$\left[{\mathrm{SiF}}_6\right]{ }^{2-}$ is known where as ${\left[{\mathrm{SiCl}}_6\right]}^{2-}$does not exist because
I) Six large chloride ions cannot be accommodated around ${\mathrm{Si}}^{4+}$
II) Ineraction between lone pair of chloride ion and ${\mathrm{Si}}^{4+}$ is not very strong
III)Silicon is less electronegative than chlorine
IV) ${\mathrm{Si}}^{4+}$ and ${\mathrm{Cl}}^{–}$ ions have same size