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 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.
F3B + :NH3 → F3B ← NH3
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.