Anomalous Behaviour of Boron - Practice Questions & MCQ

Certain important trends can be observed in the chemical behaviour of group 13 elements. The tri-chlorides, bromides and iodides of all these elements being covalent in nature are hydrolysed in water. Species like tetrahedral [M(OH)₄]^- and octahedral [M(H₂O)₆]³⁺, except in boron, exist in an aqueous medium.
The monomeric trihalides, being electron-deficient, are strong Lewis acids. Boron trifluoride easily reacts with Lewis bases such as NH₃ to complete octet around boron.

It is due to the absence of d orbitals that the maximum covalence of B is 4. Since the d orbitals are available with Al and other elements, the maximum covalence can be expected beyond 4. Most of the other metal halides (e.g., AlCl₃) are dimerised through halogen bridging (e.g., Al₂Cl₆). The metal species completes its octet by accepting electrons from halogen in these halogen bridged molecules.