atoms form multiple bonds to the central atoms, thereby satisfying the octet rule?
Halogen atoms are much more electronegative than Be or B; formal charges show these are not important Lewis structures.
Concept 10-4. The octet rule and its three major exceptions molecules with an electron deficiency, an odd number of electrons, or an expanded valence shell.