$\bullet$ 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.