In boron halides, boron involves sp2 hybridization sing one s and two p-orbitals. Each of the sp2 hybrid orbital overlaps with 2p orbital of alogen to form B-X bonds. As 2p orbital is left vacant in e valence shell. In these compounds, octet of boron is ot complete because they have only six electrons round boron. Therefore, these halides are referred to as electron eficient compounds.
