Assertion : ${\text{SiF}}_6^{2-}$ is known but ${\text{SiCl}}_6^{2-}$ is not.Reason : Size of fluorine is small and its lone pair of electrons interacts with $d$ -orbitals of $\text{Si}$ strongly.

The non-existence of ${\text{SiCl}}_6^{2-}$ is primarily due to steric hindrances: chloride ions are much larger than fluoride ions, and six of them cannot be accommodated around the Silicon atom. Additionally, the interaction of Fluorine lone pairs with Silicon's $d$ -orbitals is stronger than that of Chlorine. Both assertion and reason are true, and the reason provides the correct underlying chemical explanation.