`[SiF_6]^(2-)` is known to exist whereas `[CF_6]^(2-)` does not exist. Explain.

Explanation

Silicon can extend its coordination number beyond four because it possess vaccant d-orditals. Hence, `[SiF_6]^(2-)` exists. On the other hand, C has no vaccant d -orbitals in its valence shell and thus it cannot extend its coordination number beyond four. Hence, `[CF_6]^(2-)` does not exist.