The Valence Shell Electron Pair Repulsion model

6 The Valence Shell Electron Pair Repulsion model 

The Group 13 atom in a trichloride or a trimethyl derivative is surrounded by three bonding electron pairs. According to the VSEPR model the three valence shell electron pairs surrounding the metal atom repel one another and occupy domains near the comers of an equilateral triangle with the metal atom at the center. The observed trigonal planar structures are thus in accord with the VSEPR model, but the model provides no explanation for the observation that the bonds in the trichlorides are shorter and weaker than in the monochlorides. 

We now turn to the molecular orbital model. If the three M-Cl or M-C bonds in the trichlorides or trimethyls are to be described as a 2c, 2c bonds, we need to hybridize the valence shell s and p orbitals at the central atom in such a manner that we obtain three equivalent and orthogonal hybrid AOs pointing towards the three ligands, i.e. forming angles of 120° with respect to one another. 

We have seen that combination of valence shell s and Pz AOs yield hybrid AOs with maximum electron density on the positive or negative z-axis combination with an s orbital does not change the direction of the p orbital, it only transfers electron density from one side to the nucleus to the other. If we wish to form sp hybrid orbitals on the central atom in a trichloride that point in the directions of the three ligands, we must first construct pure p orbitals pointing in the right directions. 

According to equation (1.24) the three valence shell p orbitals may be written on the form 

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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 ... 

Having introduced methane and the tetrahedron, we now begin a systematic coverage of the VSEPR model and molecular shapes. The valence shell electron pair repulsion model assumes that electron-electron repulsion determines the arrangement of valence electrons around each inner atom. This is accomplished by positioning electron pairs as far apart as possible. Figure 9-12 shows the optimal arrangements for two electron pairs (linear),... 

The distortion produced by the lone pairs is traditionally described using the Valence Shell Electron Pair Repulsion Model (VSEPR model) (Gillespie and Hargittai 1991), which assumes that each pair of electrons in the valence shell is... 

In the valence-shell electron-pair repulsion model, or VSEPR model, we focus attention on the central atom of a molecule, such as the B atom in BF3 or the C atom in C02. We then imagine that all the electrons involved in bonds to the central atom and the electrons of lone pairs belonging to that atom lie on the surface of an invisible sphere that surrounds it (Fig. 3.3). These bonding electrons and lone pairs are regions of high electron concentration, and they repel one another. To minimize their repulsions, these regions move as far apart as possible on the surface of the sphere. Once we have identified the most distant ... 

The structures of the binary fluorides are predictable on the basis of the valence shell electron pair repulsion model (see Chapter 2). With eight valence shell electrons from the xenon atom and two additional electrons from the two fluorine atoms, there are 10 electrons surrounding the xenon atom in XeF2. Thus, the structure of XeF2 has Doah symmetry as shown here ... 

A. Schmiedekamp, D. W. J. Cruickshank, S. Skaarup, P. Pulay, I. Hargittai, J. E. Boggs, Investigation of the Basis of the Valence Shell Electron Pair Repulsion Model by ab Initio Calculation of Geometry Variations in a Series of Tetrahedral and Related Molecules. J. Am. Chem. Soc. 1979, 101, 2002-2010. 

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... 

The Valence Shell Electron-pair Repulsion Model... 

Molecular geometry and the valence-shell electron pair repulsion model... 

Once a Lewis structure is drawn, you can determine the molecular geometry, or shape, of the molecule. The model used to determine the molecular shape is referred to as the Valence Shell Electron Pair Repulsion model, or VSEPR model. This model is based on an arrangement that minimizes the repulsion of shared and unshared pairs of electrons around the central atom. 

The observed variations in the N—C bond distances and < C3—N—C angles are not in agreement with the valence-shell electron-pair repulsion model. This predicts that partial removal of the lone-pair electrons on the nitrogen atom would lead to a shortening of the N—C bonds. It would further predict that < C3—N—C should be greater than tetrahedral in the free donor and decrease on complex formation. 

While the valence angles around the aluminum atom are indistinguishable from those in [( H3)2A1F]4, the < Al— 1—Al angle is only 91°. That this angle is less than tetrahedral is in agreement with the valence-shell electron-pair repulsion model. learly the greater size of the chlorine atom reduces the importance of repulsion between the metal atoms. 

Lewis structures give the connectivity of an atom in a molecule, the bond order and the number of lone pairs and these may be used to derive structures using the valence-shell electron-pair repulsion model (see Section 1.19). 

THE VALENCE SHELL ELECTRON-PAIR REPULSION MODEL AND THE LIGAND CLOSE-PACKING MODEL... 

Schmiedekamp A, Cruickshank DWJ, Skaamp S, Pulay P, Haigittai I, Boggs JE (1979) Investigation of the basis of the valence shell electron pair repulsion model by ab initio calculation of geometry variations in a series of tetrahedral and related molecules. J Am Chem Soc 101 2002-2010... 

HOW TO Predict Molecular Geometry The Valence Shell Electron Pair Repulsion Model 44... 

Like Charges Repel It is the repulsion of the electrons in covalent bonds of the valence shell of a molecule that is central to the valence shell electron pair repulsion model for explaining molecular geometry. And, although it is not so obvious, this same factor underlies the explanations of molecular geometry that come from orbital hybridization because these repulsions are taken into account in calculating the orientations of the hybrid orbitals. 

The significant difference between the bond angles in NH3 (valence-shell electron-pair repulsion model of Gillespie (or its earlier variants) by a smaller repulsion of bonding electron pairs in NF3 as compared to... 

VSEPR The valence shell electron pair repulsion model, originally introduced by Nyholm and Gillespie (with antecedents from Sidgwick and Powell), which assumes that molecular geometry associated with a central atom is determined by the number of groups (single bonds, double bonds, triple bonds, or lone pairs) surrounding that atom. 

The character of a covalent bond, the main focus of this chapter, was identified by G.N. Lewis in 1916, before quantum mechanics was fully developed. Lewis s original theory was unable to account for the shapes adopted by molecules. The most elementary (but qualitatively quite successful) explanation of the shapes adopted by molecules is the valence-shell electron pair repulsion model (VSEPR model). In this model, which should be familiar from introductory chemistry courses, the shape of a molecule is ascribed to the repulsions between electron pairs in the valence shell. The purpose of this chapter is to extend these elementary arguments and to indicate some of the contributions that quantum theory has made to understanding why atoms form bonds and molecules adopt characteristic shapes. 

The main advantages of this model are its easy visualisation and its obvious affinity with the valence-shell-electron pair repulsion model. The difference of this approach from many earlier methods of representing non-bonding pairs by point charges is that it needs only relatively tiny charges (e.g. few hundredths of an electron) at reasonable distances (r 1 A). These small charges are generated naturally from the condition that the observed barrier... 

The geometry of the interhalides can be predicted from the valence shell electron pair repulsion model (VSEPR). Because the halogens do not form double or triple bonds, the shapes of these compounds are relatively straightforward to determine, as shown in Examples 22.8 and 22.9. 

The Valence Shell Electron Pair Repulsion model correctly predicts the structures of main group compounds and efi transition metal compounds in most cases. The structures of less symmetric molecules can be predicted semi-quantitatively. The model is particularly successful if the coordination number (including the count of nonbonding electron pairs) does not exceed six. A peculiarity of the model is the stereochemical activity of the free electron pair. Whether the VSEPR model can be successfully extended to coordination numbers larger than 6 is the topic of the following discussion. 

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