Crystal structure of bis(triphenylphosphonium) hexabromadigallate(II) in the correct space group: conformational complexity in a heteroethan

Abstract

The crystal structure of [Ph3PH]2[Ga2Br6], previously described as having a disordered anion in the space group R̄ 3, has been re-determined in the correct space group P̄ 3, where it is fully ordered. Interestingly, two-thirds of the [Ga2Br6]2− dianions have an intermediate conformation with a Br–Ga–Ga–Br torsion angle of 36.91 (1)°, while the remaining is staggered as required from adopting a site with inversion symmetry. In the lattice, [Ph3PH]+ ions lie along the same threefold axes as the dianions and are oriented such that the P–H bond is directed towards a gallium atom. The phosphonium ions lie back-to-back and interact with relatively strong T-interactions between phenyl rings on adjacent cations. DFT calculations at the B3LYP/6–311+G(fd,) level have been used to determine the barriers to rotation in [Ga2X6]2− ions. For X = Cl and X = Br, the barriers are found to be very small, with values of 4.3 and 5.1 kJ mol−1 for the two halogens.