Multinuclear NMR studies of mixtures of aluminium and gallium trihalides in benzene

The Al-27 and Ga-71 NMR spectra of mixtures Of M2X6 and M2Y6 (M = Al, Ga, X not equal Y = Cl, Br or I) in benzene are reported. All these systems show rapid inter-halogen exchange, and in some cases stable intermediates M2XnY 6-n can be characterised spectroscopically. In particular, the results are used to identify the predominant species present in solutions prepared by m ixing (i) Al2Br6 and Ga2Cl6, and (ii) Al2I6 and Ga2Br6, in benzene in vario us mole ratios. The expected M2X3Y3 intermediates are found for M = Al or G a, X not equal Y = Cl, Br, but with other mixed halide systems, unsymmetric structures predominate. and possible reasons for this are discussed. The e xperimental results are in agreement with thermodynamic predictions, and wi th arguments based on maximizing the M-X, Y electronegativity differences.