Which one of the following compounds of Group-\( 14 \) elements is not known?
\( [\mathrm{SiCl_6}]^{2-} \)
\( [\mathrm{Sn(OH)_6}]^{2-} \)
\( [\mathrm{SF_6}]^{2-} \)
\( [\mathrm{GeCl_6}]^{2-} \)