Molecular shape and the VSEPR model

In order to investigate the way in which CO bonds to metals, we must appreciate the electronic structure of the carbon monoxide molecule. 

Before constructing an orbital interaction diagram for CO, we must take note of the following  

Two species are isoelectronic if they possess the same total number of electrons. 

If two species contain the same number of electrons they are isoelectronic, CH4, [BH4] and [NH4] are isoelectronic, since each contains a total of 10 electrons. Two other series of isoelectronic species are N2, CO and [NO]+, and [SiFd -, [PFfi]- and SFg. 

The word isoelectronic is often used in the context of meaning same number of valence electrons, although strictly such usage should always be qualified e.g. HF, HCl and HBr are isoelectronic with respect to their valence electrons. 

Valence-shell electron-pair repulsion model 

The shapes of molecules containing a central / -block atom tend to be controlled by the number of electrons in the valence shell of the central atom. The valence-shell electron-pair repulsion (VSEPR) model provides a simple model for predicting the shapes of such species. The model combines original ideas of Sidgwick and Powell with extensions developed by Nyhohn and Gillespie, and may be summarized as follows  

The VSEPR model works best for simple halides of the j-block elements, but may also be applied to species with other substituents. However, the model does not take steric factors (i.e. the relative sizes of substituents) into account. 

When structures are determined by diffraction methods, atom positions are effectively located. Thus, in terms of a structural descriptor XeF2 is linear and [XeFs] is pentagonal planar. In the diagrams above we show two representations of each species, one with the lone pairs to emphasize the origin of the prediction from the VSEPR model. 

Show that the VSEPR model is in agreement with the following molecular shapes  

Pictorial representations of the HOMO and one of the LUMOs are given in Fig. 2.15 refer to end-of-chapter problem 2.21. 

HOMO — highest occupied molecular orbital. L17Mt) = lowest unoccupied molecular orbital. 

The valence-shell electron-pair repulsion ( VSEPR) model is used to rationalize or predict the shapes of molecular species. It is based on the assumption that electron pairs adopt arrangements that minimize repulsions between them. 

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MO theory heteronuclear diatomic molecules Isoelectronic molecules Molecular shape and the VSEPR model Geometrical isomerism... 

The electron-dot structures described in Sections 7.6 and 7.7 provide a simple way to predict the distribution of valence electrons in a molecule, and the VSEPR model discussed in Section 7.9 provides a simple way to predict molecular shapes. Neither model, however, says anything about the detailed electronic nature of covalent bonds. To describe bonding, a quantum mechanical model called valence bond theory has been developed. 

Strategy Use Lewis structures and the VSEPR model to determine first the electron-domain geometry and then the molecular geometry (shape). 

STRATEGY For the electron arrangement, draw the Fewis structure and then use the VSEPR model to decide how the bonding pairs and lone pairs are arranged around the central (nitrogen) atom (consult Fig. 3.2 if necessary). Identify the molecular shape from the layout of atoms, as in Fig. 3.1. 

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),... 

Tetrahedral geometry may be the most common shape in chemistry, but several other shapes also occur frequently. This section applies the VSEPR model to four additional electron group geometries and their associated molecular shapes. 

The carbon atom in CO2 has two groups of electrons. Recall from our definition of a group that a double bond counts as one group of four electrons. Although each double bond includes four electrons, all four are concentrated between the nuclei. Remember also that the VSEPR model applies to electron groups, not specifically to electron pairs (despite the name of the model). It is the number of ligands and lone pairs, not the number of shared eiectrons, that determines the steric number and hence the molecular shape of an inner atom. 

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)... 

First, use the VSEPR model described in Section 7.9 to predict the molecular shape of vinyl chloride. Then, assign polarities to the individual bonds according to the differences in electronegativity of the bonded atoms (Figure 7.4), and make a reasonable guess about the overall polarity that would result by summing the individual contributions. 

The VSEPR model works at its best in rationalizing ground state stereochemistry but does not attempt to indicate a more precise electron distribution. The molecular orbital theory based on 3s and 3p orbitals only is also compatible with a relative weakening of the axial bonds. Use of a simple Hiickel MO model, which considers only CT orbitals in the valence shell and totally neglects explicit electron repulsions can be invoked to interpret the same experimental results. It was demonstrated that the electron-rich three-center bonding model could explain the trends observed in five-coordinate speciesVarious MO models of electronic structure have been proposed to predict the shapes and other properties of non-transition element... 

Concept Mapping Design a concept map that will link both the VSEPR model and the hybridization theory to molecular shape. 

For a discussion of the shapes of alkyl complexes related to the VSEPR model of molecular geometry, see G. S. McGrady and A. J. Downs, Coord. Chem. Rev., 2000,197, 95. 

The VSEPR model generates reliable predictions of the geometries of a variety of molecular structures. Chemists use the VSEPR approach because of its simphcity. Although there are some theoretical concerns about whether electron-pair repulsion actually determines molecular shapes, the assumption that it does leads to useful (and generally reliable) predictions. We need not ask more of any model at this stage in the study of chemistry. 

The VSEPR model we discussed in Chapter 10 accounts for molecular shapes by assuming that electron groups minimize their repulsions, and thus occupy as much space as possible around a central atom. But it does not explain how the shapes arise from interactions of atomic orbitals. After all, the orbitals we examined in Chapter 7 aren t oriented toward the comers of a tetrahedron or a trigonal bipyramid, to mention just two of the common molecular shapes. Moreover, knowing the shape doesn t help us explain the magnetic and spectral properties of molecules only an understanding of their orbitals and energy levels can do that. 

Scientists choose the model that best helps them answer a particular question. If the question concerns molecular shape, chemists choose the VSEPR model, followed by hybrid-orbital analysis with VB theory. But VB theory does not adequately explain magnetic and spectral properties, and it understates the importance of electron delocalization. In order to deal with these phenomena, which involve molecular energy levels, chemists choose molecular orbital (MO) theory. 

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