Q.[SiF6]2– is known whereas [SiCl6]2– not. Give possible reasons.

Silicon, the second most abundant element in the earth's crust, occurs combined as silica and silicates rather than free, and differs from carbon chiefly in having accessible 3d orbitals, letting it expand its coordination number beyond four. This is why $SiF_6^{2-}$ readily forms (six small fluoride ions can pack tightly enough around silicon) while the analogous $SiCl_6^{2-}$ does not (the larger chloride ions cannot be accommodated six-around-one without excessive crowding) — the same d-orbital access that lets $SiCl_4$, unlike carbon's $CCl_4$, hydrolyse readily in water, going through an intermediate in which silicon first accepts a lone pair from water's oxygen into an empty d orbital. Silicon dioxide ($SiO_2$, silica) is a covalent, three-dimensional network solid in which every silicon is tetrahedrally bonded to four oxygens and every oxygen bridges to a second silicon, so the whole crystal is effectively one giant molecule; despite the very …