Abstract

Disilane (Si₂H₆) represents the second member of the silane homologous series (Si_nH_2n+2), formally analogous to ethane in carbon chemistry. This colorless, pyrophoric gas possesses a density of 2.7 g·dm⁻³ at standard temperature and pressure. Disilane exhibits a melting point of -132 °C and boiling point of -14 °C. The compound demonstrates significant reactivity differences from its carbon analog due to weaker Si-Si bonds (approximately 330 kJ·mol⁻¹) and more pronounced susceptibility to nucleophilic attack. Primary industrial applications include chemical vapor deposition processes for semiconductor manufacturing and photovoltaic device production. Thermal decomposition around 640 °C yields amorphous silicon deposits. The molecular structure conforms to D_3d symmetry with a Si-Si bond length of 2.331 Å and Si-H bond length of 1.492 Å.

Introduction

Disilane occupies a fundamental position in silicon hydride chemistry as the simplest catenated silicon compound. First identified in 1902 by Henri Moissan and Samuel Smiles through acid hydrolysis of metal silicides, the compound was initially designated "silicoethane." Systematic investigation by Alfred Stock and Carl Somiesky established disilane within the broader context of hydride silicon chemistry. Unlike its carbon analog ethane, disilane displays markedly higher reactivity and pyrophoric characteristics. This heightened reactivity stems from relatively weak silicon-silicon bonding and the availability of low-lying antibonding orbitals. The compound's significance extends to materials science applications, particularly in semiconductor manufacturing processes where it serves as a silicon source for chemical vapor deposition.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

Disilane adopts a staggered conformation with D_3d point group symmetry, isostructural with ethane. The silicon-silicon bond distance measures 2.331 Å, substantially longer than the carbon-carbon bond in ethane (1.536 Å). Silicon-hydrogen bonds extend 1.492 Å, compared to 1.095 Å for C-H bonds in ethane. The Si-Si-H bond angle is 109.3°, while H-Si-H angles measure 109.5°, consistent with nearly tetrahedral coordination at each silicon center. Molecular orbital analysis reveals a highest occupied molecular orbital primarily constituted of silicon 3p orbitals with some hydrogen character. The lowest unoccupied molecular orbital possesses significant Si-Si σ* antibonding character, facilitating homolytic cleavage of the Si-Si bond. Photoelectron spectroscopy indicates an ionization potential of 10.53 eV for the outermost electrons.

Chemical Bonding and Intermolecular Forces

The silicon-silicon bond in disilane demonstrates a bond dissociation energy of approximately 330 kJ·mol⁻¹, considerably weaker than the carbon-carbon bond in ethane (376 kJ·mol⁻¹). This bond weakness contributes substantially to the compound's enhanced reactivity. Silicon-hydrogen bonds exhibit dissociation energies near 384 kJ·mol⁻¹. Molecular orbital theory describes the Si-Si bond as a conventional single σ bond formed by sp³ hybrid orbital overlap, though with diminished overlap integral compared to carbon analogs. The molecule possesses a dipole moment of 0 D due to its center of symmetry. Intermolecular interactions are dominated by relatively weak London dispersion forces, consistent with the low boiling point of -14 °C. The polarizability of disilane measures 6.97 × 10⁻³⁰ m³, significantly higher than ethane's polarizability of 4.47 × 10⁻³⁰ m³, reflecting the more diffuse electron cloud around silicon atoms.

Physical Properties

Phase Behavior and Thermodynamic Properties

Disilane exists as a colorless gas at standard temperature and pressure with a characteristic acrid odor. The compound condenses to a mobile liquid at -14 °C and freezes at -132 °C. The density of gaseous disilane is 2.7 g·dm⁻³ at 0 °C, substantially higher than air. Liquid disilane demonstrates a density of 0.686 g·cm⁻³ at its boiling point. The vapor pressure follows the equation log₁₀(P/bar) = 4.2387 - 1071.4/(T/K) between 200-260 K. The enthalpy of vaporization measures 20.5 kJ·mol⁻¹ at the boiling point. The critical temperature is 176.3 K, with critical pressure of 34.7 bar and critical volume of 207 cm³·mol⁻¹. The standard enthalpy of formation (Δ_fH°₂₉₈) is 80.3 kJ·mol⁻¹, while the standard Gibbs free energy of formation (Δ_fG°₂₉₈) is 126.7 kJ·mol⁻¹. The entropy (S°₂₉₈) is 272.6 J·mol⁻¹·K⁻¹.

