A Brief Note on Limitations of VSEPR Theory

Although the VSEPR model is effective at forecasting molecular geometry, it is ineffective at predicting the morphologies of isoelectronic species and transition metal complexes. In this model, the relative sizes of substituents and stereochemically inactive lone pairs are ignored.. As a result, hefty d-block species that experience the stereochemical inert pair effect cannot be studied with VSEPR.

The VSEPR model is a strong tool that chemists use to predict the geometries of molecules, but it contains exceptions and limitations, just like any other theory s.

Limitations of VSEPR Theory

The following are some of the VSEPR theory’s limitations:

1. The structure of transition metal compounds and ions cannot be explained by the VSEPR theory.
2. The isoelectronic species are not explained by VSEPR theory. Elements with the same amount of electrons are called isoelectronic species. Despite having the same number of electrons, isoelectronic species have different deviations and forms.
3. The VSEPR hypothesis does not account for the effects of orbital interactions on molecule structures. As a result, actual molecule shapes are similar rather than exact to those anticipated by this theory.
4. Several compounds defy this idea since the theory ignores the active lone pairs and their corresponding substituent group sizes.
5. According to this idea, the halves of Group 2 components should have a linear structure. However, their actual structure is a twisted one.
6. The exact bond angles between the atoms in a molecule cannot be determined using VSEPR theory.

Molecule with a Linear Shape

1. In this sort of molecule, we discover two sites in the valence shell of the central atom where the electrons can be found.
2. They should be organised in such a way that repulsion is reduced to the greatest extent possible (pointing in the opposite direction).
3. As an illustration, BeF2

For Isoelectronic Species, VSEPR Fails

Elements, ions, and molecules that share the same number of electrons are known as isoelectronic species. Chemists use the VSEPR model to calculate the shape of molecules based on valence electron counts (i.e. bond pairs and lone pairs). Despite having the same number of valence electrons, two isoelectronic species can have different geometrical properties. Both IF7 and [TeF7]–, for example, have 56 valence electrons, and VSEPR theory predicts that they are pentagonal bipyramidal. The equatorial F atoms of [TeF7]– are not coplanar, according to electron diffraction and X-ray diffraction measurements. Furthermore, equatorial I-F and Te-F bonds have distinct bond lengths. Because it does not account for such abnormalities, the VSEPR model fails to accommodate the correct form for [TeF7]- and other species.

For Transition Metal Compounds, VSEPR Fails

Because it ignores the relative sizes of substituents and stereochemically inactive lone pairs, the VSEPR model fails to predict the structure of some compounds. The elements in the d-block have relatively large atomic weights and stereochemically inactive electron pairs. In other words, in these elements, the valence shell s-electrons tend to play a non-bonding role. The inert pair effect is what this is called. As a result, VSEPR does not produce accurate geometries for these transitional metal complexes. Because the centre atom can have seven electron pairs, VSEPR predicts that [SeCl6]2-, [TeCl6]2-, and [BrF6]– will adopt pentagonal bipyramidal geometries. However, because one of the electron pairs is stereochemically inactive, these molecules are discovered to be regular octahedral due to the stereochemical inert pair effect.

One of the atoms in a molecule with three or more atoms is called the central atom, and the other atoms are connected to the central atom.

VSEPR Theory Applications

The assumption behind VSEPR models is that electrons circling around a core atom will organise themselves to minimise repulsion, determining the structure of the molecule.It can anticipate the shape of almost all compounds with a central atom as long as the central atom is not a metal.

The VSEPR theory can be used to predict molecular structure. Predict the structure of a gaseous CO2 molecule, for example. Only two electron groups surround the core carbon atom in the Lewis structure of CO2. The bonds are as far apart as feasible when there are two bonding groups and no lone pairs of electrons on the central atom, and when these regions of high electron density lie on opposite sides of the core atom, electrostatic repulsion between them is decreased to a minimum.The bonding angle is 180 degrees.

A linear geometry is formed by two regions of electron density surrounding a central atom in a molecule; a trigonal planar geometry is formed by three regions; a tetrahedral geometry is formed by four regions; a trigonal bipyramidal geometry is formed by five regions; and an octahedral geometry is formed by six regions.

Conclusion

According to VSEPR theory, repulsion by a lone pair is larger than repulsion by a bonded pair. As a result, when a molecule has two interactions with differing degrees of repulsion, VSEPR theory predicts that lone pairs will occupy places that reduce repulsion. However, this theory fails to account for isoelectronic species (i.e. elements having the same number of electrons). Despite having the same number of electrons, the morphologies of the species can differ. The VSEPR theory provides little insight into transition metal compounds.