The Valence-Shell Electron-Pair Repulsion VSEPR Model

7 The Valence Shell Electron Pair Repulsion (VSEPR) model 

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]  

Qualitative prediction of deviations from ideal bond angles can be made by taking into account the differences of the sizes and shapes of the electron pair domains in a valence shell. The electron pair domains in a valence shell are not all equivalent for the three important reasons  

Non-bonding or lone pairs have larger domains than bonding pairs. 

Bonding domains in the valence shell of the central atom decrease in size with increasing electronegativity of the ligand X [-]. 

When you tie similar balloons together, they assume the arrangements shown.These arrangements are the same as those assumed by valence electron pairs about an atom. 

Beryllium fluoride, BeFj, is normally a solid.The Bep2 molecule exists in the vapor phase at high temperature. 

Central Atom with Two, Three, or Four Valence-Shell Electron Pairs 

Examples of geometries in which two, three, or fonr valence-shell electron pairs surround a central atom are shown in Eigure 10.4. 

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The tetrahedral geometry of methane is often explained with the valence shell electron pair repulsion (VSEPR) model The VSEPR model rests on the idea that an electron pair either a bonded pair or an unshared pair associated with a particular atom will be as far away from the atom s other electron pairs as possible Thus a tetrahedral geomehy permits the four bonds of methane to be maximally separated and is charac terized by H—C—H angles of 109 5° a value referred to as the tetrahedral angle... 

The major features of molecular geometry can be predicted on the basis of a quite simple principle—electron-pair repulsion. This principle is the essence of the valence-shell electron-pair repulsion (VSEPR) model, first suggested by N. V. Sidgwick and H. M. Powell in 1940. It was developed and expanded later by R. J. Gillespie and R. S. Nyholm. According to the VSEPR model, the valence electron pairs surrounding an atom repel one another. Consequently, the orbitals containing those electron pairs are oriented to be as far apart as possible. 

The molecular geometry of a complex depends on the coordination number, which is the number of ligand atoms bonded to the metal. The most common coordination number is 6, and almost all metal complexes with coordination number 6 adopt octahedral geometry. This preferred geometry can be traced to the valence shell electron pair repulsion (VSEPR) model Introduced In Chapter 9. The ligands space themselves around the metal as far apart as possible, to minimize electron-electron repulsion. 

The most widely used qualitative model for the explanation of the shapes of molecules is the Valence Shell Electron Pair Repulsion (VSEPR) model of Gillespie and Nyholm (25). The orbital correlation diagrams of Walsh (26) are also used for simple systems for which the qualitative form of the MOs may be deduced from symmetry considerations. Attempts have been made to prove that these two approaches are equivalent (27). But this is impossible since Walsh s Rules refer explicitly to (and only have meaning within) the MO model while the VSEPR method does not refer to (is not confined by) any explicitly-stated model of molecular electronic structure. Thus, any proof that the two approaches are equivalent can only prove, at best, that the two are equivalent at the MO level i.e. that Walsh s Rules are contained in the VSEPR model. Of course, the transformation to localised orbitals of an MO determinant provides a convenient picture of VSEPR rules but the VSEPR method itself depends not on the independent-particle model but on the possibility of separating the total electronic structure of a molecule into more or less autonomous electron pairs which interact as separate entities (28). The localised MO description is merely the simplest such separation the general case is our Eq. (6)... 

In this chapter a few simple rules for predicting molecular structures will be investigated. We shall examine first the valence shell electron pair repulsion (VSEPR) model, and then a purely molecular orbital treatment. 

Like so many other molecular properties, shape is determined by the electronic structure of the bonded atoms. The approximate shape of a molecule can often be predicted by using what is called the valence-shell electron-pair repulsion (VSEPR) model. Electrons in bonds and in lone pairs can be thought of as "charge clouds" that repel one another and stay as far apart as possible, thus causing molecules to assume specific shapes. There are only two steps to remember in applying the VSEPR method ... 

R. J. Gillespie, A Defense of the Valence Shell Electron Pair Repulsion (VSEPR) Model, J. Chem. Educ. 1974, 51, 367. 

Of the 20th century s development of structural chemistry, we mention the discovery of the electron-pair covalent bond by Lewis [22] which remains a fundamental tenet. It is remembered in every line we have drawn to represent a linkage and is present in most models of molecular structure, such as, for example, the valence shell electron pair repulsion (VSEPR) model [23]. 

VSEPR The valence shell electron pair repulsion (VSEPR) model is based on the observation that principles the geometrical arrangement of bonds around an atom is influenced by nonbonding... 

An important group of cations that shows electronically distorted environments are those of the main group elements in lower oxidation states. These contain nonbonding electron pairs in their valence shells, the so-called lone pairs . Such atoms are usually found displaced from the center of their coordination sphere so as to form between 3 and 5 strong bonds and a number of weaker ones. The effect can be described using the Valence Shell Electron Pair Repulsion (VSEPR) Model [43] in which it is assumed that the cation is surrounded uniformly by between 4 and 7 electron pairs occupying valence shell orbitals. One or more of these is a lone pair... 

Organic chemists find that the information obtained from MO theory, where valence electrons occupy bonding and antibonding molecular orbitals, does not always yield the needed information about the bonds in a molecule. The valence-shell electron-pair repulsion (VSEPR) model combines the Lewis concept of shared electron pairs and lone-pair electrons with the concept of atomic orbitals and adds a third principle the minimization of electron repulsion. In this model, atoms share electrons by overlapping... 

Molecular structure can be predicted by using the valence shell electron pair repulsion (VSEPR) model. 

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