Digallane
Encyclopedia
Digallane is a chemical compound of gallium
and hydrogen
with the formula Ga2H6. It is the dimer of the monomeric compound gallane. The eventual preparation of the pure compound, reported in 1989,
was hailed as a "tour de force." Digallane had been reported as early as 1941 by Wiberg; however, this claim could not be verified by later work by Greenwood and others.
chlorine atoms and thus formulated as (H2Ga(μ-Cl))2) was prepared via the hydrogenation of gallium trichloride
, GaCl3, with Me
3SiH. This step was followed by a further reduction with LiGaH4, solvent free, at −23 °C, to produce digallane, Ga2H6 in low yield.
Digallane is volatile and condenses at −50 °C into a white solid.
with 2 bridging hydrogen atoms (so-called three-center two-electron bond
s). The terminal Ga—H bond length is 152 pm, the Ga—H bridging is 171 pm and the Ga—H—Ga angle is 98°. The Ga—Ga distance is 258 pm. The 1H NMR
spectrum of a solution of digallane in toluene
shows two peaks attributable to terminal and bridging hydrogen atoms.
In the solid state, digallane appears to adopt a polymeric or oligomeric structure. The vibrational spectrum is consistent with tetramer (i.e. (GaH3)4). The vibrational data indicate the presence of terminal hydride ligands. In contrast, the hydrogen atoms are all bridging in α-alane
, a high-melting, relatively stable polymeric form of aluminium hydride
wherein the aluminium centers are 6-coordinated.
The reactions with Lewis bases are similar to those of diborane, but the larger size of gallium is indicated by its ability to form 2:1 adduct
s. Thus, with trimethylamine
both 1:1 and 2:1 adducts are formed (i.e. Me
3N·GaH3 and (Me3N)2·GaH3, respectively. With phosphine a 1:1 adduct H3P·GaH3 is formed. The trimethylamine
adducts have been known for some time.
These adducts are prepared by the reaction of LiGaH4 with trimethylammonium chloride, Me3NHCl, the 2:1 adduct is formed at low temperatures:
The monomeric structure of Me3N.GaH3 has been confirmed in both the gas and solid phases. In this regard, the 2:1 adduct contrasts with the corresponding alane
complex, Me3N.AlH3 which in the solid is dimeric with bridging hydrogen atoms.
A range of other 1:1 adducts have been prepared and their stabilities determined.
Gallium
Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...
and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
with the formula Ga2H6. It is the dimer of the monomeric compound gallane. The eventual preparation of the pure compound, reported in 1989,
was hailed as a "tour de force." Digallane had been reported as early as 1941 by Wiberg; however, this claim could not be verified by later work by Greenwood and others.
Preparation
A two-stage approach proved to be the key to successful synthesis of pure digallane. Firstly the dimeric monochlorogallane, (H2GaCl)2 (containing bridgingBridging
-Construction:* Building of bridges across a gap*Cross braces used between joists to stabilize them.-Electronics:* In electronics, using a low source impedance to drive a large load impedance for maximum voltage transfer, called impedance bridging...
chlorine atoms and thus formulated as (H2Ga(μ-Cl))2) was prepared via the hydrogenation of gallium trichloride
Gallium trichloride
Gallium trichloride is the chemical compound with the formula GaCl3. Solid gallium trichloride exists as a dimer with the formula Ga2Cl6. It is colourless and soluble in virtually all solvents, which is unusual for a metal halide. It is the main precursor to most derivatives of gallium and a...
, GaCl3, with Me
Methyl group
Methyl group is a functional group derived from methane, containing one carbon atom bonded to three hydrogen atoms —CH3. The group is often abbreviated Me. Such hydrocarbon groups occur in many organic compounds. The methyl group can be found in three forms: anion, cation and radical. The anion...
3SiH. This step was followed by a further reduction with LiGaH4, solvent free, at −23 °C, to produce digallane, Ga2H6 in low yield.
