Select the element (M) whose trihalides cannot be hydrolysed to form $[M(H_2O)_6]^{3+}$.

Boron (B) is the first element of group 13 and belongs to the second period of the periodic table. Its valence shell contains only four orbitals (one 2s and three 2p orbitals) [1]. Due to the absence of vacant d-orbitals, boron cannot expand its covalency beyond 4 [1]. Therefore, its trihalides cannot undergo hydrolysis to form the hexacoordinated complex $[B(H_2O)_6]^{3+}$. In contrast, other elements of the group such as Al, Ga, and In have vacant d-orbitals in their valence shells, allowing them to expand their coordination number to 6 and easily form $[M(H_2O)_6]^{3+}$ complex ions [1, 2].