Give important trends and anomalous properties of boron.

(N/A) Certain important trends can be observed in the chemical behaviour of group-$13$ elements. The trichlorides,bromides,and iodides of all these elements are covalent in nature and are hydrolysed in water.
Species like tetrahedral $[M(OH)_{4}]^{-}$ and octahedral $[M(H_{2}O)_{6}]^{3+}$ exist in aqueous medium,except for boron.
The monomeric trihalides,being electron-deficient,are strong Lewis acids. Boron trifluoride $(BF_{3})$ easily reacts with Lewis bases such as $NH_{3}$ to complete the octet around boron.
Due to the absence of $d$-orbitals,the maximum covalence of $B$ is $4$. Since $d$-orbitals are available in $Al$ and other elements,their maximum covalence can exceed $4$.
Most other metal halides (e.g.,$AlCl_{3}$) are dimerised through halogen bridging (e.g.,$Al_{2}Cl_{6}$). The metal species completes its octet by accepting electrons from the halogen in these bridged molecules.