The digallane molecule, Ga2H6: experimental update giving an improved structure and estimate of the enthalpy change for the reaction Ga2H6(g) --> 2GaH3(g).

Improved methods of analysis and new quantum chemical calculations have been applied to the results of earlier gas-phase electron diffraction (GED) studies of digallane to give what is judged to be the most realistic structure available to date. The principal distances (r(a3,1) in pm) and interbond angles (angle(a3,1) in deg) are as follows (t = terminal, b = bridging): r(Ga...Ga) 254.9(2), r(Ga-H(t)) 155.0(6), r(Ga-H(b)) 172.3(6), angleGa-H(b)-Ga 95.4(5), and angleH(t)-Ga-H(t) 128.6(9). Scrutiny of the IR spectra of solid Ar matrices doped with the vapour above solid samples of gallane at temperatures in the range 190-220 K reveals the presence of not only Ga(2)H(6) as the major component, but also a significant fraction of the monomer GaH(3). Analysis of the relative proportions of the two molecules evaporating from the solid at different temperatures has led to a first experimental estimate of 59 +/- 16 kJ mol(-1) for the enthalpy change associated with the reaction Ga(2)H(6)(g) --> 2GaH(3)(g). Together with a value of 52 kJ mol(-1) delivered by fresh calculations at the MP2 level, this implies that the stability of the dimer with respect to dissociation has been overrated by earlier theoretical treatments.

Gas electron diffraction study of the vapour over dimethylamine-gallane leading to an improved structure for dimeric dimethylamidogallane, [Me2NGaH2]2: a cautionary tale.

Dimethylamine-gallane is relatively slow to decompose in a closed system and vaporises at low temperature primarily as Me2(H)N.GaH3 molecules which can be trapped in a solid Ar matrix and characterised by their IR spectrum. Under the conditions needed to secure a useful gas electron diffraction (GED) pattern, however, the vapour was found to consist of dimeric dimethylamidogallane molecules, [Me2NGaH2]2, formed from the secondary amine adduct by elimination of H2, and the most reliable structure for which has been determined. Salient structural parameters (r(hl) structure) were found to be: r(Ga-N) 202.6(2), r(Ga-H) 155.6(8), r(N-C) 148.0(3), r(C-H) 111.2(6) pm; Ga-N-Ga 90.7(1), C-N-C 109.3(5), N-C-H 109.9(10) and H-Ga-H 119.4(42) degrees.

Synthesis and structural characterisation of primary amine adducts of gallane, RH2N.GaH3, and of their decomposition products, [RHNGaH2]n(R = Me, n= 3; R =tBu, n= 2).

Primary amine-gallane adducts, RH(2)N.GaH(3)[R = Me (1) and (t)Bu (2)], have been isolated for the first time from the reaction of [RNH(3)]Cl with LiGaH(4) in Et(2)O solution at ca. 273 K and characterised by their vibrational and (1)H NMR spectra. The structures of single crystals, grown at low temperatures and determined by X-ray diffraction, reveal pseudo-polymeric arrays of RH(2)N.GaH(3) molecules with Ga-N distances of 2.0424(18) and 2.058(5)A for 1 and 2, respectively. The adducts decompose at or near ambient temperatures with the elimination of H(2) and formation of the corresponding monoalkylamido derivative [RHNGaH(2)](n). The crystal structures of these at 150 K consist of either [MeHNGaH(2)](3) molecules with a "skew-boat" conformation for the cyclic Ga(3)N(3) skeleton (3) or [(t)BuHNGaH(2)](2) molecules with a four-membered Ga(2)N(2) core (4), with Ga-N distances averaging 1.986(8) and 1.991(3)A for 3 and 4, respectively. The crystalline solids 1, 3 and 4 feature intermolecular N-H...H-Ga interactions with H...H distances estimated to fall in the range 2.1-2.3 A. The significance of these is discussed.

Decomposition of monochlorogallane, [H2GaCl]n, and adducts with amine and phosphine bases: formation of cationic gallane derivatives.

Thermal decomposition of monochlorogallane, [H2GaCl]n, at ambient temperatures results in the formation of subvalent gallium species. To Ga[HGaCl3], previously reported, has now been added a second mixed-valence solid, Ga4[HGaCl3]2[Ga2Cl6] (1), the crystal structure of which at 150 K shows a number of unusual features. Adducts of monochlorogallane, most readily prepared from the hydrochloride of the base and LiGaH4 in appropriate proportions, include not only the 1:1 molecular complex Me3P.GaH2Cl (2), but also 2:1 amine complexes which prove to be cationic gallane derivatives, [H2Ga(NH2R)2]+Cl-, where R = tBu (3a) or sBu (3b). All three of these complexes have been characterized crystallographically at 150 K.

Matrix reactivity of Al and Ga atoms (M) in the presence of silane: generation and characterization of the eta2-coordinated complex M.SiH4, the insertion product HMSiH3, and the MI species MSiH3 in a solid argon matrix.

Matrix isolation experiments give evidence for the formation of the loosely bonded metal-silane complex M.SiH(4) by the spontaneous reaction of Al or Ga atoms (M) with silane in a solid Ar matrix at 12 K; however, Ga(2) appears to insert spontaneously into an Si--H bond to form HGaGaSiH(3), probably with the structure HGa(micro-SiH(3))Ga. In M.SiH(4) the metal atom is eta(2)-coordinated by the silane, resulting in a species with C(2v) symmetry. The complex has a distinctive photochemistry: it can be converted on photolysis at lambda approximately 410 or approximately 254 nm to its tautomer, HMSiH(3), which also has a doublet ground electronic state and from which it can be regenerated with lambda approximately 580 nm radiation. Broadband UV-visible photolysis (lambda=200-800 nm) results in decomposition of HMSiH(3), the univalent species MSiH(3) being the only detectable product. The experimental data collected for several silane isotopomers (SiH(4), SiD(4), and SiD(3)H) and different reagent concentrations, together with the results of sophisticated quantum chemical calculations, are used to explore in detail the properties of the detected species and the reaction pathways compassing their formation.

Molecular Structures of Methyldifluoroarsine, CH(3)AsF(2), and Dimethylfluoroarsine, (CH(3))(2)AsF, in the Gas Phase As Determined by Electron Diffraction and ab InitioCalculations.

The structures of gaseous CH(3)AsF(2) and (CH(3))(2)AsF have been determined by electron diffraction incorporating vibrational amplitudes derived from ab initio force fields scaled by experimental frequencies and, for the difluoride, restrained by microwave constants. The following parameters (r(alpha) degrees structure, distances in pm, angles in degrees) have been determined for CH(3)AsF(2): r(As-C) = 194.6(4), r(As-F) = 173.1(1), angleCAsF = 95.2(1), angleFAsF = 97.0(1). For (CH(3))(2)AsF structural refinement gives r(As-C) = 195.1(1), r(As-F) = 175.4(1), angleCAsF = 95.3(5), and angleCAsC = 96.9(8). For the series (CH(3))(3)As, (CH(3))(2)AsF, CH(3)AsF(2), and AsF(3), both As-C and As-F bond lengths are shortened with increasing numbers of F atoms, but the angles CAsF and FAsF are almost invariant.