$\bullet$ Due to presence of d-orbital in Si, Ge and Sn they form species like $SiF_6^{2-} , [GeCl_6]^{2-} , [Sn(OH)_6]^{2-}$
$\bullet$ $SiCl_6^{2-}$ does not exist because six large chloride ions cannot be accommodated around $Si^{4+}$ due to limitation of its size.