Theories of Covalent Bonding

1. Theories of Covalent Bonding • Lewis + VSEPR theories lead to • predictions of: • bonding pairs • electron pairs • molecular shape • But is silent about the types of orbitals • used for bonding.

2. Theories of Covalent Bonding: models of how atomic orbitals interact to form bonds in molecules Valence Bond (VB) Theory and Orbital Hybridization Atomic orbitals spatially overlap (interact) to form localized bonds between 2 nuclei Molecular Orbital (MO)Theory and Electron Delocalization Atomic orbitals overlap to form delocalized bonds - bonds that connect more than 2 nuclei.

3. Resonance: Delocalized Electron-Pair Bonding Valence Bond Description of Ozone-Localized Bonding O3 can be drawn in 2 ways - Molecular Orbital Description of Ozone-Delocalized Bonding 2s(a) + 2s(b) +2s(c) A bonding m.o. Ob Oa Oc

4. The Central Themes of VB Theory Basic Principle • A covalent bond forms when the orbitals of two atoms overlap and are occupied by a pair of electrons that have the highest probability of being located between the nuclei. Themes • These overlapping orbitals can have up to two electrons that must have opposite spins (Pauli principle). • The greater the orbital overlap, the stronger (more stable) the bond. • The valence orbitals in a molecule are different from those in isolated atoms. (Recall that the 4s orbital is filled before the 3d of metal atoms but not for metal compounds.)

5. Hydrogen, H2 Hydrogen fluoride, HF Fluorine, F2 Figure 11.1 Orbital overlap and spin pairing in three diatomic molecules

6. The sphybrid orbitals in gaseous BeCl2 Figure 11.2 Promote to create two half filled orbitals that participate in bond formation Promotion Filled 2s orbital can’t bond to Cl • Hybridization: process where a valence e is promoted to empty orbital followed by orbital “mixing” to give the desired spatial arrangements of bonding e’s and unshared e pairs. • Increased energies of hybridized bonds greater than e promotion energies. Why are sp hybrids invoked? Because if Be made one bond with its 2s and one bond with a 2p orbital, then the two Be-Cl bonds would have different strengths & lengths. But both bonds are identical.

7. Key Points Types of HybridOrbitals sp sp2 sp3 sp3d sp3d2 Hybrid Orbitals • The number of hybrid orbitals obtained equals the number of atomic orbitals mixed. • The type of hybrid orbitals obtained varies with the types of atomic orbitals mixed. Shapes: linear triangular tetrahedral trig. bipyram. Octahedral # orbitals: 2 3 4 5 6

8. The two sp hybrid orbitals in gaseous BeCl2 Figure 11.2 orbital box diagrams with orbital contours Note the two “leftover” p orbitals of Be Region of overlap

9. The three sp2 hybrid orbitals in BF3 Figure 11.3 Promotion Note the single left over Unhybridized p orbital on B Region of overlap

10. The four sp3 hybrid orbitals in CH4 Figure 11.4 Promotion

11. Carbon-diamond structure

12. The four sp3 hybrid orbitals in NH3 Figure 11.5 Promotion not required! Lone pair

13. The sp3 hybrid orbitals in H2O Figure 11.5 Lone pairs

14. The five sp3d hybrid orbitals in PCl5 Figure 11.6

15. Figure 11.7 The six sp3d2hybrid orbitals in SF6

17. Step 1 Step 2 Step 3 Figure 10.1 Figure 10.12 Table 11.1 Figure 11.8 The conceptual steps from molecular formula to the hybrid orbitals used in bonding. Molecular shape and e- group arrangement Molecular formula Lewis structure Hybrid orbitals

18. PROBLEM: Use partial orbital diagrams to describe mixing of atomic orbitals on the central atoms leads to hybrid orbitals in each of the following: PLAN: Use the Lewis structures to ascertain the arrangement of groups and shape of each molecule. Postulate the hybrid orbitals. Use partial orbital box diagrams to indicate the hybrid for the central atoms. SAMPLE PROBLEM 11.1 Postulating Hybrid Orbitals in a Molecule (a) Methanol, CH3OH (b) Sulfur tetrafluoride, SF4 SOLUTION: (a) CH3OH The groups around C are arranged as a tetrahedron. O also has a tetrahedral arrangement with 2 nonbonding e- pairs.

19. hybridized C atom hybridized O atom single C atom single O atom hybridized S atom S atom SAMPLE PROBLEM 11.1 Postulating Hybrid Orbitals in a Molecule continued (b) SF4 has a seesaw shape with 4 bonding and 1 nonbonding e- pairs.

20. both C are sp3 hybridized s-sp3 overlaps to s bonds sp3-sp3 overlap to form a s bond relatively even distribution of electron density over all s bonds The s bonds in ethane. Figure 11.9 Rotation about C-C bond allowed. • (Greek sigma) bonds have axial symmetry and good overlap