Silicon Tetrachloride (SiCl4)

Silicon tetrachloride, $\text{SiCl}_4$, is the most important and best studied of the silicon halides. Like carbon tetrachloride, it is a simple, symmetric tetrahedral $\text{MCl}_4$ molecule, with the silicon atom $sp^3$ hybridised and each of the four chlorine atoms occupying an equivalent corner of the tetrahedron. It is commonly prepared by direct chlorination of silicon, or by heating a mixture of silica ($\text{SiO}_2$) with carbon and chlorine.

Despite this structural similarity to $\text{CCl}_4$, $\text{SiCl}_4$'s chemical behaviour towards water is completely different. $\text{CCl}_4$ is essentially unreactive towards water even on prolonged contact, whereas $\text{SiCl}_4$ hydrolyses readily, fuming visibly in moist air and reacting vigorously with liquid water, to give silicic acid and hydrochloric acid:

The reason for this striking difference lies in the availability (or lack) of low-lying $d$ orbitals on the central atom. Hydrolysis proceeds through a nucleophilic-attack mechanism in which an incoming water molecule's oxygen lone pair attacks the central atom; this requires the central atom to be able to expand its coordination number, even temporarily, to five in a transition state. Carbon, restricted to its $n=2$ valence shell, has no accessible $d$ orbitals and so cannot expand its octet or accommodate a fifth incoming group — hydrolysis is kinetically blocked (even though it would be thermodynamically favourable) and $\text{CCl}_4$ simply does not react with water under ordinary conditions. Silicon, however, has vacant, energetically accessible $3d$ orbitals, which allow it to accept the incoming water molecule's lone pair and pass through a five-coordinate transition state on the way to the tetrahedral hydrolysis product — so the reaction proceeds rapidly. …