Spectroscopic Characteristics

Infrared spectroscopy of disilane reveals characteristic absorptions at 2149 cm⁻¹ (Si-H asymmetric stretch), 2105 cm⁻¹ (Si-H symmetric stretch), and 975 cm⁻¹ (Si-Si stretch). Raman spectroscopy shows a strong polarized band at 2183 cm⁻¹ assigned to the totally symmetric Si-H stretching vibration. The Si-Si stretching vibration appears at 437 cm⁻¹ in the Raman spectrum. Nuclear magnetic resonance spectroscopy demonstrates a ¹H NMR chemical shift of 3.51 ppm relative to tetramethylsilane, while ²⁹Si NMR exhibits a resonance at -99.5 ppm. Ultraviolet photoelectron spectroscopy indicates ionization potentials at 10.53 eV, 13.35 eV, and 15.72 eV corresponding to electron ejection from molecular orbitals with predominant Si-Si bonding, Si-H bonding, and silicon lone pair character, respectively. Mass spectrometric analysis shows a parent ion at m/z 62 (Si₂H₆⁺) with characteristic fragmentation patterns including m/z 60 (Si₂H₄⁺), m/z 31 (SiH₃⁺), and m/z 30 (SiH₂⁺).

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Disilane undergoes thermal decomposition according to first-order kinetics with an activation energy of 213 kJ·mol⁻¹. The primary decomposition pathway involves homolytic cleavage of the Si-Si bond to form two silyl radicals (SiH₃•), which subsequently undergo various recombination and disproportionation reactions. The decomposition rate constant is 1.6 × 10⁻⁴ s⁻¹ at 640 °C. Disilane reacts rapidly with oxygen, exhibiting spontaneous ignition in air due to formation of highly reactive silylperoxy radicals. Halogenation occurs vigorously with chlorine and bromine, yielding silicon tetrahalides and hydrogen halides. Hydrolysis proceeds readily in aqueous systems, generating silanol compounds and hydrogen gas. Reaction with alcohols follows similar pathways, producing alkoxysilanes. Disilane participates in dehydrogenative coupling reactions with transition metal catalysts, forming higher silanes and polysilanes. The compound serves as a reducing agent in various chemical transformations, particularly toward metal halides and oxides.

Acid-Base and Redox Properties

Disilane demonstrates weak Brønsted acidity with estimated pK_a values exceeding 35 in aqueous solution. Proton abstraction occurs with strong bases such as alkyllithium compounds, yielding disilanyl anions. The compound functions as a moderate reducing agent with a standard reduction potential estimated at -0.6 V for the Si₂H₆/Si₂H₅• couple. Electrochemical oxidation proceeds irreversibly at platinum electrodes with an oxidation peak potential of +0.85 V versus standard hydrogen electrode in acetonitrile solution. Disilane undergoes oxidative addition reactions with various transition metal complexes, forming silyl metal derivatives. The compound is unstable in both strongly acidic and basic aqueous media, decomposing to silicon oxides and hydrogen gas. Stability in non-aqueous solvents varies considerably, with greatest stability observed in hydrocarbon and ether solvents.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The classical preparation of disilane involves hydrolysis of magnesium silicide (Mg₂Si) with dilute hydrochloric or sulfuric acid. This method produces a mixture of silanes (SiH₄, Si₂H₆, Si₃H₈) with disilane typically comprising 20-30% of the product distribution. The reaction proceeds through intermediate metal silicide hydrolysis products that undergo disproportionation. Reduction of hexachlorodisilane (Si₂Cl₆) with lithium aluminum hydride in diethyl ether provides a more selective route, yielding disilane in 45-60% yield after purification. The reaction follows the stoichiometry: 3Si₂Cl₆ + 4LiAlH₄ → 3Si₂H₆ + 4LiCl + 4AlCl₃. Thermal decomposition of silane at 450-500 °C generates disilane along with hydrogen gas through the reaction: 2SiH₄ → Si₂H₆ + H₂. This process requires careful temperature control to prevent further decomposition to elemental silicon. Purification typically employs low-temperature fractional distillation under inert atmosphere.

