Molecular Orbitals in 9 Chemical Bonding OUTLINE 9-1 Molecular Orbitals 9-5 Heteronuclear Diatomic 9-2 Molecular Orbital Energy Level Molecules Diagrams 9-6 Delocalization and the Shapes of 9-3 Bond Order and Bond Stability Molecular Orbitals 9-4 Homonuclear Diatomic Molecules OBJECTIVES After you have finished studying this chapter, you should be able to A computer representation of one of • Describe the basic concepts of molecular orbital theory the ␲ molecular orbitals of benzene. • Relate the shapes and overlap of atomic orbitals to the shapes and energies of the resulting molecular orbitals • Distinguish among bonding, antibonding, and nonbonding orbitals • Apply the Aufbau Principle to find molecular orbital descriptions for homonuclear diatomic molecules and ions • Apply the Aufbau Principle to find molecular orbital descriptions for heteronuclear diatomic molecules and ions with small ⌬(EN) values • Find the bond order in diatomic molecules and ions • Relate bond order to bond stability • Use the MO concept of delocalization for molecules in which valence bond theory would postulate resonance e have described bonding and molecular geometry in terms of valence bond W theory. In valence bond theory, we postulate that bonds result from the W sharing of electrons in overlapping orbitals of different atoms. These orbitals may be pure atomic orbitals or hybridized atomic orbitals of individual atoms. We describe electrons in overlapping orbitals of different atoms as being localized in the bonds between the two atoms involved. We then use hybridization to help account for the geometry of a molecule. In molecular orbital theory, we postulate that the combination of atomic orbitals on different atoms forms molecular orbitals In some polyatomic molecules, a (MOs), so that electrons in them belong to the molecule as a whole. molecular orbital may extend over only a fraction of the molecule. Valence bond and molecular orbital theories are alternative descriptions of chemical bonding. They have strengths and weaknesses, so they are complementary. Valence bond 354 CHAPTER 9: Molecular Orbitals in Chemical Bonding 2Ϫ Polyatomic ions such as CO3 , theory is descriptively attractive, and it lends itself well to visualization. Molecular orbital 2Ϫ ϩ SO4 , and NH4 can be described by (MO) theory gives better descriptions of electron cloud distributions, bond energies, and the molecular orbital approach. magnetic properties, but its results are not as easy to visualize. The valence bond picture of bonding in the O2 molecule involves a double bond. OO This shows no unpaired electrons, so it predicts that O2 is diamagnetic. Experiments show, however, that O2 is paramagnetic; therefore, it has unpaired electrons. Thus, the valence bond structure is inconsistent with experiment and cannot be accepted as a description of the bonding. Molecular orbital theory accounts for the fact that O2 has two unpaired elec- trons. This ability of MO theory to explain the paramagnetism of O2 gave it credibility as a major theory of bonding. We shall develop some of the ideas of MO theory and apply them to some molecules and polyatomic ions. 9-1 MOLECULAR ORBITALS We learned in Chapter 5 that each solution to the Schrödinger equation, called a wave function, represents an atomic orbital. The mathematical pictures of hybrid orbitals in valence bond theory can be generated by combining the wave functions that describe two or more atomic orbitals on a single atom. Similarly, combining wave functions that describe atomic orbitals on separate atoms generates mathematical descriptions of molecular An early triumph of molecular orbitals. orbital theory was its ability to An orbital has physical meaning only when we square its wave function to describe the account for the observed electron density. Thus, the overall sign on the wave function that describes an atomic paramagnetism of oxygen, O2. orbital is not important, but when we combine two orbitals, the signs of the wave func- According to earlier theories, O was 2 tions are important. When waves are combined, they may interact either constructively expected to be diamagnetic, that is, to have only paired electrons. or destructively (Figure 9-1). Likewise, when two atomic orbitals overlap, they can be in phase (added) or out of phase (subtracted). When they overlap in phase, constructive inter- action occurs in the region between the nuclei, and a bonding orbital is produced. The energy of the bonding orbital is always lower (more stable) than the energies of the combining orbitals. When they overlap out of phase, destructive interaction reduces the probability of finding electrons in the region between the nuclei, and an antibonding orbital is produced. This is higher in energy (less stable) than the original atomic orbitals. The overlap of two atomic orbitals always produces two MOs: one bonding and one anti- bonding. We can