Abstract

Indium trihydride (InH₃), systematically named trihydridoindium or indigane, represents an inorganic metal hydride compound with limited stability under standard conditions. This compound exhibits a molar mass of 117.842 g/mol and decomposes at temperatures above -90°C. The molecular structure adopts trigonal planar geometry in isolated monomeric form, though solid-state polymerization occurs through bridging hydride ligands. Experimental characterization primarily occurs through matrix isolation techniques and laser ablation methods coupled with infrared spectroscopy. Indium trihydride demonstrates significant thermal lability and serves primarily as a chemical intermediate rather than a compound of practical utility. Research interest focuses on its fundamental bonding characteristics and comparison with Group 13 hydride analogs.

Introduction

Indium trihydride belongs to the class of inorganic metal hydrides within Group 13 of the periodic table. This compound occupies a position intermediate between the more stable aluminum hydride and the less stable thallium hydride analogs. The compound's significance lies primarily in its role as a model system for understanding metal-hydrogen bonding in main group elements, particularly those exhibiting both metallic and non-metallic characteristics. Unlike its boron and aluminum counterparts, indium trihydride lacks practical industrial applications due to its inherent thermal instability, though it serves as a precursor to more stable adduct complexes and finds use in specialized research contexts involving metal hydride chemistry.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

Monomeric indium trihydride exhibits trigonal planar molecular geometry consistent with VSEPR theory predictions for AX₃E₀ systems. The indium atom employs sp² hybridization with bond angles of approximately 120° between hydrogen ligands. The electronic configuration of indium ([Kr]4d¹⁰5s²5p¹) facilitates the formation of three covalent bonds through promotion to an sp² hybridized state with an empty p orbital perpendicular to the molecular plane. This empty orbital contributes to the compound's Lewis acidic character and propensity for adduct formation. Molecular orbital calculations predict a highest occupied molecular orbital primarily localized on hydrogen atoms and a lowest unoccupied molecular orbital centered on the indium atom.

Chemical Bonding and Intermolecular Forces

The In-H bonds in indium trihydride demonstrate predominantly covalent character with partial ionic contribution due to the electronegativity difference between indium (1.78) and hydrogen (2.20). Experimental determinations from matrix-isolated samples indicate bond lengths of approximately 168 pm in monomeric form, slightly longer than the 148-160 pm range observed in aluminum hydride. The compound exhibits weak intermolecular forces in gaseous phase, primarily van der Waals interactions with minimal dipole-dipole contributions due to the non-polar nature of the symmetric trigonal planar structure. Solid-state forms display significant bridging interactions through three-center two-electron bonds, creating polymeric networks analogous to aluminum hydride.

Physical Properties

Phase Behavior and Thermodynamic Properties

Indium trihydride decomposes at -90°C, precluding measurement of conventional phase transition temperatures. The compound exists as a colorless gas under matrix isolation conditions but has not been isolated in pure bulk form due to spontaneous decomposition. Theoretical calculations predict a standard enthalpy of formation of 99.2 kJ/mol and a free energy of formation of 156.9 kJ/mol, indicating thermodynamic instability relative to elemental indium and molecular hydrogen. No reliable density measurements exist for the pure compound, though computational models suggest a gas-phase density of approximately 5.24 g/L at standard temperature and pressure.

Spectroscopic Characteristics

Infrared spectroscopy of matrix-isolated InH₃ reveals characteristic stretching vibrations at 1706.7 cm⁻¹ for the asymmetric stretch and 834.5 cm⁻¹ for the symmetric stretch. Deuterated analogs (InD₃) show corresponding vibrations at 1233.7 cm⁻¹ and 595.8 cm⁻¹ respectively, consistent with expected isotopic shifts. Raman spectroscopy demonstrates a strong polarized band at 834 cm⁻¹ corresponding to the symmetric stretching mode. No ultraviolet-visible absorption data are available due to the compound's instability, though theoretical predictions suggest absorption maxima below 200 nm corresponding to σ→σ* transitions. Mass spectrometric analysis shows parent ion peaks at m/z 118 and 119 corresponding to InH₃⁺ and InH₄⁺ fragments.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Indium trihydride undergoes rapid thermal decomposition according to the equation: 2InH₃ → 2In + 3H₂. This decomposition proceeds with an activation energy of approximately 96 kJ/mol and follows second-order kinetics in condensed phases. The compound functions as a reducing agent toward electrophilic substrates but with less vigor than aluminum hydride analogs. Coordination chemistry dominates its reactivity pattern, with spontaneous formation of adducts containing InH₃L and InH₃L₂ stoichiometries when exposed to Lewis bases such as amines, phosphines, and N-heterocyclic carbenes. Hydrolysis occurs rapidly with water, producing indium hydroxide and hydrogen gas.

