10.2 Important Trends and Anomalous Properties of Boron

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 $\left[\mathrm{M}(\mathrm{OH})_4\right]^{-}$and octahedral $\left[\mathrm{M}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{3+}$, except in boron, exist in aqueous medium.

The monomeric trihalides, being electron deficient, are strong Lewis acids. Boron trifluoride easily reacts with Lewis bases such as $\mathrm{NH}_3$ to complete octet around boron.
$\mathrm{F}_3 \mathrm{~B}+: \mathrm{NH}_3 \rightarrow \mathrm{~F}_3 \mathrm{~B} \leftarrow \mathrm{NH}_3$
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., $\mathrm{AlCl}_3$ ) are dimerised through halogen bridging (e.g., $\mathrm{Al}_2 \mathrm{Cl}_6$ ). The metal species completes its octet by accepting electrons from halogen in these halogen bridged molecules.

Example 10.3: Boron is unable to form $\mathrm{BF}_6^{3-}$ ion. Explain.

Solution: Due to non-availability of d orbitals, boron is unable to expand its octet. Therefore, the maximum covalence of boron cannot exceed 4.