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VSEPR and more than six electron pairs

It is considerably less easy to draw a distinction between apparently reasonable seven coordinate geometries. There are several possibilities, including the pentagonal bipyramid and the capped octahedron.

The pentagonal bipyramid (left) and the monocapped octahedron.

Iodine heptafluoride, IF7, is a good example of a pentagonal bipyramidal geometry. The molecule XeF6 is an interesting case. As with IF7, application of VSEPR rules suggests seven electron pairs. These are made up from six bonding pairs and one lone pair. In fact, the structure of XeF6 is based upon a distorted octahedron, probably towards a monocapped octahedron. It is difficult to settle the geometry of the lowest energy configurtion because the geometry of XeF6 changes rapidly with time, that is, it is fluxional. The effect of this fluxional process is to average all the fluorine positions.

With higher coordination numbers the situation is more complex. For instance, the idealized geometry for eight electron pairs (epitomized by the anion [XeF8]2-) is a square antiprism but the energy of other coordination geometries may be very similar or more stable in particular cases.

The idealized coordination geometry for eight electron pairs.

A VSEPR tutorial on the WWW

VSEPR tutorial on the WWW, URL:
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