Q.What are electron deficient compounds? Is BCl3 an electron deficient species? Explain.

A compound is electron deficient if its central atom's Lewis structure has fewer than an octet of electrons around it. BCl3 is a classic example — boron ends up with only 6 electrons, making it a strong Lewis acid.

What is an electron deficient compound?

Most stable Lewis structures obey the octet rule — the central atom is surrounded by 8 electrons (4 bond pairs, or a mix of bond and lone pairs). An electron deficient compound is one where the central atom, after forming all its normal covalent bonds, is surrounded by FEWER than 8 electrons — it has an incomplete octet and an empty valence orbital available to accept an electron pair. Such compounds are typically formed by elements of Group 13 (boron family), which have only 3 valence electrons.

Is BCl3 electron deficient?

Yes. Boron's ground state configuration is [He] 2s² 2p¹ — it has only 3 valence electrons. In BCl3, boron forms 3 covalent (sp² hybridised) bonds, one to each Cl atom, using all 3 of its valence electrons.

• Total electrons around B in the Lewis structure = 3 bond pairs × 2 electrons = 6 electrons
• This is 2 electrons short of a full octet, so BCl3 is electron deficient.
• Geometrically, BCl3 is trigonal planar (sp² hybridised, bond angle 120°) with an empty, unhybridised 2p orbital on boron perpendicular to the molecular plane.

Consequence …