VSEPR model

The generalizations of the VSEPR model are useful, but there are limitations to its use. In this section, we give examples that illustrate some problems. The isoelectronic species IF7 and [TeFy] are predicted by the VSEPR model to be pentagonal bipyramidal and this is observed. However, electron diffraction data for IF7 and X-ray diffraction data for Me4N] TeF7] reveal that the equatorial F atoms are not coplanar, a result that cannot be predicted by the VSEPR model. Moreover, in IF7, the I-Fax and I F q distances are 179 and 186 pm respectively, and in [TeF7], the Te-Fax bond distance is 179 pm and the Te-F q distances lie in the range 183 to 190 pm. 

It is important to note that whereas the VSEPR model may be applicable to p-block species, it is not appropriate to apply it to d-electron configurations of transition metal compounds (see Chapters 20-24). 

If the presence of a lone pair of electrons influences the shape of a molecule or ion, the lone pair is stereochemically active. If it has no effect, the lone pair is stereochemically inactive. The tendency for the pair of valence s electrons to adopt a non-bonding role in a molecule or ion is termed the stereochemical inert pair effect. 

Valence shell electron pair repulsion (VSEPR) model 

The simplest way of looking at the VSEPR approach is to regard each valence electron pair as being represented by a point, the points being constrained to move over the surface of a sphere drawn around the central atom. As the points (i.e.valence shell electron pairs) repel each other, the most stable molecular arrangement will be that in which the points are as far apart as possible. If each valence shell electron pair is called P, then the following geometries of the Ps around the central atom M are at once predicted  

Comparison with the geometries which are discussed in Chapter 3 will show that for main group elements these predictions are rather good. So, [BFj NMe3], in which there are four valence electron pairs around the boron, is approximately tetrahedral, the distortion being consistent with the 

On the whole, the VSEPR method predicts the geometries of main group compounds and complexes rather well. This is not the same thing as saying that it provides a correct explanation of molecular geometry. Indeed, in the opinion of some, its status and use is just that of prediction, that of an aid to getting the right answer. The VSEPR method only really works for main 

Most workers who have considered this question have concluded that there is. Crucial is the way that a single entity, the electron distribution in a molecule, is divided up to give distinct lone and bonding pairs. What has usually been done is to adopt some criterion which seems to lead to a division as close as possible to that made, more qualitatively, by the VSEPR model itself. 

When structures are determined by diffraction methods, atom positions are effectively located. Thus, in terms of a molecular structure, Xep2 is linear and [XeF5] is pentagonal planar. In the diagrams above, two representations of each species are shown, one with the lone parrs to emphasize the origin of the prediction from the VSEPR model. 

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

Worked example 2.8 VSEPR molecules with double bonds 

Is the VSEPR model consistent with a linear or bent structure for [NOi]  

N is in group 15 and has five valence electrons. Allow the positive charge to be localized on the nitrogen centre an N centre has four valence electrons. O is in group 16 and has six valence electrons an atom of O requires two electrons to complete its octet. All four electrons in the valence shell of the N centre are involved in bonding, forming two double bonds in [NO]. Since there are no lone pairs on the N atom, the VSEPR model is consistent with a linear structure  

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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... 

Multiple bonds are treated as a single unit m the VSEPR model Formaldehyde is a trigonal planar molecule m which the electrons of the double bond and those of the two single bonds are maximally separated A linear arrangement of atoms m carbon diox ide allows the electrons m one double bond to be as far away as possible from the elec Irons m the other double bond... 

Valence shell electron pair repulsion (VSEPR) model (Section 110) Method for predicting the shape of a molecule based on the notion that electron pairs surrounding a central atom repel one another Four electron pairs will arrange them selves in a tetrahedral geometry three will assume a trigo nal planar geometry and two electron pairs will adopt a linear arrangement... 

VSEPR model, the dihalides of Be and Mg and the heavier halides of Ca and Sr are essentially linear. However, the other dihalides are appreciably bent, e.g. Cap2 145°, Srp2 -- 120°, Bap2 108° SrCl2 - 130°, BaCh - 115° BaBri -115° Bah 105°. The uncertainties on these bond angles are often quite large ( 10°) and the molecules are rather flexible, but there seems little doubt that the equilibrium geometry is substantially non-linear. This has been interpreted in terms of sd (rather than sp) hybridization or by a suitable id hoc modification of the VSEPR theory. ... 

R. J. Gillespie and I. Hargittai The VSEPR Model of Molecular Geometry, Allyn and Bacon, 1991. 

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. 

