AbstractThe geometry, vibrational spectra and charge distribution of Br5+ were calculated by the use of the local density functional (LDF) method. The results show that for free Br5+ the lowest energy configuration is a skew structure with the three central Br atoms forming an angle of 168.6° und the two terminal Br atoms exhibiting a dihedral angle of 82°. This skew configuration is in contrast to the planar trans configuration of C2h symmetry found for Br5+ in solid Br5+MF6− (M = As, Sb). The small energy difference of 1.2 kcal mol−1 between the skew and the trans configurations, combined with crystal packing effects, can account for the planar trans configuration of Br5+ in solid Br5+MF6−. The computed vibrational spectra were used to select the most likely set from three sets of previously published and widely diverging spectra. Contrary to previous STO‐3G calculations for Cl5+, the present LDF calculations for Br5+ and Cl5+ result in charge distributions which agree with a previously proposed simple valence bond model for pentahalogen(1+) cations.