Question

$[SiF_6^2]-$ is known whereas $[SiCl_6]^{-2}$ is not known.

Answer 1

The size of fluorine is small therefore can be easily arranged around Silicon to form [SiF6]^2- while the size of chlorine is large enough which create steric hindrance to each other therefore cannot be arranged around silicon in excess.

Answer 2

$\huge\boxed{\underline{\mathcal{\red{A}\green{N}\pink{S}\orange{W}\blue{E}\purple{R}}}}$

❤⭐MAIN REASONS:- ⭐❤

1. six large chloride ions cannot be accomodated around ${Si}^4+$ due to $\purple{\tt{Limitation}}$of its size.
2. Interaction between lone pair of chloride ion and${Si^4+}$ is not very strong.