Chapter 9 Covalent Bonding: Orbitals

1. Chapter 9Covalent Bonding: Orbitals Zumdahl & Zumdahl M. Todd Tippetts, Ph.D.

2. Valence Bond Theory Atoms overlap their atomic obitals to form covalent bonds Bond occurs when two electrons with spins paired are shared by two overlapping atomic orbitals One electron from each of the two atoms in the bond Portions of two orbitals overlap to occupy the same region of space

3. One pair of electrons can occupy this overlapping area Electron density is maximizes in overlapped region H2 bonds form because atomic valence orbitals overlap Each hydrogen contributed 1s orbital

4. 4 Valence Bond Theory HF involves overlaps between the s orbital on H and the 2p orbital of F FIG. 9.15 The formation of the hydrogen molecule according to valence bond theory. FIG. 9.16 The formation of the hydrogen fluoride molecule according to valence bond theory. For clarity, only the half-filled 2p orbital of fluorine is shown. The other 2p orbitals of fluorine are filled and cannot participate in bonding. Chem FAQ: Explain the bonding in simple molecules in terms of overlapping of half-filled atomic orbitals.FIG. 9.15 The formation of the hydrogen molecule according to valence bond theory. FIG. 9.16 The formation of the hydrogen fluoride molecule according to valence bond theory. For clarity, only the half-filled 2p orbital of fluorine is shown. The other 2p orbitals of fluorine are filled and cannot participate in bonding. Chem FAQ: Explain the bonding in simple molecules in terms of overlapping of half-filled atomic orbitals.

5. 5 VB Theory And H2S Assume that the unpaired e- in S and H are free to form a paired bond We may assume that the H-S bond forms between an s and a p orbital FIG. 9.17 Bonding in H2S.We expect the hydrogen 1s orbitals to position themselves so that they can best overlap with the two partially filled 3p orbitals of sulfur, which gives a predicted bond angle of 90�. The experimentally measured bond angle of 92� is very close to the predicted angle. FIG. 9.17 Bonding in H2S.We expect the hydrogen 1s orbitals to position themselves so that they can best overlap with the two partially filled 3p orbitals of sulfur, which gives a predicted bond angle of 90�. The experimentally measured bond angle of 92� is very close to the predicted angle.

6. 6 According to Valence Bond Theory: Which orbitals overlap in the formation of NH3? Ground state of nitrogen 2s ?? 2p _? ? _?__

7. 7 Difficulties With VB Theory So Far: Most experimental bond angles do not support those predicted by mere atomic orbital overlap For example: C 1s22s22p2 and H 1s1 Experimental bond angles in methane are 109.5� and all are the same p orbitals are 90� apart, and not all valence e- in C are in the p orbitals How can multiple bonds form? Chem FAQ: What are hybrid orbitals? Chem FAQ: What are hybrid orbitals?

8. 8 Hybridization The mixing of atomic orbitals to allow formation of bonds that have realistic bond angles The new shapes that result are called �hybrid orbitals� The number of hybrid orbitals required = the number of bonding domains + the number of non-bonding domains on the atom

9. Hybrid between s and p Orbitals

10. 10 What Shall We Call These New Orbitals? Since we have annexed the spaces previously defined by atomic orbitals, we name the hybrid as a combination of the orbitals used to form the new hybrid Name tell what type of atomic orbitals, and how many of each sp, sp2, sp3 etc. One atomic orbital is used for every hybrid formed (orbitals are conserved)

11. 11 Hybrids From s & p Atomic Orbitals explain VSEPR Geometry

12. Hybrids From s & p Atomic Orbitals explain VSEPR Geometry

13. Hybrids From s & p Atomic Orbitals explain VSEPR Geometry

14. Hybrid Orbitals in BeH2 Ground state of Be 2s?? 2p _ ___ [He]2s2 No half filled orbitals available for bonding For hybrid sp hybrid orbitals sp ? ? 2p ___ Now bond can form between 1s orbital of hydrogen and sp hybrid orbital or berylllium sp ?? ?? 2p ___

16. 16 Consider the CH4 molecule Ground state for carbon 2s ? ? 2p ? _ ? ___ [He]2s2 2p2 Form sp3 hybrid orbitals ? ?_ ? ? . Each sp3 hybrid can now overlap with 1s orbital of hydrogen ?? ??_ ?? ?? . Chem FAQ: Use hybridization to describe molecular geometries. Use VSEPR theory to predict hybridization.Chem FAQ: Use hybridization to describe molecular geometries. Use VSEPR theory to predict hybridization.

