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.
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