Give important trends and anomalous properties of boron. 

Certain important trends can be observed in the chemical behaviour of group-$13$ elements. The trichlorides, 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.

It is due to the absence of $d$-orbitals that the maximum covalence of $B$ is $4$ . Since the $d$ orbitals are available with $\mathrm{Al}$ 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.