Acid-Base and Redox Properties

Indium trihydride exhibits weak Brønsted acidity with an estimated pKa of 35-40 in aprotic solvents, significantly less acidic than water but more acidic than ammonia. The compound functions as a Lewis acid through its vacant orbital on indium, forming stable adducts with donor molecules. Redox properties include reduction potentials approximately -0.34 V for the In³⁺/In couple in aqueous solution, though direct electrochemical characterization of InH₃ remains challenging due to decomposition. The compound demonstrates stability in inert atmospheres but rapidly oxidizes upon exposure to oxygen, forming indium oxides and water.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Matrix isolation techniques represent the primary method for generating monomeric indium trihydride. Laser ablation of metallic indium targets in the presence of hydrogen gas (typically at pressures of 1-10 torr) produces gaseous InH₃, which is subsequently trapped in solid argon or nitrogen matrices at temperatures below 20 K. Chemical synthesis routes involve metathesis reactions such as the treatment of indium(III) chloride with lithium aluminum hydride: InCl₃ + 3LiAlH₄ → InH₃ + 3LiCl + 3AlH₃. This reaction proceeds at -78°C in ethereal solvents but yields polymeric indium hydride rather than the monomeric form. Alternative routes utilize organometallic precursors such as trialkylindium compounds with hydrogenolysis agents.

Industrial Production Methods

No industrial production methods exist for indium trihydride due to its thermal instability and lack of commercial applications. Research-scale preparations remain confined to specialized laboratories equipped with matrix isolation apparatus or low-temperature reaction systems. Scale-up considerations are not relevant given the compound's decomposition characteristics above -90°C. Economic factors do not apply to this compound, though related indium hydride complexes find niche applications in semiconductor manufacturing and specialty reducing agents.

Analytical Methods and Characterization

Identification and Quantification

Infrared spectroscopy serves as the primary identification method for indium trihydride, with characteristic bands at 1706.7 cm⁻¹ and 834.5 cm⁻¹ providing definitive evidence of its formation. Matrix isolation techniques coupled with Fourier-transform infrared spectroscopy offer detection limits in the nanomole range. Mass spectrometry provides complementary identification through parent ion detection at m/z 118 (InH₃⁺) and characteristic fragmentation patterns. No chromatographic methods exist for separation or purification due to decomposition issues. Quantitative analysis relies on volumetric measurement of hydrogen gas evolved during hydrolysis or combustion.

Purity Assessment and Quality Control

Purity assessment presents significant challenges due to the compound's instability. Infrared spectroscopy remains the most reliable method for assessing sample integrity, with the absence of extraneous peaks indicating relative purity. Common impurities include elemental indium, indium oxides, and various indium hydride oligomers. No pharmacopeial or industrial specifications exist for this compound. Storage requires maintenance at cryogenic temperatures (-196°C) under inert atmosphere to prevent decomposition.

Applications and Uses

Industrial and Commercial Applications

Indium trihydride possesses no significant industrial or commercial applications due to its inherent instability. Related indium hydride complexes find limited use in chemical vapor deposition processes for indium-containing semiconductor materials, though these typically employ organometallic precursors rather than the pure hydride. The compound's primary utility remains confined to fundamental research in main group hydride chemistry.

Research Applications and Emerging Uses

Research applications focus primarily on comparative studies of Group 13 hydrides, providing insights into periodic trends in metal-hydrogen bonding. The compound serves as a model system for theoretical calculations of main group element bonding, particularly for elements exhibiting the inert pair effect. Emerging research explores its potential as a precursor to nanostructured indium materials through controlled decomposition pathways. Patent literature contains no significant intellectual property related specifically to indium trihydride, reflecting its limited practical utility.

Historical Development and Discovery

The existence of indium trihydride was first postulated in the mid-20th century based on periodic trends and analogies with boron and aluminum hydrides. Experimental verification occurred through matrix isolation studies in the 1970s, with definitive infrared characterization reported by researchers at several institutions including the University of Cambridge and Iowa State University. Key methodological advances involved the development of laser ablation techniques coupled with cryogenic matrix isolation, allowing generation and characterization of otherwise unstable species. The compound's polymeric structure in solid state was elucidated through comparative studies with aluminum hydride analogs in the 1990s. Recent research has focused on stabilized adducts rather than the pure compound.

Conclusion

Indium trihydride represents a chemically interesting but practically limited compound within the Group 13 hydride series. Its molecular structure exemplifies trigonal planar geometry with covalent metal-hydrogen bonding, while its solid-state behavior demonstrates bridging interactions leading to polymeric forms. The compound's significant thermal instability precludes most practical applications but provides valuable insights into the bonding characteristics of post-transition metal hydrides. Future research directions may explore stabilized derivatives and potential applications in materials synthesis through controlled decomposition processes. Fundamental studies continue to elucidate the relationship between electronic structure and stability across the main group hydride series.