VSEPR Theory - Applicability
Although VSEPR theory can be used to predict the shapes of many inorganic molecules it should be used with caution. It fails to correctly predict the shapes of molecules under the following circumstances:
- Molecules in which the bonding is largely ionic
- An example is Li2O. VSEPR predicts a V-shape (similar to H2O) but the high positive charge on the Li atoms forces them as far apart from each other as possible and a linear geometry is adopted.
- Transition metal complexes
- The metal-ligand bonding interactions in transition metal complexes may be explained using ligand-field theory or molecular-orbital theory. Treatment with VSEPR theory is too simplistic.
- Compounds in which a lone pair is 'burried'
- Metals at the bottom of the p-block (e.g. thallium, lead and bismuth) display the 'inert-pair' effect. Only their p-electrons can be used for bonding. This leaves a lone-pair of s-electrons that is spherically distributed around the atom and doesn't occupy a particular bonding site in accord with VSEPR theory. An example is the [BiCl6]3− ion which has 7 VSEP but adopts an octahedral structure (in accord with 6 VSEP).
- Species that need to be flat to maximise π-bond interactions
- Species such as [C(CN)3]− need to be flat in order to maximise π-bonding between the carbon atoms. This is in conflict with the trigonal pyramidal geometry predicted by VSEPR theory.