Valence shell electron pair repulsion rule

We have already assumed that electron pairs, whether in bonds or as nonbonding pairs, repel other electron pairs. This is manifested in the tetrahedral and trigonal geometry of tetravalent and trivalent carbon compounds. These geometries correspond to maximum separation of the electron-pair bonds. Part of this repulsion is electrostatic, but there is another important factor. The Pauli exclusion principle states that only two electrons can occupy the same point in space and that they must have opposite spin quantum numbers. Equivalent orbitals therefore maintain maximum separation, as found in the sp, sjf, and sp hybridization for tetra-, tri-, and divalent compounds of the second-row elements. The combination of Pauli exclusion and electrostatic repulsion leads to the valence shell electron-pair repulsion rule (VSEPR), which states that bonds and unshared electron pairs assume the orientation that permits maximum separation. 

You recall from Lecture 7 that the VSEPR (valence shell electron pair repulsion) rules predict the shapes of main-element molecules with nice compact reasoning, by simply counting the number of electron pairs, bond pairs and lone pairs around the central atom. In transition metal (TM) complexes, the lone pairs on the TM do not occupy space in the same manner as they do in the case of the main group elements, so what matters for the 3D shape is only the number of bond pairs the TM maintains with the ligands. This number is also known as the coordination number (cn) by the historical referral of Werner to these complexes as coordination complexes. 

The Lewis structures encountered in Chapter 2 are two-dimensional representations of the links between atoms—their connectivity—and except in the simplest cases do not depict the arrangement of atoms in space. The valence-shell electron-pair repulsion model (VSEPR model) extends Lewis s theory of bonding to account for molecular shapes by adding rules that account for bond angles. The model starts from the idea that because electrons repel one another, the shapes of simple molecules correspond to arrangements in which pairs of bonding electrons lie as far apart as possible. Specifically ... 

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)... 

The remaining two are the unchanged 2py and orbitals. There are six electrons to be placed in these orbitals, and for the linear structure, the lowest two, A and B, will be filled, and the Ipy, and 2p will have a single electron each. Hund s rule suggests that the triplet state with parallel spins will be lower in energy than the singlet state (Fig. 7.1). For an related, alternative valence-shell electron-pair repulsion (VESPR) theory treatment see Chapter 11 by Platz in this volume. 

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. 

If an attempt were made to apply the rules of valence shell electron pair repulsion theory to radicals, it would not be clear how to treat the single electron. Obviously, a single electron should not be as large as a pair of electrons, but it is expected to result in some repulsion. Therefore, it is difficult to predict whether a radical carbon should be sp2 hybridized with trigonal planar geometry (with the odd electron in a p orbital), sp3 hybridized with tetrahedral geometry (with the odd electron in an sp3 AO), or somewhere in between. Experimental evidence is also somewhat uncertain. Studies of the geometry of simple alkyl radicals indicate that either they are planar or, if they are pyramidal, inversion is very rapid. 

Some simple rules were supported by empirial evidence, valence shell electron pair repulsion model (VSEPR) and MO calculations, both semiempirical and ab initio. These rules could explain those features of molecular geometry which have been characterized by structural investigations using spectroscopic and diffraction techniques. 

The geometric structure of the covalent binary halides, whether neutral or complexed ions, can be explained on the basis of the Nyhotm-Gillespie rules known as the Valence Shell Electron Pair Repulsion Model (VSEPR) theory the geometrical arrangements of the bonds around an atom in a species depends on the total number of electron pairs in the valence shell of the central atom, including both bonding... 

For xenon fluorides and oxides, for example, the same models can be apphed as for interhalogen and halogen oxy species. Furthermore, the very successful valence shell electron pair repulsion (VSEPR) rules for molecule and ion shapes are as effective for noble gas compounds and their relatives as for classical octet compounds. 

The introduction of Valence Bond theory has motivated the search for structural regularities that can be interpreted by models of local electronic features, such as the powerful model of Valence Shell Electron Pair Repulsion [93,94] theory. Alternative approaches, based on Molecular Orbital theory, have led to the discovery of important rules, such as the Woodward-Hoffmann orbital symmetry rules [95] and the frontier orbital approach of Fukui [96,97], As a result of these advances and the spectacular successes of ab initio computations on molecular... 

VSEPR (valence-shell electron-pair repulsion) A theory that allows the prediction of shapes of simple polyatomic molecules by applying a set of rules. 

The valence shell electron pair repulsion theory states that electron pairs aroimd the central atom of the molecule arrange themselves to minimize electronic repulsion the electrons orient themselves as far as possible from each other. Two electron pairs around the central atom lead to a linear arrangement of the attached atoms three indicate a trigonal planar arrangement, and four result in a tetrahedral geometry. Both lone pair and bonding pair electrons must be taken into accoimt when predicting structure. Molecules with fewer than four and as many as five or six electron pairs around the central atom also exist. They are exceptions to the octet rule. 

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