A silanide is a chemical compound containing an anionic silicon(IV) centre, the parent ion being . The hydrogen atoms can also be substituted to produced more complex derivative anions such as tris(trimethylsilyl)silanide (hypersilyl), tris(''tert''-butyl)silanide, tris(pentafluoroethyl)silanide, or triphenylsilanide. The simple silanide ion can also be called trihydridosilanide or silyl hydride.

Formation

The simplest trihydridosilanides can be produced from a triphenylsilanide in a reaction with hydrogen or at standard conditions. The triphenylsilanide can be made in a reaction of Ph₃SiSiMe₃ with the metal ''tert''-butoxy compound. Reacting hydrogen with potassium triphenylsilyl can yield potassium silanide. Other method to form silanides are to heat a heavy metal

silicide A silicide is a type of chemical compound that combines silicon and a (usually) more electropositive element. Silicon is more electropositive than carbon. Silicides are structurally closer to borides than to carbides. Similar to borides and carb ...

with hydrogen, or react the dissolved metal with

silane Silane is an inorganic compound with chemical formula, . It is a colourless, pyrophoric, toxic gas with a sharp, repulsive smell, somewhat similar to that of acetic acid. Silane is of practical interest as a precursor to elemental silicon. Sila ...

. Atomic metals can react directly with silane to yield unstable molecules with formulae. These can be condensed into a noble gas matrix. With titanium this also yields molecules with hydrogen bridging between silicon and titanium.

Properties

The silanide ion has an effective ionic radius of 2.26 Å. In salts at room temperature the ion's orientation is not stable, and it rotates. But at lower temperatures (under 200K) silanide becomes fixed in orientation. The ordered structure forms the β- phase, whereas the higher temperature and more symmetrical disordered structure is called α- phase. The β- phase is about 15% more compact than the α-phase. The silanide ion has C_3v symmetry. The silicon to hydrogen bond length is 1.52 Å and the H-Si-H bond angle is 92.2°, not far off a right angle. In a range of compounds, the stretching force constant for the Si-H bond is 1.9 to 2.05 N cm^–1, which is much softer than that of silane's 2.77 N cm^–1. Silanide salts are very easily damaged by air or water. Heating to under 414K results in the release of hydrogen and the formationn of a Zintl-phase MSi. If an alkali silande is rapidly heated to 500K another irreversible reaction occurs: :.

Use

Trihydridosilanides have been investigated as

hydrogen storage Hydrogen storage can be accomplished by several existing methods of holding hydrogen for later use. These include mechanical approaches such as using high pressures and low temperatures, or employing chemical compounds that release H2 upon demand ...

materials. Potassium silanide can reversibly gain or lose hydrogen over several hours at 373K. However this does not work for sodium silanide. The rate of hydrogen exchange may be improved by a catalyst. Unwanted reactions may reduce the number of times this process can happen.

List

Under high hydrogen pressure, pentacoordinated and hexacoordinated silicon hydride ions are stabilised including and . More complex derivatives include silanimine -, With a double bond between silicon and the metal a silylene complex is formed. With a triple bond, M≡SiH forms with metals such as molybdenum and tungsten. With less hydrogen, a polyanionic hydride SiH)⁻can be formed.

References

{{Reflist Silanes Anions

Formation

Properties

Use

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Related

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