All about VSEPR

What is VSEPR Theory?

Valence Shell Electron Pair Repulsion (VSEPR) is a chemical model used to predict the shape of a single covalent molecule. The theory predicts the geometric configuration of a molecule by calculating the number of valence electrons and coordination number of the central atom, and constructs a reasonable Lewis structure to represent the position of all bonds and lone pairs of electrons in the molecule.

What is the theoretical basis of VSEPR?

The geometric configuration of a molecule or ion is mainly determined by the repulsion between the electron pairs associated with the central atom. The electron pairs can be either bonded or unbonded (called lone pairs). Molecules will try their best to avoid the repulsion of electron pairs to maintain stability. When repulsion cannot be avoided, the entire molecule tends to form a structure with the weakest repulsion. The number of electron pairs determines the overall geometric shape they will adopt.

What is molecular shape table?

The following is a table of common molecular shapes predicted by the valence shell electron pair repulsion theory:

Number of electron pairs	Hybridization type	Shape of hybrid orbitals	Number of lone electron pairs	Molecular shape	Examples
2	sp	Linear	0	Linear	BeCl2 CO2
3	sp2	Trigonal planar	0	Trigonal planar	BCl3
			1	Bent(V-shaped)	SO2
4	sp3	Tetrahedral	0	Tetrahedral	CCl4
			1	Trigonal pyramidal	NH3
			2	Bent(V-shaped)	H2O
5	sp3d	Trigonal bipyramidal	0	Trigonal bipyramidal	PCl5
			1	Seesaw	TeCl4
			2	T-shaped	ClF3
			3	Linear	XeF2
6	sp3d2	Octahedral	0	Octahedral	SF6
			1	Square pyramidal	IF5
			2	Square planar	ICl4
			3	T-shaped	ClF3
			2	Square planar	XeF4
7	sp3d3	Pentagonal bipyramidal	0	Pentagonal bipyramidal	IF7
			1	Pentagonal pyramidal	IF5

How to use VSEPR Theory to speculate the spatial structure of molecules?

The following is a method for inferring the spatial structure of molecules using the valence electron pair repulsion theory of AB_n type molecules:

1) Determine the number of σ bond electron pairs of the central atom through the chemical formula

2) Calculate the number of lone electron pairs of the central atom

3) Determine the number of valence electron pairs of the central atom

4) Determine the spatial orientation of the valence electron pairs based on the number of valence electron pairs of the central atom

5) Use the VSEPR model and the lone electron pairs and multiple bonds of the central atom to infer the spatial structure of the molecule.

What are the exceptions to the VSEPR theory?

In some compounds, the VSEPR theory cannot correctly predict the spatial configuration of molecules.

1) Transition metal compounds

The geometric configuration of many transition metal compounds cannot be explained by the VSEPR theory, which can be attributed to the lack of lone pairs in the valence electrons and the interaction between the core d electrons and the ligands

2) Group IIA halides

The gas phase structure of triatomic halides of heavier alkaline earth metals (for example: calcium, strontium, barium halides) is not linear as predicted, but V-shaped. This may be because the ligands interact with the inner electrons of the metal atoms, polarizing the inner electron cloud so that it is not completely spherically symmetrical, thus causing a change in the molecular configuration.

3) Some AX₂E₂ molecules

An example is the lithium oxide molecule Li₂O, whose intermediate configuration is straight rather than curved, which can be attributed to the fact that if the configuration is curved, there will be a strong repulsion between lithium atoms.

Another example is the bond angle of the Si-O-Si bond of disilyl (O(SiH₃)₂) is 144.1°, which is quite different from the bond angles in other molecules. A more reasonable explanation is that the positions of the lone pairs of electrons are different. When the central atom has a large electronegativity, like in O(SiH₃)₂, the localization of the lone pair of electrons is not obvious, and the repulsion is weak. This combination leads to repulsion between strong ligands, making the bond angle of the Si-O-Si bond larger than expected.

4) Some AX₆E₁ molecules

Some AX₆E₁ molecules, such as compounds containing Te(IV) or Bi(III) ions such as TeCl₆, TeBr₆, BiCl₆, BiBr₆ and BiI₆, are regular octahedral structures; their lone pair electrons do not affect their configuration. One rational explanation is that the crowded arrangement of ligand atoms leaves no space for lone pairs of electrons; another rational explanation is the inert electron pair effect.