17. 17 Bonding in CH4 The 4 hybrid orbitals are evenly distributed around the C The H s-orbitals overlap the sp3 hybrid orbitals to form the bonds. FIG. 9.22 Formation of the bonds in methane. Each bond results from the overlap of a hydrogen 1s orbital with an sp3 hybrid orbital on the carbon atom. FIG. 9.22 Formation of the bonds in methane. Each bond results from the overlap of a hydrogen 1s orbital with an sp3 hybrid orbital on the carbon atom.

18. 18 Bonding in NH3 The 4 hybrid orbitals are evenly distributed around the N The H s-orbitals overlap the sp3 hybrid orbitals to form three bonds bonds. The remaining lone pair occupies the last hybrid orbital FIG. 9.22 Formation of the bonds in methane. Each bond results from the overlap of a hydrogen 1s orbital with an sp3 hybrid orbital on the carbon atom. FIG. 9.22 Formation of the bonds in methane. Each bond results from the overlap of a hydrogen 1s orbital with an sp3 hybrid orbital on the carbon atom.

19. Hybridization for form sp2 orbitals

20. Ethene and Double Bonds

22. Orbitals used for bonding in Ethene

23. Formation of sp Hybrid Orbitals

24. Hybrid Orbitals is CO2 molecule

25. CO2 Molecule

26. Triple bond in N2 molecule consists of one sigma bond and 2 pi bonds Sigma bond stems from sp hybrid overlap Pi bonds come from unhybridized p orbital overlap

27. 27 Expanded Octet Hybridization Can be predicted from the geometry as well In these situations, d orbitals are be needed to provide room for the extra electrons One d orbital is added for each pair of electrons in excess of the standard octet

28. Expanded Octet hybridization dsp3 hybridization gives rise to trigonal bipyramid geometry of PCl5 28

29. d2sp3 hybridization gives rise to octahedral geometry

30. Consider SF6 Ground state for sulfur 3s ? ? 3p ? ? ? ?_ 3d _ _ _ _ _ Six hybrid orbitals needed sp3d2 ? ? ? ? ? ? . 3d _ _ _ Each sp3d2 hybrid can now overlap with 2p orbital of fluorine ?? ??_ ?? ?? . ?? ??

31. Bonding is XeF4 Placing lone pairs at axial positions lets them be as far as possible from one another Square planar geometery

32. Hybrid orbital can also hold nonbonding electrons Usually results in polar molecules

33. Consider the SF4 molelcule Four bonding + 1 nonbonding pairs around sulfer Five hybrid orbitals needed sp3d ?? ? ? ? ? . 3d _ _ _ _ Four half filled orbitals available to overlap with 2p orbital of fluorine sp3d ?? ?? ? ? ?? ? ? . 3d _ _ _ _

34. Geometry of SF4 sp3d requires trigonal bipyramid geometry Nonbonding pair goes on equatorial position Distorted tetrahedron geometry

35. 35 Bonding Types Two types of bonds result from orbital overlap: sigma s bonds from head-on overlap lie along the bond axis account for the first bond Can freely rotate around bond Chem FAQ: Use VB theory to describe double bonds. Use VB theory to describe triple bonds. Chem FAQ: Use VB theory to describe double bonds. Use VB theory to describe triple bonds.

36. pi p bonds from lateral overlap by adjacent p or d orbitals pi bonds are perpendicular to bond axis account for the second and third bonds in a multiple bond Cannot undergo rotation around bond

37. Bonding in Ethene C2H4 Carbon forms sp2 hybrid orbitals, and one unhybridized p orbital

39. 39 Sigma and Pi Bonding FIG. 9.30 The carbon�carbon double bond.FIG. 9.30 The carbon�carbon double bond.

40. 40 H-C=C -H Each C has a triple bond and a single bond Requires 2 hybrid orbitals, sp unhybridized p orbitals used to form the pi bond FIG. 9.33 The carbon�carbon triple bond in acetylene. (a) The sp hybrid orbitals on the carbon atoms are used to form sigma bonds to the hydrogen atoms and to each other. This accounts for one of the three bonds between the carbon atoms. (b) Sideways overlap of unhybridized 2px and 2py orbitals of the carbon atoms produces two p bonds. (c) The two p bonds in acetylene after they�ve formed surround the s bond. FIG. 9.33 The carbon�carbon triple bond in acetylene. (a) The sp hybrid orbitals on the carbon atoms are used to form sigma bonds to the hydrogen atoms and to each other. This accounts for one of the three bonds between the carbon atoms. (b) Sideways overlap of unhybridized 2px and 2py orbitals of the carbon atoms produces two p bonds. (c) The two p bonds in acetylene after they�ve formed surround the s bond.