Figure 7.5 (page 177) shows the geometries predicted by the VSEPR model for molecules of the types AX2 to AX. The geometries for two and three electron pairs are those associated with species in which the central atom has less than an octet of electrons. Molecules of this type include BeF2 (in the gas state) and BF3, which have the Lewis structures shown below ... 

In many molecules and polyatomic ions, one or more of the electron pairs around the central atom are unshared. The VSEPR model is readily extended to predict the geometries of these species. In general—... 

Geometries of molecules such as these can be predicted by the VSEPR model The results are shown in Figure 7.8 (page 181). The structures listed include those of all types of molecules having five or six electron pairs around the central atom, one or more of which may be unshared. Note that—... 

The VSEPR model is readly extended to species in which double or triple bonds are present A simple principle applies Insofar as molecular geometry is concerned, a multiple bond behaves like a single bond. This makes sense. The four electrons in a double bond, or the six electrons in a triple bond, must be located between the two atoms, as are the two electrons in a single bond. This means that the electron pairs in a multiple bond must occupy the same region of space as those in a single bond. Hence the extra electron pairs in a multiple bond have no effect on geometry. 

The VSEPR model applies equally well to molecules in which there is no single central atom. Consider the acetylene molecule, C2H2. Recall that here the two carbon atoms are joined by a triple bond ... 

Four-coordinate metal complexes may have either of two different geometries (Figure 15.3). The four bonds from the central metal may be directed toward the comers of a regular tetrahedron. This is what we would expect from VSEPR model (recall Chapter 7). Two common tetrahedral complexes are Zn(NH3)42+ and C0CI42. ... 

VSEPR model Valence Shell Electron Pair Repulsion model, used to predict molecular geometry states that electron pairs around a central atom tend to be as far apart as possible, 180-182... 

In some respects arenediazonium ions show analogies to acetylene. Acetylene has two deformation vibrations, v4 at 613.5 cm-1 and v6 at 729.6 cm-1, as shown in Figure 7-1 (Feldmann et al., 1956). The fact that the symmetrical vibration v4 has a lower frequency than v6 can be understood from BartelPs valence-shell electron-pair repulsion (VSEPR) model (1968) on the basis of a <pseudo-Jahn-Teller> effect. 

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 VSEPR model was first explored by the British chemists Nevil Sidgwick and Herbert Powell and has been developed by the Canadian chemist Ronald Gillespie. 

A molecule with only two atoms attached to the central atom is BeCl2. The Lewis structure is CI — Be — CE, and there are no lone pairs on the central atom. To be as far apart as possible, the two bonding pairs lie on opposite sides of the Be atom, and so the electron arrangement is linear. Because a Cl atom is attached by each bonding pair, the VSEPR model predicts a linear shape for the BeCL molecule, with a bond angle of 180° (4). That shape is confirmed by experiment. 

A sulfur hexafluoride molecule, SF6, has six atoms attached to the central S atom and no lone pairs on that atom (8). According to the VSEPR model, the electron arrangement is octahedral, with four pairs at the corners of a square on the equator and the remaining two pairs above and below the plane of the square (see Fig. 3.2). An F atom is attached to each electron pair, and so the molecule is predicted to be octahedral. All its bond angles are either 90° or 180°, and all the F atoms are equivalent. 

The second rule of the VSEPR model concerns the treatment of multiple... 

According to the VSEPR model, regions of high electron concentration take up positions that maximize their separations electron pairs in a multiple bond are treated as a single unit. The shape of the molecule is then identified from the relative locations of its atoms. 

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. 

When there is more than one central atom in a molecule, we concentrate on each atom in turn and match the hybridization of each atom to the shape at that atom predicted by VSEPR. For example, in ethane, C2H6 (38), the two carbon atoms are both central atoms. According to the VSEPR model, the four electron pairs around each carbon atom take up a tetrahedral arrangement. This arrangement suggests sp hybridization of the carbon atoms, as shown in Fig. 3.14. Each... 

STRATEGY Use the VSEPR model to identify the shape of the molecule and then assign the hybridization consistent with that shape. All single bonds are cr-bonds and multiple i bonds are composed of a cr-bond and one or more TT-bonds. Because the C atom is attached to three atoms, we anticipate that its hybridization scheme is sp1 and that one unhybridized p-orbital remains. Finally, we form cr- and Tr-bonds by allowing the 1 orbitals to overlap. 

Use the VSEPR model to identify The C atom is bonded to 3 atoms and has no lone the electron arrangements around pairs therefore, it has a trigonal planar arrangement,... 

Explain the basis of the VSEPR model of bonding in terms of repulsions between electrons (Section 3.1). 

Using the VSEPR model, predict the shapes of each of the following molecules and identify the member of each pair with the higher boiling point (a) PBr3 or PF3 (b) S02 or C02 ... 

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