(ii) Silicon atoms has vacant 3d orbtials in its valence shell. It is in a position to accept electron pairs from six `F^(-)` ions and expand its co-ordination number upto 6. Therefore, the compound `SiF_(6)^(2-)` can exist. on the other hand, carbon has no vacant 3d orbitals and cannot expand its co-ordination number upto six. tehrefore, the compound `CF_(6)^(2-)` does not exist.