41. Summary of Multiple Bonds Molecular skeleton held together by s bonds. First bond between two atoms always s. Hybrid orbitals are used to form s bonds, and to hold nonbonding electrons Number of hybrid orbitals needed = # atoms bonded + # of nonbonding pairs p bonds are formed using non-hybridized p or d orbitals Double bond is one s and one p bond Triple bond consists of one s and two p bonds

42. 42 Molecular Orbital Theory Modification of VB theory that considers that the orbitals may exhibit interference. Waves may interfere constructively or destructively Bonding orbitals stabilize, antibonding destabilize. FIG. 9.35 Interaction of 1s atomic orbitals to produce bonding and antibonding molecular orbitals. These are s-type orbitals because the electron density is concentrated along the imaginary line that passes through both nuclei. The antibonding orbital has a nodal plane between the nuclei where the electron density drops to zero. Chem FAQs: What is a molecular orbital? What are bonding and antibonding molecular orbitals? FIG. 9.35 Interaction of 1s atomic orbitals to produce bonding and antibonding molecular orbitals. These are s-type orbitals because the electron density is concentrated along the imaginary line that passes through both nuclei. The antibonding orbital has a nodal plane between the nuclei where the electron density drops to zero. Chem FAQs: What is a molecular orbital? What are bonding and antibonding molecular orbitals?

43. MO diagram for H2 Show atomic energy level diagram for each atom Show molecular orbitals (bonding and antibonding*) 1 MO for each Atomic orbital. Show electron occupancy of the orbitals. FIG. 9.36a Molecular orbital descriptions of H2 (a) Molecular orbital energy level diagram for H2. FIG. 9.36a Molecular orbital descriptions of H2 (a) Molecular orbital energy level diagram for H2.

44. 44 Filling MO diagrams �Electrons fill the lowest-energy orbitals that are available. No more than two electrons, with spins paired, can occupy any orbital. Electrons spread out as much as possible, with spins unpaired, over orbitals that have the same energy. Chem FAQ: How do I compute bond orders from an MO diagram? Chem FAQ: How do I compute bond orders from an MO diagram?

45. H2 vs He2

46. Molecular Orbitals Using p Orbitals

47. Two px orbitals overlap for form sigma bonding and antibonding molecular orbitals

48. Two p orbitals overlap to form pi bonding and anti-bonding orbitals Can happen both to py pair and to pz pair, resulting in two bonding and two anti-bonding orbitals

49. Molecular Orbital Diagram for B2

50. 50 Diatomic MO diagrams differ by group A) I - V B) VI-VIIIA FIG. 9.38 Approximate relative energies of molecular orbitals in second period diatomic molecules. (a) Li2 through N2. (b) O2 through Ne2.FIG. 9.38 Approximate relative energies of molecular orbitals in second period diatomic molecules. (a) Li2 through N2. (b) O2 through Ne2.

51. Molecular Orbitals Explains Paramagnetic O2 Paramagnetic; weakly attracted to magnetic field Usually a result of unpaired electron Simple Lewis structure has no unpaired electrons However, MO treatment shows two unpaired electrons in p* orbitals

52. Molecular Orbital Diagrams for B2 to F2

53. MO Diagram for Group I-V

54. MO Diagram for Group VI-VIII

56. MO�s and Free Radicals NO.

57. Figure 9.44: Resonance Structures for O3 and NO3

58. Figure 9.45: (a) benzene molecule (b) two resonance structures for benzene molecule

59. 59 Delocalized Electrons Lewis structures use resonance to explain that the actual molecule appears to have several equivalent bonds, rather than different possible structures MO theory shows the electrons being delocalized in the structure FIG. 9.42 Benzene. (a) The -bond framework. All atoms lie in the same plane. (b) The unhybridized p orbitals at each carbon prior to side-to-side overlap. (c) The double doughnut-shaped electron cloud formed by the delocalized p electrons. FIG. 9.42 Benzene. (a) The -bond framework. All atoms lie in the same plane. (b) The unhybridized p orbitals at each carbon prior to side-to-side overlap. (c) The double doughnut-shaped electron cloud formed by the delocalized p electrons.