`[SiF_(6)]^(2-)` is known where as `[SiCl_(6)]^(2-)` not. Reason is

Explanation

The main reasons are :
(i) Due to smaller size of F as compared to Cl, six small F atoms can be easily accomodated around Si atom but six large Cl atoms cannot.
(ii) The lone pair of electrons on F are present in a smaller 2p-orbital but in Cl they are present on a larger 3p-orbital. Therefore, interaction of F lone pair electrons with d-orbitals of silicon are stronger than that of Cl lone pairs.