Chapter 11: Theories of Covalent Bonding

1. Chapter 11: Theories of Covalent Bonding 11.1 Valence Bond (VB) Theory and Orbital Hybridization 11.2 The Mode of Orbital Overlap and the Types of Covalent Bonds 11.3 Molecular Orbital (MO) Theory and Electron Delocalization

2. Central Themes of Valence Bond Theory Basic Principle of Valence Bond Theory:a covalent bond forms when the orbitals from two atoms overlap and a pair of electrons occupies the region between the nuclei. 1) Opposing spins of the electron pair. The region of space formed by the overlapping orbitals has a maximum capacity of two electrons that must have opposite spins.

3. Central Themes of Valence Bond Theory Basic Principle of Valence Bond Theory:a covalent bond forms when the orbitals from two atoms overlap and a pair of electrons occupies the region between the nuclei. 1) Opposing spins of the electron pair. The region of space formed by the overlapping orbitals has a maximum capacity of two electrons that must have opposite spins. 2) Maximum overlap of bonding orbitals. The bond strength depends on the attraction of nuclei for the shared electrons, so the greater the orbital overlap, the stronger the bond.

4. Central Themes of Valence Bond Theory 3) Hybridization of atomic orbitals. To explain the bonding in simple diatomic molecules such as HF it is sufficient to propose the direct overlap of the s and p orbitals of isolated ground state atoms. In cases such as methane CH4 where 4 hydrogen atoms are bonded to a central carbon atom it is impossible to obtain the correct bond angles. Pauling proposed that the valence atomic orbitals in the molecule are different from those in the isolated atoms.We call this Hybridization!

5. Fig. 11.1

6. Hybrid Orbital Types - Periodic Groups Hybrid Orbital Types Groups in the Periodic Table Associated SP Group IIA Alkaline Earth Elements SP2 Group IIIA Boron Family SP3 Group IVA Carbon Family** SP3d Group VA Nitrogen Family SP3d2 Group VIA Oxygen Family ** The exception is carbon which can have: SP, SP 2, SP 3 hybrid orbitals

7. The sp Hybrid Orbitals in Gaseous BeCl2 Fig. 11.2 A&B

8. Fig. 11.2 C&D

9. Fig. 11.3

10. Fig. 11.4

11. The sp3 Hybrid Orbitals in NH3 and H2O Fig. 11.5

12. The sp3d Hybrid Orbitals in PCl5 Fig. 11.6

13. The sp3d2 Hybrid Orbitals in SF6 Sulfur Hexafluoride -- SF6 Fig. 11.7

14. Fig. 11.8

15. Postulating the Hybrid Orbitals in a Molecule Problem: Describe how mixing of atomic orbitals on the central atoms leads to the hybrid orbitals in the following: a) Methyl amine, CH3NH2b) Xenon tetrafluoride, XeF4 Plan: From the Lewis structure and molecular shape, we know the number and arrangement of electron groups around the central atoms, from which we postulate the type of hybrid orbitals involved. Then we write the partial orbital diagram for each central atom before and after the orbitals are hybridized.

16. Postulating the Hybrid Orbitals in a Molecule Problem: Describe how mixing of atomic orbitals on the central atoms leads to the hybrid orbitals in the following: a) Methyl amine, CH3NH2b) Xenon tetrafluoride, XeF4 Plan: From the Lewis structure and molecular shape, we know the number and arrangement of electron groups around the central atoms, from which we postulate the type of hybrid orbitals involved. Then we write the partial orbital diagram for each central atom before and after the orbitals are hybridized. Solution: a) For CH3NH2: The shape is tetrahedral around the C and N atoms. Therefore, each central atom is sp3hybridized. The carbon atom has four half-filled sp3 orbitals: 2s 2p sp3 Isolated Carbon Atom Hybridized Carbon Atom

17. Postulating the Hybrid Orbitals in a Molecule Problem: Describe how mixing of atomic orbitals on the central atoms leads to the hybrid orbitals in the following: a) Methyl amine, CH3NH2b) Xenon tetrafluoride, XeF4 Plan: From the Lewis structure and molecular shape, we know the number and arrangement of electron groups around the central atoms, from which we postulate the type of hybrid orbitals involved. Then we write the partial orbital diagram for each central atom before and after the orbitals are hybridized. Solution: a) For CH3NH2: The shape is tetrahedral around the C and N atoms. Therefore, each central atom is sp3hybridized. The carbon atom has four half-filled sp3 orbitals: 2s 2p sp3 Isolated Carbon Atom Hybridized Carbon Atom

18. The N atom has three half-filled sp3orbitals and one filled with a lone pair. 2s sp3 2p .. H C N H H H H

19. b) The Xenon atom has filled 5 s and 5 p orbitals with the 5 d orbitals empty. Isolated Xe atom 5 d 5 s 5 p Hybridized Xe atom: sp3d2 5 d

20. .. .. b) continued:For XeF4. for Xenon, normally it has a full octet of electrons,which would mean an octahedral geometry, so to make the compound, two pairs must be broken up, and bonds made to the four fluorine atoms. If the two lone pairs are on the equatorial positions, they will be at 900 to each other, whereas if the two polar positions are chosen, the two electron groups will be 1800 from each other. Thereby minimizing the repulsion between the two electron groups. F F F F 1800 Xe Xe F F F F Square planar

21. Fig. 11.9

22. Fig. 11.10

23. Fig. 11.11

24. Restricted Rotation of -Bonded Molecules A) Cis - 1,2 dichloroethylene B) trans - 1,2 dichloroethylene Fig. 11.12

25. An Analogy between Light Waves and Atomic Wave Functions Fig. 11.13

26. Fig. 11.14

27. Filling Molecular Orbitals with Electrons 1) Orbitals are filled in order of increasing Energy ( Aufbau principle ) 2) An orbital has a maximum capacity of two electrons with opposite spins ( Pauli exclusion principle ) 3) Orbitals of equal energy are half filled, with spins parallel, before any is filled ( Hund’s rule )

28. Fig. 11.15

29. Fig. 11.16

30. Fig. 11.17

31. Fig. 11.18

32. Fig. 11.19

33. Fig. 11.20

34. Fig. 11.22

35. Fig. 11.23

36. Fig. 11.24