Important Trends and Anomalous Properties of Boron

Important Trends and the Anomalous Behaviour of Boron

Boron, as the first (and smallest) member of Group 13, behaves quite differently from the rest of its group — a pattern the NCERT syllabus repeats for the first member of several p-block groups.

Hydrolysis of trihalides. All the tri-chlorides, tri-bromides and tri-iodides of the group are covalent and hydrolyse readily in water. Every member EXCEPT boron gives the tetrahedral $[M(OH)_4]^-$ ion and the octahedral $[M(H_2O)_6]^{3+}$ ion in aqueous solution. Boron cannot form either of these, because it has no d orbitals to expand its coordination number beyond 4 — it simply cannot accommodate the extra ligands needed for a coordination number of 6.

Lewis acidity from electron deficiency. The monomeric trihalides of Group 13 elements have only six electrons around the central atom, making them electron-deficient and therefore strong Lewis acids. Boron trifluoride is the standout example — it reacts eagerly with Lewis bases such as ammonia to complete its octet:

The covalence-4 ceiling. Because boron has no accessible d orbitals, its maximum covalence is fixed at 4 — it can never form a species such as $BF_6^{3-}$. Aluminium and the heavier members of the group, having accessible d orbitals, are not bound by this limit and can expand their covalence beyond 4. …