Silanide

A silanide is a chemical compound containing an anionic silicon(IV) centre, the parent ion being SiH−3. The hydrogen atoms can also be substituted to produce more complex derivative anions such as tris(trimethylsilyl)silanide (hypersilyl),^[1] tris(tert-butyl)silanide, tris(pentafluoroethyl)silanide, or triphenylsilanide.^[2] The simple silanide ion can also be called trihydridosilanide or silyl hydride.

Silanide

Names
Other names Trihydridosilanide Trihydridosilicate(1-) Trihydridosilicate(IV)
Identifiers
CAS Number	15807-96-2 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:30561
ChemSpider	19684660
Gmelin Reference	266
PubChem CID	166628
CompTox Dashboard (EPA)	DTXSID801317131
InChI InChI=1S/H3Si/h1H3/q-1 Key: LNVJLOIIRUIQCP-UHFFFAOYSA-N
SMILES [SiH3-]
Properties
Chemical formula	SiH−3
Molar mass	31.109 g·mol−1
Related compounds
Related compounds	Methyl anion, Germyl, Stannyl, Phosphinide, Arsinide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Formation

The simplest trihydridosilanides can be produced from a triphenylsilanide in a reaction with hydrogen or PhSiH₃ at standard conditions. The triphenylsilanide can be made in a reaction of Ph₃SiSiMe₃ with the metal tert-butoxy compound.^[3]

Reacting hydrogen with potassium triphenylsilyl K(Me₆TREN)SiPh₃ can yield potassium silanide.^[4]

Other method to form silanides are to heat a heavy metal silicide with hydrogen,^[5] or react the dissolved metal with silane.^[3]

Atomic metals can react directly with silane to yield unstable molecules with HMSiH₃ formulae. These can be condensed into a noble gas matrix. With titanium this also yields molecules with hydrogen bridging between silicon and titanium.^[6]

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.^[7] 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.^[8]

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.^[8] 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.^[8]

Silanide salts are very easily damaged by air or water.^[7]

Heating to under 414K results in the release of hydrogen and the formation of a Zintl-phase MSi. If an alkali silande is rapidly heated to 500K another irreversible reaction occurs:

46KSiH₃ → K₈Si₄₆ + 38KH + 50H₂.^[9]

Use

Trihydridosilanides have been investigated as hydrogen storage materials.^[10] Potassium silanide can reversibly gain or lose hydrogen over several hours at 373K. However this does not work for sodium silanide.^[5] The rate of hydrogen exchange may be improved by a catalyst. Unwanted reactions may reduce the number of times this process can happen.^[11]

List

name	formula	Crystal system	space group	unit cell	volume	density	comment	references
tetramethyl-1,4,7,10-tetraaminocyclododecane lithium silanide	Li(Me4TACD)SiH3						colourless; unstable	[3]
trisilylamine	N(SiH3)3						mp -105 °C; planar	[12]
tetramethyl-1,4,7,10-tetraaminocyclododecane sodium silanide	Na(Me4TACD)SiH3	tetragonal	P4/n	a=9.77 c=9.45 Z=2	901	1.041	colourless	[3]
	Na8(OC2H4OC2H4OCH3)6(SiH3)2						H is bridge	[13]
trisilylphosphine	P(SiH3)3							[14]
Potassium silanide	KSiH3	cubic		a=7.23	377.9	1.241	pale yellow	[7][15]
	β-KSiH3	orthorhombic	Pnma	a = 8.800, b = 5.416, c = 6.823, Z = 4	325.2			[16]
tetramethyl-1,4,7,10-tetraaminocyclododecane potassium silanide	K(Me4TACD)SiH3•2C6H6	tetragonal	P42/mnm	a=12.3401 c=14.9372 Z=2	2274.6	1.10	colourless	[3]
	[K(18-crown-6)SiH3·THF]							[17]
	[K(18-crown-6)SiH3·HSiPh3]						H is bridge	[17]
	Cp2(Me3P)TiSiH3						purple	[6]
	[(C5H5)2TiSiH2]2	tetragonal	P42/mnm	a = 8.018, c = 16.113, Z = 2			olive green; Ti-SiH2-Ti-SiH2- ring	[18]
	[Cp2Ti(μ-HSiH2)]2						dark blue	[19]
	Cp2Ti(μ-HSiH2)(μ-H)TiCp2						dark yellowish green	[19]
	HCrSiH3							[6]
	[Cp(OC)2Fe]2SiH2	triclinic	P1	a=6.318 b=10.653 c=12.453 α=67.884 β=75.35 γ=72.79 Z=2	732.1	1.742	light yellow	[20]
	[(μ2-CO)Cp2(OC)2Fe2]SiH2						dark red	[20]
	[(μ2-CO)Cp2(OC)2Fe2][Cp(OC)2Fe]SiH						dark red	[20]
	HNiSiH3							[6]
	HZnSiH3							[6]
	[(dtbpCbz)GeSiH3]2•C6H18	monoclinic	P21/n	a 16.144 b 15.0369 c 21.974 β 91.927°				[21]
trisilylarsine	As(SiH3)3							[14]
rubidium silanide	RbSiH3	cubic		a=7.52	425.3	1.824	yellow	[7]
tetramethyl-1,4,7,10-tetraaminocyclododecane rubidium silanide	Rb(Me4TACD)SiH3•2C6H6	tetragonal	P42/mnm	a=12.3934 c=14.9632 Z=2	2298.3	1.223	yellow	[3]
	K0.5Rb0.5SiH3	cubic	P43m	a=12.832	2112.7			[22]
	Mo(CO)(H)(SiH3)(depe)2							[6]
	[Cp(OC)2Ru]2SiH2						beige mp 25	[20]
trisilylstibine	Sb(SiH3)3							[14]
caesium silanide	CsSiH3	cubic		a=7.86	485.6	2.243	yellow	[3][7]
	Cs0.5K0.5SiH3	cubic	P43m	a=13.0965	2246.3			[22]
	Cs0.5Rb0.5SiH3	cubic	P43m	a=13.2982	2351.7			[22]
bis(di-tert-butylphenyl)di-tert-butylcanozalide	[(dtbpCbz)BaSiH3]8		P4/nnc	a=38.7375 c=44.8635				[21]
	[Cp2SmSiH3]3						orange	[6]
	(C5Me5)Sm(SiH3)(THF)(C5Me5)K(THF)						dark red	[23]
	(C5Me5)Eu(SiH3)(THF)(C5Me5)K(THF)	orthorhombic	Pna21	a=19.320 b=16.742 c=10.027 Z=4	3240.0	1.406	orange-red	[23]
	(C5Me5)Yb(SiH3)(THF)(C5Me5)K(THF)	orthorhombic	Pna21	a=19.321 b=16.496 c=9.926 Z=4	3163.7		dark red	[23]
	Cp(iPr3P)Os(H)(Br)SiH3						yellow	[6]
	trans-(Cy3P)2HPtSiH3							[6]

Related

Under high hydrogen pressure, pentacoordinated and hexacoordinated silicon hydride ions are stabilised including SiH−5 and SiH2−6.^[24]

More complex derivatives include silanimine -NHSiH₃,^[25]

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 ^∞
₁[(SiH)⁻] can be formed.^[26]

General organic compounds are termed silylium ions.