Industrial Production Methods

Industrial production of disilane primarily utilizes the disproportionation of monosilane over solid catalysts. The process employs aluminosilicate or zeolite catalysts at temperatures between 200-300 °C and pressures of 10-30 bar. Typical reactor configurations employ fixed-bed or fluidized-bed systems with careful temperature control to optimize disilane selectivity. The reaction follows the general scheme: 3SiH₄ → Si₂H₆ + SiH₄ + H₂, though higher silanes also form in smaller quantities. Product separation utilizes cryogenic distillation columns operating at temperatures from -30 °C to -100 °C. Annual global production estimates range between 10-20 metric tons, primarily for electronic applications. Production costs are substantially higher than for monosilane due to lower yields and more complex purification requirements. Major manufacturers employ stringent safety protocols due to the compound's pyrophoric nature and high reactivity.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography with flame ionization detection provides the primary method for disilane quantification, employing capillary columns with dimethylpolysiloxane stationary phases. Typical retention indices range between 400-450 relative to n-alkanes. Detection limits approach 0.1 ppm(v) with linear response across concentration ranges from 1 ppm to 100%. Fourier transform infrared spectroscopy offers rapid identification through characteristic absorption patterns between 2000-2200 cm⁻¹ and 900-1000 cm⁻¹. Quantitative IR analysis employs the integrated absorbance of the 2149 cm⁻¹ band with a detection limit of approximately 5 ppm(m). Mass spectrometric detection provides unambiguous identification through the parent ion cluster at m/z 58-64 and characteristic fragmentation patterns. Raman spectroscopy allows non-destructive analysis with prominent bands at 2183 cm⁻¹ and 437 cm⁻¹. Gas detection tubes specific for silicon hydrides enable field measurements with detection limits around 1 ppm(v).

Purity Assessment and Quality Control

Electronic grade disilane specifications typically require minimum purity of 99.995% with particular attention to oxygen-containing impurities. Moisture content must not exceed 1 ppm(v) to prevent decomposition during storage and handling. Carbon-containing impurities including methane and silmethyl derivatives are limited to 5 ppm(v) total. Metallic impurities are controlled below 100 ppb(w) for critical electronic applications. Analytical methods for impurity determination employ gas chromatography with mass spectrometric detection, cavity ring-down spectroscopy for moisture analysis, and inductively coupled plasma mass spectrometry for metallic contaminants. Stability testing demonstrates that high-purity disilane decomposes at rates below 0.1% per month when stored in properly passivated containers at room temperature. Storage vessels typically employ stainless steel with interior electropolishing and chemical passivation to minimize catalytic decomposition.

Applications and Uses

Industrial and Commercial Applications

Disilane serves as a silicon source in chemical vapor deposition processes for semiconductor manufacturing. The compound offers advantages over monosilane in specific applications due to lower deposition temperatures and improved step coverage. Plasma-enhanced chemical vapor deposition utilizing disilane enables fabrication of amorphous silicon thin films at temperatures below 300 °C, suitable for glass substrates. Solar cell manufacturing employs disilane in the production of hydrogenated amorphous silicon layers for photovoltaic devices. The compound finds application in thin-film transistor production for display technologies, particularly in active matrix liquid crystal displays. Epitaxial silicon growth processes utilize disilane for selective deposition on patterned wafers. Disilane serves as a precursor for silicon carbide and silicon nitride coatings through reactions with appropriate carbon and nitrogen sources. The compound finds limited application in specialty chemical synthesis as a silylating agent and reducing agent.

Historical Development and Discovery

The initial observation of disilane occurred in 1902 through the work of Henri Moissan and Samuel Smiles, who identified the compound among products from acid treatment of magnesium silicide. Their investigation built upon earlier observations by Friedrich Wöhler and Heinrich Buff, who had noted hydrogen evolution from metal silicide-acid reactions but had not characterized the silicon-containing products. Moissan and Smiles designated the compound "silicoethane" and recognized its relationship to ethane. Systematic investigation of disilane chemistry commenced with Alfred Stock's pioneering work on silicon hydrides in the period 1916-1933. Stock developed improved synthetic methods and purification techniques that enabled detailed characterization of disilane and higher silanes. Carl Somiesky extended this work through meticulous studies of silane pyrolysis and disproportionation reactions. The mid-20th century saw increased interest in disilane as a potential rocket fuel, though this application was abandoned due to handling difficulties. The emergence of semiconductor technology in the 1970s revitalized interest in disilane as a deposition precursor, leading to improved synthetic methods and safety protocols.

Conclusion

Disilane represents a fundamental compound in silicon chemistry with distinctive properties arising from relatively weak silicon-silicon bonding and enhanced reactivity compared to carbon analogs. The compound's molecular structure, characterized by D_3d symmetry and elongated bonds compared to ethane, provides insight into the nature of silicon-silicon bonding. Thermal decomposition pathways and reaction mechanisms demonstrate the compound's propensity for homolytic bond cleavage and radical-mediated transformations. Industrial applications leverage these properties in chemical vapor deposition processes for electronic materials fabrication. Current research directions focus on developing more efficient synthetic routes, understanding surface reaction mechanisms during deposition processes, and exploring potential applications in energy storage materials. The compound continues to offer fundamental insights into catenated silicon chemistry while maintaining practical significance in advanced materials manufacturing.