Question

Glass made up of $SiO_{2}$ reacts with HF to form $SiF_{6}^{2-}$ anion. The analogous $CF_{6}^{2-}$ anion does not form. Why?

Answer 1

i) (SiF$_{6}$)⁻² is possible but (CF$_{6}$)⁻² not possible

ii) Carbon doesn't have empty d- orbitals it cannot accommodate more electrons in it

iii) Carbon valence  is maximum  4 electons while in case of silicon it can be beyond 4 or may be 6

For this reason only (SiF$_{6}$)⁻² anion is possible whereas (CF$_{6}$)⁻² anion is not possbile .

Though there are six fluorine ions in each compounds, the ionic characteristics depends on the valency of the molecule to which they make bonds as well as to the type of formation.