Boron is unable to form `BF_(6)^(3-)` ions due to

Explanation

(a) Standard electrode potential values for two half-cell reactions suggest that aluminium has high tendency to form `Al_((aq))^(3+)` ions, whereas `Tl^(3+)` is not unstable in solution but is a powerful oxidising agent also. Thus `Tl^(+)` is more stable in solution than `Tl^(3+)`. Aluminium being able to form `+ 3` ions easily, is more electropositive than thallium.
(b) Anhydrous aluminium chloride is partially hydrolysed with atmospheric moisture to liberate `HCl` gas. Moist `HCl` appears white in colour.
`AlCl_(3) + 3H_(2)O rarr Al(OH)_(3) + 3HCl`
( c) Due to non-availability of d-orbitals, boron is unable to expand its octet. Therefore, the maximum covalency of boron cannot exceed `4`.