Question

In 1961, Goggin et al. (Trans. Faraday Soc., 1961, 57, 1877-1884) used infrared (IR) spectroscopy to ascertain the structure of SOF_4. They reported a total of 4 observable vibrations corresponding to S-F stretching modes. These observations validated the prediction of molecular geometry by VSEPR. With this information in hand, use group theory and symmetry selection rules to determine which of the following two shapes is correct. The correct structure will lead to the correct number of S-F stretching modes observed experimentally:

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Answer 1

The sulfur oxygen bond is 1.409Å. A S-F bond on the axis has length 1.596Å and the S-F bond on the equator of the distorted trigonal bipyramidal has length 1.539Å. The angle between the equatorial fluorine atoms is 112.8°. The angle between axial fluorine and oxygen is 97.7°. The angle between oxygen and equatorial fluorine is 123.6° and between axial and equatorial fluorine is 85.7°. The fluorine atoms only produce one NMR line, because they exchange positions, according to the VSEPR. According to the Bent's rule, an electron pair of a more electropositive ligand constitutes greater repulsion; which is why oxygen is in the equator. Therefore, the correct shape is trigonal bipyramidal.