GALLANE - SYNTHESIS, PHYSICAL AND CHEMICAL-PROPERTIES, AND STRUCTURE OF THE GASEOUS MOLECULE GA2H6 AS DETERMINED BY ELECTRON-DIFFRACTION

The elusive binary hydride of gallium, [GaH3]n, has at last been synthesized in rigorously conditioned all-glass apparatus by the reaction between monochlorogallane, [H2GaCl]2, and lithium tetrahydridogallate, LiGaH4, near -30-degrees-C. The compound, which decomposes to the elements at ambient temperatures, has been characterized by chemical analysis, by its vibrational and H-1 NMR spectra, and by chemical trapping with trimethylamine. The infrared spectra of the vapor species at low pressures (sampled directly at ambient temperatures or trapped in a solid inert matrix at ca. 20 K, and including the results of deuteration experiments) leave little doubt that the principal component is the diborane-like molecule H2Ga(mu-H)2GaH2. Such a conclusion is endorsed by electron-diffraction measurements, carried out on the vapor at ca. 255 K; these are consistent with an r(a) structure featuring the following parameters: r(Ga...Ga) 258.0 (0.2), r(Ga-H(t)) 151.9 (3.5), and r(Ga-H(b)) 171.0 (3.8) pm; and angle Ga-H(b)-Ga 97.9 (3.2)degrees (H(t) = terminal H atom; H(b) = bridging H atom). The molecule Ga2H6 is highly susceptible to aggregation. The vibrational spectra of the solid suggest the presence of an oligomer [GaH3]n where n > 2 and possibly equal to 4, but still retaining terminal Ga-H bonds (cf. alpha-AlH3). A similar species is probably a major constituent of toluene-d8 solutions of the gallane; at temperatures < -30-degrees-C, such a solution displays two distinct H-1 magnetic resonances with relative intensities 2:1 attributable to Ht and Hb atoms. The reactions of gallane appear mostly to parallel those of diborane. Thus, symmetrical cleavage of the Ga(mu-H)Ga bridges occurs with NMe3 (at -95-degrees-C) or PH3 (under matrix-isolation conditions) to give the corresponding molecular adduct LnGaH3 (L = Nme3, n = 1 or 2; L = PH3, n = 1), whereas NH3 causes unsymmetrical cleavage at -95-degrees-C with the formation of [H2Ga(NH3)4]+GaH4-. Quantitative metathesis with HCI brings about Ga-H/Ga-Cl exchange with the production of H-2, while stepwise insertion into the Ga-H bonds is the path taken by the reaction with C2H4, affording Ga-Et derivatives.