- Ga2Cl6 + 4 Me3SiH → (H2GaCl)2 + 4 Me3SiCl
- 1/2 (H2GaCl)2 + LiGaH4 → Ga2H6 + LiCl
Digallane is volatile and condenses at −50 °C into a white solid.
Structure and bonding
Electron diffraction measurements of the vapour at 255 K established that digallane is structurally similar to diboraneDiborane
Diborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air at room temperature...
with 2 bridging hydrogen atoms (so-called three-center two-electron bond
Three-center two-electron bond
A three-center two-electron bond is an electron-deficient chemical bond where three atoms share two electrons. The combination of three atomic orbitals form three molecular orbitals: one bonding, one non-bonding, and one anti-bonding. The two electrons go into the bonding orbital, resulting in a...
s). The terminal Ga—H bond length is 152 pm, the Ga—H bridging is 171 pm and the Ga—H—Ga angle is 98°. The Ga—Ga distance is 258 pm. The 1H NMR
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
spectrum of a solution of digallane in toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...
shows two peaks attributable to terminal and bridging hydrogen atoms.
In the solid state, digallane appears to adopt a polymeric or oligomeric structure. The vibrational spectrum is consistent with tetramer (i.e. (GaH3)4). The vibrational data indicate the presence of terminal hydride ligands. In contrast, the hydrogen atoms are all bridging in α-alane
Aluminium hydride
Aluminium hydride is an inorganic compound with the formula AlH3. It is a colourless solid that is pyrophoric. Although rarely encountered outside of research laboratory, alane and its derivatives are used as reducing agents in organic synthesis.-Structure:...
, a high-melting, relatively stable polymeric form of aluminium hydride
Aluminium hydride
Aluminium hydride is an inorganic compound with the formula AlH3. It is a colourless solid that is pyrophoric. Although rarely encountered outside of research laboratory, alane and its derivatives are used as reducing agents in organic synthesis.-Structure:...
wherein the aluminium centers are 6-coordinated.
Reactions
Digallane decomposes at ambient temperatures:- Ga2H6 → 2 Ga + 3 H2
The reactions with Lewis bases are similar to those of diborane, but the larger size of gallium is indicated by its ability to form 2:1 adduct
Adduct
An adduct is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is considered a distinct molecular species...
s. Thus, with trimethylamine
Trimethylamine
Trimethylamine is an organic compound with the formula N3. This colorless, hygroscopic, and flammable tertiary amine has a strong "fishy" odor in low concentrations and an ammonia-like odor at higher concentrations...
both 1:1 and 2:1 adducts are formed (i.e. Me
Methyl group
Methyl group is a functional group derived from methane, containing one carbon atom bonded to three hydrogen atoms —CH3. The group is often abbreviated Me. Such hydrocarbon groups occur in many organic compounds. The methyl group can be found in three forms: anion, cation and radical. The anion...
3N·GaH3 and (Me3N)2·GaH3, respectively. With phosphine a 1:1 adduct H3P·GaH3 is formed. The trimethylamine
Trimethylamine
Trimethylamine is an organic compound with the formula N3. This colorless, hygroscopic, and flammable tertiary amine has a strong "fishy" odor in low concentrations and an ammonia-like odor at higher concentrations...
adducts have been known for some time.
These adducts are prepared by the reaction of LiGaH4 with trimethylammonium chloride, Me3NHCl, the 2:1 adduct is formed at low temperatures:
- LiGaH4 + Me3NHCl → LiCl + H2+ Me3N.GaH3
The monomeric structure of Me3N.GaH3 has been confirmed in both the gas and solid phases. In this regard, the 2:1 adduct contrasts with the corresponding alane
Aluminium hydride
Aluminium hydride is an inorganic compound with the formula AlH3. It is a colourless solid that is pyrophoric. Although rarely encountered outside of research laboratory, alane and its derivatives are used as reducing agents in organic synthesis.-Structure:...
complex, Me3N.AlH3 which in the solid is dimeric with bridging hydrogen atoms.
A range of other 1:1 adducts have been prepared and their stabilities determined.