Hunt Practical Methods of Attack

In the pursuit of gallane and related hypersensitive compounds, we have found that operations must be carried out at pressures 10-4 mm 

To determine the composition of a compound that is more or less short-lived at normal temperatures, it is necessary to devise a method of chemical analysis that can be carried out on site, that is, with the aid of a glass vacuum line. The prime desideratum is to find a suitable reaction of the compound that can be engineered to proceed quantitatively in an evacuated, preconditioned glass ampoule to deliver stable products that are amenable to direct assay, for example, by weighing, tensimetric measurements, or elemental analysis. A good illustration is provided by hydridogallium ftis(tetrahydroborate) (see Section IV.A), which decomposes at or just above room temperature in accordance with Eq. (11) (48-50)  

In this case a Toepler pump can be used to remove the volatile products derived from a known mass of the gallane, and an efficient trap cooled to 77 K to separate the condensable B2H6 from the noncondensable H2 the two fractions are then assayed tensimetrically. The mass balance is completed simply by weighing the residue of elemental gallium. Similar measures can be adopted to determine the stoicheiometry of a reaction engaging a gallane with another compound, e.g., NH3, NMe3, or HC1. 

It will be evident from the preceding account that not all physical methods lend themselves equally well to the detection and specification of the gallium hydrides. There are, for example, obvious difficulties in trying to grow single crystals of a low-melting, thermally unstable compound, and, although the case of hypofluorous acid (62) shows what can be achieved ultimately, X-ray methods do not obviously commend 

Some of the more robust molecules, like HGa(BH4)2 (48-50) and H2GaB3H8 (63), can be identified by their mass spectra, and UV photoelectron spectroscopy has also been turned to account in studies of gaseous tetrahydroborate molecules like A1(BH4)3 (64) and HGa(BH4)2 (50), notably for the light it sheds on the mode of coordination of the BH4 ligand. Compared with the three primary methods, however, such sources of supplementary information are barely in the running. 

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