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.

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

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

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.)

Figure 11.1 Orbital overlap and spin pairing in three diatomic molecules Hydrogen, H 2 Hydrogen fluoride, HF Fluorine, F 2

Figure 11.2 The sp hybrid orbitals in gaseous BeCl 2 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. Promotion Promote to create two half filled orbitals that participate in bond formation Filled 2s orbital can’t bond to Cl

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. Key Points spsp 2 sp 3 sp 3 dsp 3 d 2 Types of Hybrid Orbitals Shapes: linear triangular tetrahedral trig. bipyram. Octahedral # orbitals:

Figure 11.2 The two sp hybrid orbitals in gaseous BeCl 2 orbital box diagrams with orbital contours Note the two “leftover” p orbitals of Be Region of overlap

Figure 11.3 The three sp 2 hybrid orbitals in BF 3 Promotion Note the single left over Unhybridized p orbital on B Region of overlap

Figure 11.4 The four sp 3 hybrid orbitals in CH 4 Promotion

Carbon-diamond structure

Figure 11.5 The four sp 3 hybrid orbitals in NH 3 Lone pair Promotion not required!

Figure 11.5 The sp 3 hybrid orbitals in H 2 O Lone pairs

Figure 11.6 The five sp 3 d hybrid orbitals in PCl 5

Figure 11.7 The six sp 3 d 2 hybrid orbitals in SF 6

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

SAMPLE PROBLEM 11.1Postulating Hybrid Orbitals in a Molecule SOLUTION: 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. (a) Methanol, CH 3 OH(b) Sulfur tetrafluoride, SF 4 (a) (a) CH 3 OHThe groups around C are arranged as a tetrahedron. O also has a tetrahedral arrangement with 2 nonbonding e - pairs.

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

Figure 11.9 The  bonds in ethane. both C are sp 3 hybridized s-sp 3 overlaps to  bonds sp 3 -sp 3 overlap to form a  bond relatively even distribution of electron density over all  bonds  (Greek sigma) bonds have axial symmetry and good overlap Rotation about C-C bond allowed.