Explain:
Silicon forms `Si F_6^(2-)` but corresponding fluoro compound of carbon is not known.

Explanation

Silicon atom having vacant 3d orbitals in its valence shell can accept electron pairs from six F- ions and expand its co-ordination no. upto 6 for which `SiF_6^(-2)` exist. But carbon atom has no 3d orbital and can not expand its co-ordination no and so `CF_6^(2-)` does not exist.