The molecular structures of pentaborane(9) with halogen substituents in apical and basal positions, determined by electron diffraction and theoretical calculationsElectronic supplementary information (ESI) available: tables of calculated geometric parameters and energies, interatomic distances and vibration amplitudes and least-squares correlation matrices; experimental and final weighted difference molecular scattering intensities for 1- and 2-Br?B5H8. See http://www.rsc.org/suppdata/dt/b4/b402688h/