Pentagonal bipyramidal VSEPR

It is noteworthy that the to-bonded structure for ArF6 differs from that predicted by VSEPR theory. ArF6 is predicted to be of octahedral (Oh) symmetry, with three mutually perpendicular F i- Ar -h F triads and an s-type lone pair. In contrast, VSEPR predicts a pentagonal bipyramid (or other seven-vertex polyhedron) with some or all F-Ar-F angles less than 90°. The calculated equilibrium structure is in agreement with the co-bonding model. 

The structures of these ions normally conform to those predicted by the VSEPR theory, as shown in Fig. 17.2.2. Since the anion XY has two more electrons than the cation XY+, they have very different shapes. The anion IFj- is planar with lone pairs occupying the axial positions of a pentagonal bipyramid. In [Me4N](IF6), IFg is a distorted octahedron (C3V symmetry) with a sterically active lone pair, whereas both BrFg and ClFg are octahedral. The anion IF " has the expected square antiprismatic structure. 

Lewis structures as well, except that the three lone pairs of electrons on each fluorine atom have been omitted. VSEPR theory predicts that a species with six bonding pairs of electrons, such as IF6, should be octahedral. Possible structures of species with seven bonding pairs of electrons, like IFt, were not covered in Section 2.3, but a reasonable and a symmetrical structure would be a pentagonal bipyramid. 

The generalizations of the VSEPR model are useful, but there are hmitations to its use. In this section, we give examples that illustrate some problems. The isoelectronic species IF7 and [TeFy] are predicted by VSEPR theory to be pentagonal bipyramidal and this is observed. However, electron diffraction data for IF7 and X-ray diffraction data for [Me4N][TeF7] reveal that the equatorial F atoms are not coplanar, a result that cannot be predicted by the VSEPR model. Moreover, in IF7, the I-F x and I-F q distances are 179 and 186 pm respectively, and in [TeF7], the Te-Fju bond distance is 179 pm and the Te-Fgq distances lie in the range 183 to 190 pm. 

The pentagonal bipyramidal geometry in IF is difficult to rationalize in terms of the VSEPR model since the smallest FIF valence angles could be larger than 72° if the coordination polyhedron was a distorted capped octahedron [3]. See Fig. 18.3. 

The fact that heptacoordinated species in their ground states exMbit pentagonal bipyramidal structures with an unpuckered equatorial plane, cannot be rationalize by VSEPR theory (7,2) in terms of a repelling points on a sphere (POS) model which should result in either a monocapped octahedron or a monocapped trigonal prism. Furthermore, it cannot be explained by conventional bonding schemes involving localized electron orbitals of die central atom to enforce the coplanarity of a central atom and five equatorial ligands. The best explanation to account for this planarity is the... 

One possibility for the electron-group geometry for seven electron groups is pentagonal-bipyramidal, as found in the Hy molecule. Write the VSEPR notation for this molecule. Sketch the structure of the molecule, labeling all the bond angles. 

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