Silicon compounds are compounds containing the element silicon (Si). As a carbon group element, silicon often forms compounds in the +4 oxidation state, though many unusual compounds have been discovered that differ from expectations based on its valence electrons, including the silicides and some silanes. Metal silicides, silicon halides, and similar inorganic compounds can be prepared by directly reacting elemental silicon or silicon dioxide with stable metals or with halogens. Silanes, compounds of silicon and hydrogen, are often used as strong reducing agents, and can be prepared from aluminium–silicon alloys and hydrochloric acid. Several inorganic compounds have been formed with silicon and other nonmetals such as sulfur and nitrogen; most of these compounds are highly incompatible with water. One of the most useful and successfully marketed inorganic silicon compounds is silicon carbide. Naturally occurring silicon is found in silicate and aluminosilicate minerals. One of the most common silicon compounds found in the Earth's crust is silicon dioxide or silica, which often occurs as quartz. Organosilicon compounds are fairly stable due to the similarity in strength of the Si–C bond to the C–C bond. Organosilicates include silicone polymers.
Silicides
Many metal silicides are known, most of which have formulas that cannot be explained through simple appeals to valence: their bonding ranges from metallic to ionic and covalent. Some known stoichiometries are M6Si, M5Si, M4Si, M15Si4, M3Si, M5Si2, M2Si, M5Si3, M3Si2, MSi, M2Si3, MSi2, MSi3, and MSi6. They are structurally more similar to the borides than the carbides, in keeping with the diagonal relationship between boron and silicon, although the larger size of silicon than boron means that exact structural analogies are few and far between. The heats of formation of the silicides are usually similar to those of the borides and carbides of the same elements, but they usually melt at lower temperatures. Silicides are known for all stable elements in groups 1–10, with the exception of beryllium: in particular, uranium and the transition metals of groups 4–10 show the widest range of stoichiometries. Except for copper, the metals in groups 11–15 do not form silicides. Instead, most form eutectic mixtures, although the heaviest stable ones – mercury, thallium, lead, and bismuth – are completely immiscible with liquid silicon. Usually, silicides are prepared by direct reaction of the elements. For example, the alkali metals and alkaline earth metals react with silicon or silicon oxide to give silicides. Nevertheless, even with these highly electropositive elements true silicon anions are not obtainable, and most of these compounds are semiconductors. For example, the alkali metal silicides (M+)4(Si4−4) contain pyramidal tricoordinate silicon in the Si4−4 anion, isoelectronic with white phosphorus, P4. Metal-rich silicides tend to have isolated silicon atoms (e.g. Cu5Si); with increasing silicon content, catenation increases, resulting in isolated clusters of two (e.g. U3Si2) or four silicon atoms (e.g. [K+]4[Si4]4−) at first, followed by chains (e.g. CaSi), layers (e.g. CaSi2), or three-dimensional networks of silicon atoms spanning space (e.g. α-ThSi2) as the silicon content rises even higher. The silicides of the group 1 and 2 metals usually are more reactive than the transition metal silicides. The latter usually do not react with aqueous reagents, except for hydrofluoric acid; however, they do react with much more aggressive reagents such as liquid potassium hydroxide, or gaseous fluorine or chlorine when red-hot. The pre-transition metal silicides instead readily react with water and aqueous acids, usually producing hydrogen or silanes:
Na2Si + 3 H2O → Na2SiO3 + 3 H2 Mg2Si + 2 H2SO4 → 2 MgSO4 + SiH4 Products often vary with the stoichiometry of the silicide reactant. For example, Ca2Si is polar and non-conducting and has the anti-PbCl2 structure with single isolated silicon atoms, and reacts with water to produce calcium hydroxide, hydrated silicon dioxide, and hydrogen gas. CaSi with its zigzag chains of silicon atoms instead reacts to give silanes and polymeric SiH2, while CaSi2 with its puckered layers of silicon atoms does not react with water, but will react with dilute hydrochloric acid: the product is a yellow polymeric solid with stoichiometry Si2H2O.
Silanes
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