Introduction

The VSEPR model is a powerful tool for chemists to predict the shapes of molecules; yet like many other theories, it has exceptions too. In the discussion below, we will talk more in details about the various limitations of VSEPR and also give explanations to why there are such exceptions. 

VSEPR fails for isoelectronic species

Isoelectronic species are elements, ions and molecules that share the same number of electrons. According to the VSEPR model, chemists determine the shape of the molecules based on numbers of valence electrons (i.e. bond pairs and lone pair). However, when it comes to isoelectronic species, not all of them share the same shape despite the fact that they do have the same numbers of valence electrons. For example, IF₇ and [TeF₇]^- both have 56 valence electrons and are predicted to be pentagonal bipyramidal. However, electron diffraction data and X-ray diffraction data reveal that the equitorial F atoms are not coplanar. The VSEPR model fails to accommodate the correct shape for [TeF7]- because this model does not allow chemists to see if an atom is coplanar or not. Moreover, the bond distance between I-F equitorial and Te-F equitorial are different. Therefore these two compounds do not share the same shape even though theoretically they should have shared the same structure. 

VSEPR fails for transition metal compounds

Another reason that VSEPR model fails to predict the structure of certain compounds is because it does not take relative sizes of the substituents and stereochemically inactive lone pairs into account. Elements in the d-block have relatively high atomic mass and they tend to have stereochemically inactive pair of electrons. In other words, this groups of elements have the tendency for valence shell s electrons to adopt a non-bonding role in a molecule. As a result, VSEPR do not give the correct shape of these transitional metal complex compounds because this model suggests shapes based on the number of electron pairs around the central atoms. For example, [SeCl₆]^2- , [TeCl₆]^2- ,and [BrF₆]^- are supposed to be pentagonal bipyramidal according to VSEPR since the central atom can have seven electron pairs. However, due to the stereochemical inert pair effect, these molecules are found to be regular octahedral because one of the seven electron pairs is stereochemically inactive.  

Re 

References

1. Cotton, F. Albert, and Geoffrey Wilkinson. Advanced Inorganic Chemistry; a Comprehensive Text. [New York]: Interscience, 1962. Print.
2. Housecroft, Catherine E., and A. G. Sharpe. Inorganic Chemistry. Harlow, England: Pearson Prentice Hall, 2008. Print.

Problems

1. What is the shape of [XeF₈]^{2- ?}(Hint: Xenon contains one pair of stereochemically inactive lone pair)
2. Give some examples of isoelectronic species.
3. Why are d-block elements exceptions to VSEPR? 

Answers

1. Antiprismatic
2. IF₇ and [TeF₇]^-
3. Because of stereochemical inert pair effect

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