Digallane

Digallane
Names
	IUPAC name digallane(6)
	Other names Di-μ-hydrido-tetrahydridodigallium; Gallane dimer
Identifiers
CAS Number	12140-58-8 ;
3D model (JSmol)	Interactive image;
	InChI InChI=1S/2Ga.6H Key: GFQCQFDOQMRGIQ-UHFFFAOYSA-N;
	SMILES [GaH2]1[H][GaH2][H]1;
Properties
Chemical formula	Ga2H6
Molar mass	145.494 g/mol
Boiling point	-50°C (sublimes)
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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	Infobox references

Digallane (systematically named digallane(6) and di-μ-hydrido-bis(dihydridogallium)) is an inorganic compound with the chemical formula GaH
₂(H)
₂GaH
₂ (also written [{GaH
₂(μ-H)}
₂] or [Ga
₂H
₆]). It is the dimer of the monomeric compound gallane. The eventual preparation of the pure compound, reported in 1989,[1][2] was hailed as a "tour de force."[3] Digallane had been reported as early as 1941 by Wiberg;[4] however, this claim could not be verified by later work by Greenwood and others.[5]

Preparation

A two-stage approach proved to be the key to successful synthesis of pure digallane. Firstly the dimeric monochlorogallane, (H₂GaCl)₂ (containing bridging chlorine atoms and thus formulated as (H₂Ga(μ-Cl))₂) was prepared via the hydrogenation of gallium trichloride, GaCl₃, with Me₃SiH. 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 diborane with 2 bridging hydrogen atoms[2] (so-called three-center two-electron bonds). 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.[2]

In the solid state, digallane appears to adopt a polymeric or oligomeric structure. The vibrational spectrum is consistent with tetramer (i.e. (GaH₃)₄).[2] 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. Digallane decomposes at ambient temperatures:

Ga₂H₆ → 2 Ga + 3 H₂

References

Anthony J. Downs; Michael J. Goode; Colin R. Pulham (1989). "Gallane at last!". Journal of the American Chemical Society. 111 (5): 1936–1937. doi:10.1021/ja00187a090.
Pulham C.R.; Downs A.J.; Goode M.J; Rankin D.W.H. Roberson H.E. (1991). "Gallane: Synthesis, Physical and Chemical Properties, and Structure of the Gaseous Molecule Ga₂H₆ As Determined by Electron Diffraction". Journal of the American Chemical Society. 113 (14): 5149–5162. doi:10.1021/ja00014a003.
N.N. Greenwood (2001). "Main group element chemistry at the millennium". J. Chem. Soc., Dalton Trans. (14): 2055–2066. doi:10.1039/b103917m.
Wiberg E.; Johannsen T. (1941). "Über einen flüchtigen Galliumwasserstoff der Formel Ga₂H₆ und sein Tetramethylderivat". Naturwissenschaften. 29 (21): 320. Bibcode:1941NW.....29..320W. doi:10.1007/BF01479551. S2CID 44840674.
Shriver, D. F.; Parry, R. W.; Greenwood, N. N.; Storr, A; Wallbridge, M. G. H. (1963). "Some Observations Relative to Digallane". Inorg. Chem. 2 (4): 867–868. doi:10.1021/ic50008a053.

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[1] Anthony J. Downs; Michael J. Goode; Colin R. Pulham (1989). "Gallane at last!". Journal of the American Chemical Society. 111 (5): 1936–1937. doi:10.1021/ja00187a090.

[Pulham-2] Pulham C.R.; Downs A.J.; Goode M.J; Rankin D.W.H. Roberson H.E. (1991). "Gallane: Synthesis, Physical and Chemical Properties, and Structure of the Gaseous Molecule Ga₂H₆ As Determined by Electron Diffraction". Journal of the American Chemical Society. 113 (14): 5149–5162. doi:10.1021/ja00014a003.

[3] N.N. Greenwood (2001). "Main group element chemistry at the millennium". J. Chem. Soc., Dalton Trans. (14): 2055–2066. doi:10.1039/b103917m.

[4] Wiberg E.; Johannsen T. (1941). "Über einen flüchtigen Galliumwasserstoff der Formel Ga₂H₆ und sein Tetramethylderivat". Naturwissenschaften. 29 (21): 320. Bibcode:1941NW.....29..320W. doi:10.1007/BF01479551. S2CID 44840674.

[5] Shriver, D. F.; Parry, R. W.; Greenwood, N. N.; Storr, A; Wallbridge, M. G. H. (1963). "Some Observations Relative to Digallane". Inorg. Chem. 2 (4): 867–868. doi:10.1021/ic50008a053.