Digallane

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Digallane
IUPAC name digallane(6)
Other names Di-μ-hydrido-tetrahydridodigallium
Identifiers
CAS number [12140-58-8]
Properties
Molecular formula Ga2H6
Molar mass 145.494 g/mol
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)
Infobox disclaimer and references

Digallane is a chemical compound of gallium and hydrogen with the formula Ga2H6. 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]

Contents

[edit] 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 to give a white solid.

[edit] 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] 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 to 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-coordinate.

[edit] Reactions

Digallane decomposes at ambient temperatures:

2 GaH3 → 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 adducts. Thus, with trimethylamine both 1:1 and 2:1 adducts are formed i.e. Me3N.GaH3 and (Me3N)2N.GaH3, respectively. With phosphine a 1:1 adduct H3P.GaH3. The trimethylamine adducts have been known for some time. [6] [7] 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 complex, Me3N.AlH3 which in the solid is dimeric with bridging hydrogen atoms. [8] A range of other 1:1 adducts have been prepared and their stabilities determined.[9]

[edit] References

  1. ^ Anthony J. Downs, Michael J. Goode, and Colin R. Pulham (1989). "Gallane at last!". Journal of the American Chemical Society 111 (5): 1936–1937. doi:10.1021/ja00187a090. 
  2. ^ a b c d 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.: 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. doi:10.1007/BF01479551. 
  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. 
  6. ^ N. N. Greenwood, A. Storr, and M. G. H. Wallbridge (1963). "Trimethylamine Adducts of Gallane and Trideuteriogallane Gallane, (CH3)3NGaH3". Inorg. Chem. 2 (5): 1036–1039. doi:10.1021/ic50009a036. 
  7. ^ D. F. Shriver, C. E. Nordman (1963). "The Crystal Structure of Trimethylamine Gallane, (CH3N)2GaH3". Inorg. Chem. 2 (6): 1298–1300. doi:10.1021/ic50010a047. 
  8. ^ Brain P.T., Brown H.E., Downs A.J., Greene T.M., Johnsen E, Parsons S, Rankin D.W.H, Smart B.A, Tang C.Y. (1998). "Molecular structure of trimethylamine–gallane, Me3N·GaH3<: ab initio calculations, gas-phase electron diffraction and single-crystal X-ray diffraction studies". J. Chem. Soc., Dalton Trans. (21): 3685. doi:10.1039/a806289g. 
  9. ^ Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.