Electron Diffraction Study of Rhenium Fluorides. II. Structure, Pseudorotation, and Anharmonic Coupling of Modes in ReF7

Abstract

Significant departures from $D_{\text{5h}}$ symmetry are indicated by the gas‐phase electron diffraction data for ReF₇. Observed scattered intensities, corrected for heavy‐atom effects as discussed in the preceding paper, are adequately represented by structures with static deformations of either $C_2$ or $C_s$ symmetry of the following character. A puckering of the ring of five fluorines (maximum out‐of‐plane displacement ≈ 9°) is accompanied by a movement of each axial fluorine from the reference axis of ≈ 8° in the direction which provides maximum avoidance of fluorines on the rhenium coordination sphere. Equally compatible with the data is the much more reasonable dynamic pseudorotation model in which vibrational displacements carry the molecule from $C_2$ to $C_s$ to $C_2$ configurations. The tenfold character of the hindering potential and modest displacements preclude an appreciable barrier. A pronounced skewing of the envelope of axial–equatorial, $F_a{F}_e$ , distances reveals a coupling in phase of the $e_1\prime$ axial bend to the $e_2″$ ring puckering vibration through the $S^2{\mathrm{(e}}_2{\mathrm{″)\; S(e}}_1\mathrm{\prime )}$ cubic term in the potential‐energy function. The skewing, which moves the maximum of the $F_a{F}_e$ distribution peak inside the distribution center of gravity, results in an apparent “anharmonic shrinkage” of the nonbonded peak by an amount exceeding the well‐known “Bastiansen–Morino” harmonic shrinkage. It is to be noted that the equatorial pseudorotation problem closely resembles that for cyclopentane in reduced mass, in displacement, and therefore, presumably, in frequency. The structure itself may be understood in terms of bond–bond repulsions thrusting the equatorial atoms out of plane; the out‐of‐plane displacement, in turn, induces an axial bend. The actual structure, however, exhibits too large an axial bend to conform to the simple repelling points‐on‐a‐sphere model of Gillespie and too small a deviation from $D_{\text{5h}}$ to correspond to the close packing of hard atoms around the rhenium.