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Rhenium heptafluoride, ReF sub 7 is one of only two stable binary compounds MX sub 7 with a heptacoordinated metal atom M and halide atom X. Its low-temperature crystal structure, as determined by high-resolution powder neutron diffraction, reveals the molecular structure, which has been the subject of speculation and debate for many years. Here it is shown within experimental error that at 1.5 kelvin the lowest energy configuration of ReF sub 7 has symmetry C sub s (m) and is a distorted pentagonal bipyramid. The deviation of the two axial Re-F bonds from collinear and the puckering of the ring of equatorial fluorine atoms are similar to what has been postulated as one of the conformations af the pseudorotational motion observed at higher temperatures.
ReF sub 7 and IF sub 7 are the only two stable binary compounds of the type MX sub 7 . OsF sub 7 can also occur, but decomposes above 170 K (1). The molecular structures of these compounds pose several questions (2) and are subject to steric crowding and fluxionality. Up to hexacoordination, the valence-shell electron-pair-repulsion theory (VSEPR) developed by Gillespie and Nyholm (3) correctly predicts the molecular structure. Furthermore, an extension of this approach, the "principle of repelling points on a sphere," works independently of the repulsive force law invoked for MX sub n with n <= 9 with the exception of n = 7 (4). For heptacoordination compounds, the latter predicts structures whose symmetry ranges from D sub 5h for soft repulsions that distort to lower symmetries such as a monocapped trigonal prism or a monocapped octahedron as the repulsions harden, with C sub 2v and C sub 3 symmetry (5), respectively. Gas-phase and solution spectroscopic studies of IF sub 7 ReF sub 7 indicate that the favored arrangement is based on the pentagonal bipyramid (6). The presence of five ligands in the equatorial ring of a pentagonal bipyramid results in steric crowding. Puckering of the ring may relieve the strain, but the odd number of ligands is thought to cause fluxionality and pseudorotarion of the ligands (5). A determination of the crystal structure of a heptafluoride at low temperature presents an opportunity to investigate the arrangement of this unusual coordination number without the problems posed by fluxionality....