(4)

Maximum covalency of boron is four due to the absence of $\mathrm{d}$-orbitals. Hence, its trihalide cannot be hydrolysed to produce an ion of the form ${\left[\mathrm{M}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{3+}$.

(3)

Boron does not have vacant $d$-orbitals in its valence shell, so it cannot expand its covalency beyond 4 i.e., 'B' cannot form the ions like ${\mathrm{MF}}_6^{3-}.$