Orthosilicate (ion)

Structure of the anion

The orthosilicate ion, or silicon tetroxide anion, is SiO4−
4. The name of the ion is frequently simplified to silicate, as it forms the basis of most mineral-forming silicate oxoanions. An orthosilicate compound is a compound that contains this group.

In its ionic form, orthosilicate is difficult to study. It is an extremely strong base and does not persist in solution, tending to be consumed by water to form extremely weak silicic acid (H₄SiO₄; pK_a2 = 13.2 at 25 °C), a very difficult-to-control reaction that typically leaves only minuscule amounts of the ion and large amounts of hydrated silica condensate.^[1] Its salts, however, are extremely abundant in the form of nesosilicate minerals.^[2] Olivine, which may be referred to as magnesium or iron(II) (ortho-)silicate, is the most abundant mineral in the earth's mantle.

The ion has a classic tetrahedral shape, with one silicon surrounded by four oxygen centres.^[3] The Si–O bond is 162 pm long.^[4]

Organic chemistry

Although very important in inorganic chemistry and geochemistry, the orthosilicate ion is rarely seen in organic chemistry. Two silicate compounds, however, are used in organic synthesis: tetraethyl orthosilicate or TEOS is used to link polymers, and is especially important in the manufacture of aerogels. Tetramethyl orthosilicate or TMOS is used as an alternative to TEOS, and also has a number of other uses as a reagent. TEOS is preferred over TMOS as TMOS decomposes to produce high concentrations of toxic methanol. Inhaling TMOS can result in toxic build-up of silica in the lungs.

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