Limitations of VSEPR theory

The generalizations of the VSEPR model are useful, but there are limitations to its use. In this section, we give examples that illustrate some problems. The isoelectronic species IF₇ and [TeF₇]^- 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–Fax and I–Feq distances are 179 and 186pm respectively, and in [TeF₇]^-, the Te–Fax bond distance is 179pm and the Te–Feq distances lie in the range 183 to 190 pm.  Among species in which VSEPR theory appears to fail are

[SeCl₆]^2-, [TeCl₆] ^2- and [BrF₆] ^- . When characterized as alkali metal salts, these anions are found to possess regular octahedral structures in the solid state, whereas VSEPR theory suggests shapes based on there being seven electron pairs around the central atom. Although these structures cannot readily be predicted, we can rationalize themin terms of having a stereochemically inactive pair of electrons. Stereochemically inactive lone pairs are usually observed for the heaviest members of a periodic group, and the tendency for valence shell s electrons to adopt a non-bonding role in a molecule is called the stereochemical inert pair effect. Similarly, [SbCl₆] ^- and [SbCl₆]^3- both possess regular octahedral structures. Finally, consider [XeF₈]^2-, [IF8] ^– and [TeF₈]^2-. As expected from VSEPR theory, [IF₈]^- and [TeF₈] ^2- are square antiprismatic; this structure is related to the cube but with one face of the cube rotated through 458.  However, [XeF₈]^2- also adopts this structure, indicating that the lone pair of electrons is stereochemically inactive. It is important to note that whereas VSEPR theory may be applicable to p-block species, it is not appropriate for those of the d-block.

   If the presence of a lone pair of electrons influences the shape of a molecule or ion, the lone pair is stereochemically active. If it has no effect, the lone pair is stereochemically inactive. The tendency for the pair of valence s electrons to adopt a non-bonding role in a molecule or ion is termed the stereochemical inert pair effect. 1.20 Molecular shape: geometrical Isomerism.

  In this section we discuss geometrical isomerism. Examples are taken from both p- and d-block chemistry.  If two species have the same molecular formulae and the same structural framework, but differ in the spatial arrangement of different atoms or groups about a central atom or a double bond, then the compounds are geometrical isomers.