Valence Shell Electron Pair Repulsion VSEPR

5 Valence Shell Electron Pair Repulsion (VSEPR) 

When molecules have unshared pairs of electrons (sometimes referred to as lone pairs) in addition to the bonding pairs, repulsion is somewhat different than described earlier. The reason is that unshared pairs are not localized between two positive nuclei as are bonding pairs. As a result, there is a difference in the repulsion between two bonding pairs and the repulsion occurring between a bonding pair and an unshared pair. Likewise, there is an even greater repulsion between two unshared pairs. Thus, with regard to repulsion, 

For NH3, there is only one unshared pair of electrons so its interaction with the three bonding pairs produces less reduction of the bond angle than in the case of water. The observed H-N-H angle is about 107° in accord with this expectation. 

In the SF4 molecule there are six valence electrons from the central atom and four from the four F atoms (one from each). Thus, there are 10 electrons around the central atom (five pairs), which will be directed in space toward the corners of a trigonal bipyramid. However, because there are only four F atoms with a bonding pair of electrons to each, the fifth pair of electrons must an unshared pair on the sulfur atom. With there being five pairs of electrons around the central atom in SF4, there are two possible structures, which can be shown as 

However, the correct structure is on the left with the unshared pair of electrons in an equatorial position. In that structure, the unshared pair of electrons is in an orbital 90° from two other pairs and 120° from two other pairs. In the incorrect structure on the right, the unshared pair is 90° from three pairs and 180° from one pair of electrons. Although it might not seem as if there is more space in the equatorial positions, the repulsion there is less than in the axial positions. An unshared pair of electrons is not restricted to motion between two atomic centers, 

1 VALENCE SHELL ELECTRON PAIR REPULSION (VSEPR) 

In the VSEPR method the resulting structure depends on the calculated number of electron pairs around the central atom of the molecule. The resulting geometry is given by Table 8.6 

Electronic Motion in the Mean Field Atoms and Molecules 

Example 4. Water molecule. First, some guesses before using the minimal model. The hydrogen atom has a single electron and, therefore valency one, the oxygen atom has valency two (two holes in the valence shell). We expect, therefore, that the compounds of the two elements will have the following chemical bond patterns (that saturate their valencies) H-O-FI, H-O-O-H, etc. Now our minimal model comes into play. Even quite simple Hartree-Fock calculations show that the system H-O-O-H is less stable than H-O-H + O. Thus, the minimal model predicts, in accordance with what we see in the oceans, that the H2O compound called water is the most stable. 

The minimal model (within the STO 6-31G basis set) predicts three harmonie vibrational frequeneies of the water moleeule antisymmetric stretching 4264 cm symmetric stretching 4147 cm and bending 1770 cm It is not easy, though, to predict the corresponding experimental frequencies. We measure the energy differences between consecutive vibrational levels (see Chapter 6, p. 235), which are 

---

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... 

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... 

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. 

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 molecular geometry of a complex depends on the coordination number, which is the number of ligand atoms bonded to the metal. The most common coordination number is 6, and almost all metal complexes with coordination number 6 adopt octahedral geometry. This preferred geometry can be traced to the valence shell electron pair repulsion (VSEPR) model Introduced In Chapter 9. The ligands space themselves around the metal as far apart as possible, to minimize electron-electron repulsion. 

The other approach to molecular geometry is the valence shell electron-pair repulsion (VSEPR) theory. This theory holds that... 

Valence The highest-energy electrons in an atom, which an atom loses, gains, or shares in forming a chemical bond. Valence shell electron-pair repulsion (VSEPR) A procedure based on electron repulsion in molecules that enables chemists to predict approximate bond angles. 

Molecular Geometry The Valence Shell Electron-Pair Repulsion (VSEPR) Model... 

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)... 

We now have three substances remaining methane, CH4, methyl fluoride, CH3F, and krypton difluoride, KrF2. We also have two types of intermolecular force remaining dipole-dipole forces and London forces. In order to match these substances and forces we must know which of the substances are polar and which are nonpolar. Polar substances utilize dipole-dipole forces, while nonpolar substances utilize London forces. To determine the polarity of each substance, we must draw a Lewis structure for the substance (Chapter 9) and use valence-shell electron pair repulsion (VSEPR) (Chapter 10). The Lewis structures for these substances are ... 

The shape of a molecule has quite a bit to do with its reactivity. This is especially true in biochemical processes, where slight changes in shape in three-dimensional space might make a certain molecule inactive or cause an adverse side effect. One way to predict the shape of molecules is the valence-shell electron-pair repulsion (VSEPR) theory. The... 

---