illustrate this basic principle by considering the combination of the 1s atomic orbitals on two different atoms (Figure 9-2). When these orbitals are occupied by electrons, the shapes of the orbitals are plots of electron density. These plots show the regions in molecules where the probabilities of finding electrons are the greatest. In the bonding orbital, the two 1s orbitals have reinforced each other in the region between the two nuclei by in-phase overlap, or addition of their electron waves. In the antibonding orbital, they have canceled each other in this region by out-of-phase overlap, or subtraction of their electron waves. We designate both molecular orbitals as sigma (␴) See the Saunders Interactive molecular orbitals (which indicates that they are cylindrically symmetrical about the General Chemistry CD-ROM, internuclear axis). We indicate with subscripts the atomic orbitals that have been combined. Screen 10.9, Molecular Orbital Theory. # ␴ The star ( ) denotes an antibonding orbital. Thus, two 1s orbitals produce a 1s (read ␴# “sigma-1s”) bonding orbital and a 1s (read “sigma-1s-star”) antibonding orbital. The right-hand side of Figure 9-2 shows the relative energy levels of these orbitals. All sigma 9-1 Molecular Orbitals 355 Figure 9-1 An illustration of constructive and destructive ϩ ϩ interference of waves. (a) If the two Ϫ Ϫ identical waves shown at the left are added, they interfere constructively ϩ to produce the more intense wave at the right. (b) Conversely, if they are Ϫ subtracted, it is as if the phases (signs) of one wave were reversed ϩ ϩ and added to the first wave. This Ϫ Ϫ causes destructive interference, resulting in the wave at the right with zero amplitude; that is, a (a) In-phase overlap (add) (b) Out-of-phase overlap (subtract) straight line. antibonding orbitals have nodal planes bisecting the internuclear axis. A node, or nodal plane, is a region in which the probability of finding electrons is zero. Another way of viewing the relative stabilities of these orbitals follows. In a bonding molecular orbital, the electron density is high between the two atoms, where it stabilizes the arrangement by exerting a strong attraction for both nuclei. By contrast, an anti- bonding orbital has a node (a region of zero electron density) between the nuclei; this allows for a strong net repulsion between the nuclei, which makes the arrangement less stable. Electrons are more stable (have lower energy) in bonding molecular orbitals than in the individual atoms. Placing electrons in antibonding orbitals, on the other hand, requires an increase in their energy, which makes them less stable than in the individual atoms. Nodal Molecular plane orbitals σ # 1s Out-of-phase Atomic (antibonding) Atomic overlap (subtract) orbital orbital and σ # (antibonding) In-phase 1s 1s 1s overlap (add) 1s 1s Energy σ σ 1s (bonding) 1s (bonding) Atomic orbitals Molecular orbitals Figure 9-2 Molecular orbital (MO) diagram for the combination of the 1s atomic orbitals ␴ on two identical atoms (at the left) to form two MOs. One is a bonding orbital, 1s (blue), resulting from addition of the wave functions of the 1s orbitals. The other is an antibonding ␴# orbital, 1s (red), at higher energy resulting from subtraction of the waves that describe the combining 1s orbitals. In all ␴-type MOs, the electron density is symmetrical about an imaginary line connecting the two nuclei. The terms “subtraction of waves,” “out of phase,” and “destructive interference in the region between the nuclei” all refer to the formation of an antibonding MO. Nuclei are represented by dots. 356 CHAPTER 9: Molecular Orbitals in Chemical Bonding Out-of-phase σ# 2p (antibonding) overlap (subtract) x and In-phase overlap (add) 2px 2px Energy Figure 9-3 Production of ␴ and σ 2px 2p (bonding) ␴# molecular orbitals by overlap of x 2px Atomic orbitals 2px orbitals on two atoms. (head-on overlap) Molecular orbitals How we name the axes is arbitrary. We For any two sets of p orbitals on two different atoms, corresponding orbitals such as designate the internuclear axis as the x ␴ ␴# px orbitals can overlap head-on. This gives p and p orbitals, as shown in Figure 9-3 for direction. the head-on overlap of 2px orbitals on the two atoms. If the remaining p orbitals overlap ␲ (py with py and pz with pz ), they must do so sideways, or side-on, forming pi ( ) molecular ␲ orbitals. Depending on whether all p orbitals overlap, there can be as many as two p and ␲# two p orbitals. Figure 9-4 illustrates the overlap of two corresponding 2p orbitals on ␲ ␲# two atoms to form 2p and 2p molecular orbitals. There is a nodal plane along the inter- This would involve rotating Figures nuclear axis for all pi molecular orbitals. If one views a sigma molecular orbital along the 9-2, 9-3, and 9-4 by 90° so that the internuclear axis, it appears to be symmetrical around the axis like a pure s atomic orbital.