(a) At 297 K, the ¹¹B NMR spectrum of a CD₂Cl₂ solution of [Ph₄As][B₆H₇] shows one doublet (δ–18.0 ppm, J = 147 Hz). In the ¹H NMR spectrum, two signals are observed (δ–5.5 ppm, broad; δ +1.1 ppm, 1:1:1:1 quartet). At 223 K, the ¹¹B NMR spectrum exhibits signals at δ–14.1 and –21.7 ppm (relative integrals 1: 1). Lowering the temperature has little effect on the ¹H NMR spectrum. Draw the solid state structure of [B₆H₇]^– and rationalize the solution NMR spectroscopic data.

(b) The reaction of Ga metal with NH₄F at 620K liberates H₂ and NH₃ and yields an ammonium salt X in which gallium is in oxidation state +3. The solid state structure of X consists of discrete cations lying between sheets composed of vertex-sharing GaF₆-octahedra; sharing of vertices occurs only in one plane. Suggest an identity for X. Write a balanced equation for the reaction of Ga and NH₄F to give X. Explain with the aid of a diagram how the stoichiometry of X is maintained in the solid state structure.