Experiment 15 The Valence Shell Electron Pair Repulsion (VSEPR) Theory of Directed Valency: An exercise Attempts to understand and predict the shapes of molecules using either the valencebond theory or the molecular orbital theory are quite different. However, the VSEPR theory has the advantage that the structures of molecules can be understood and rationalized in terms of the repulsions between the electron pairs in valence shells without making any use of the concept of hybrid orbitals. By learning a few rules, students can learn to predict quite favorably the geometry of many molecules. According to the VSEPR theory, the arrangement of bonds around any one atomic center depends on the number of electron pairs surrounding this atom. Electron pairs repel each other and try to stay as far apart from other electron pairs as possible. Using simple geometry, it can be shown that the most probable arrangements of 2, 3, 4, 5 and 6 electron pairs around a central atom are linear, trigonal (equilateral triangle), tetrahedral, trigonal bipyramid, and octahedral, respectively. These general bonding classifications are illustrated in Figure 151. Knowing these bonding classifications and their relation to electron pairs surrounding a central atom A, we may predict the shapes of many molecules. The prescription given below has been selected because it allows for accurate prediction of the shapes of a greater number of molecules than do the more simple methods. 1 1 Two such methods briefly state: (a) Write the Lewis structure for the compound and from this determine the number of shared and lone pairs. (b) Add the group numbers of and the number of ligands and divide by two to obtain the number of electron pairs.
Method for Determining Ligand Lone Pair Configurations in Space 2 1. Find the total number of valence electrons for the central atom plus the ligand atoms. If charged species are involved, add or subtract the magnitude of charge from the above total depending on whether the charge is or +. 2. The number of bonding electrons to be distributed in pairs about the central atom is just twice the number of ligand atoms. Hence, the total number of bonds should equal the number of ligands. 3. Find the total number of remaining ligand electrons, those that are needed to satisfy the octet rule for the ligands. 4. Compute the number of nonbinding electrons by subtracting the sum of the bonding and remaining ligand electrons from the total number of valence electrons. 5. Distribute the bonding and nonbonding electrons in pairs about the central atom. Remember the bonding electrons are associated with ligands and the nonbonding electrons are lone pairs. 6. Once the number of bonding and nonbonding electrons pairs distributed about the central atom is known, use Figure 151 to predict the approximate geometry. Familiarize yourself with the method by proving the CH 4 is tetrahedral because C has four bonding pairs; PCl 5 is trigonal bipyramid because it has five bonding pairs; and H 2 O is tetrahedral because it has two bonding and two nonbonding pairs. 2 The present method is quite adequate for making predictions when the molecule contains only single bonds. If other than single bonds are involved, the number of shared electrons is not restricted by the theory to only two per bond. Such cases are discussed in the references listed on page.
Application of the Method The following binary molecules and ions are to be analyzed according to the outline given below. BeCl 2 SiCl 4 SF 6 ICl 5 BF 3 2 SiCl 6 ICl O 3 PCl 3 SCl 2 ICl 2 XeF 4 PCl 5 SeCl 4 ICl 3 CF 4 + 2 CH 3 CH 3 CO 3 PH 2 1. Using electron pair repulsion theory, determine the approximate geometry of the molecule. 2. Draw a sketch of the molecule showing bonding electron pairs, ligands and nonbonding electron pairs. 3. Construct a model of the molecule if necessary, (pipe cleaners may be used in place of toothpicks and may also be used to represent electrons). 4. From the molecular geometry, give the values of the angles between bonds and predict how these ideal angles would be distorted in a real molecule due to the unequal interaction of the lone pairlone pair, lone pairbond pair, and bond pairbond pair interactions. 5. Use Table 151 and determine the approximate distance between the central atom and the ligands. 6. Predict whether or not the molecule would have a dipole moment.
Table 151 Some Covalent Radii Atom Covalent Radius (pm) Be 90 B 82 P 106 Si 111 S 102 I 133 Cl 99 F 72 C (single) 77 Se 116 H 32 O (single) 73 Xe 131 To be done after the discussion of valence bond theory: 7. Rationalize the bonding in each of the above molecules by invoking the type of hybridization which best explains the molecular geometry that is observed.
VSEPR total electron pairs 2 sp orbital hybridization AX 2 linear AX 3 trigonal planar AX 2 E Bent 3 sp 2 AX 4 tetrahedral AX 3 E trigonal pyramidal AX 2 E 2 bent 4 sp 3 AX 5 trigonal bipyramidal AX 4 E seesaw AX 3 E 2 Tshaped AX 2 E 3 Linear 5 sp 3 d AX 6 octahedral AX 5 E square pyramidal AX 4 E 2 square planar 6 sp 3 d 2
Fill in the following table Electron Dot Structures and VSEPR Name Molecule Bond class Dot structure Electron Molecular Bond Net dipole Geometry Geometry distance Include angles BeCl 2 SiCl 4 BF 3 PCl 3 PCl 5 CH 3
SF 6 SiCl 6 2 SCl 2 SeCl 4 CH 3 + ICl 5 ICl 2
ICl 3 CO 3 2 O 3 XeF 4 CF 4 PH 2