Lewis-Gillespie VSEPR

Valence shell electron pair repulsion theory (VSEPR) provides a method for predicting the shape of molecules, based on the electron pair electrostatic repulsion. It was described by Sidgwick and Powell" in 1940 and further developed by Gillespie and Nyholm in 1957. In spite of this method s very simple approach, based on Lewis electron-dot structures, the VSEPR method predicts shapes that compare favorably with those determined experimentally. However, this approach at best provides approximate shapes for molecules, not a complete picture of bonding. The most common method of determining the actual stmctures is X-ray diffraction, although electron diffraction, neutron diffraction, and many types of spectroscopy are also used. In Chapter 5, we will provide some of the molecular orbital arguments for the shapes of simple molecules. 

The VSEPR model is probably the most successful and the most widely used model for predicting the shapes of simple non-ionic molecules. It builds directly on the Lewis formula of the molecule, but has been influenced by quanmm mechanics in so far as the electrons are allowed to move. A succinct description of the model has been given in a recent textbook by Gillespie and Hargittai [9] ... 

One of the goals of the ELF approach is to provide an interpretative tool of quantum chemical calculations in terms of purely chemical concepts without the recourse to the nature of the approximate wave functions. The ELF recovers many features of the simple chemical models based on a spatial distribution of the valence electron, namely, the valence theory of G. N. Lewis, the mesomery concept of C. K. Ingold, and the VSEPR model of R. J. Gillespie and R. S. Nyholm. The partition of the electron density based on a statistical criterion provides a quantum mechanical support to the hypotheses which are explicitly or implicitly assumed in these models. Indeed, this statistical approach provides at least formally a mathematical bridge between quantum mechanics and chemistry which enables to critically think about the content and the definition of many chemical concepts related to the... 

Lewis s original theory could not take into account the shape adopted by molecules. Gillespie and Nyhohn (1957) developed the currently accepted modem theory of chemical bond formation (MO and VB theories), which uses the valence-shell electron pair repulsion model (VSEPR) to account for molecular structure (Gillespie, 1970). VSEPR states that molecular shape is caused by repulsions between electron pairs in the valence shell. 

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