Theories of Covalent BondingLewis + 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

O3 can be drawn in 2 ways - OO O

OO O

Valence Bond Description of Ozone-Localized Bonding

Molecular Orbital Description of Ozone-Delocalized Bonding

Oa

Ob

Oc

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, H2

Hydrogen fluoride, HF

Fluorine, F2

Figure 11.2

The sp hybrid orbitals in gaseous BeCl2

•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 its2s 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

sp sp2 sp3 sp3d sp3d2

Types of Hybrid Orbitals

Shapes: linear triangular tetrahedral trig. bipyram. Octahedral# orbitals: 2 3 4 5 6

Figure 11.2 The two sp hybrid orbitals in gaseous BeCl2

orbital box diagrams with orbital contours

Note the two “leftover” p orbitals of BeRegion of overlap

Figure 11.3 The three sp2 hybrid orbitals in BF3

PromotionNote the single left overUnhybridized p orbital on B

Region of overlap

Figure 11.4 The four sp3 hybrid orbitals in CH4

Promotion

Carbon-diamond structure

Figure 11.5The four sp3 hybrid orbitals in NH3

Lone pairPromotion notrequired!

Figure 11.5 The sp3 hybrid orbitals in H2O

Lone pairs

Figure 11.6 The five sp3d hybrid orbitals in PCl5

Figure 11.7

The six sp3d2 hybrid orbitals in SF6

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 10.12

Step 2 Step 3

Table 11.1

SAMPLE PROBLEM 11.1 Postulating 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, CH3OH (b) Sulfur tetrafluoride, SF4

(a) (a) CH3OH H

CH H

OH

The groups around C are arranged as a tetrahedron.

O also has a tetrahedral arrangement with 2 nonbonding e- pairs.

SFF

F

F

SAMPLE PROBLEM 11.1 Postulating Hybrid Orbitals in a Molecule

continued

2p

2s single C atomsingle C atom

sp3

hybridized hybridized C atomC atom

2p

2s single O atomsingle O atom

sp3

hybridized hybridized O atomO atom

(b) SF4 has a seesaw shape with 4 bonding and 1 nonbonding e- pairs.

3p

3s

3d

S atomS atomsp3d

3d

hybridized hybridized S atomS atom

Figure 11.9 The bonds in ethane.

both C are sp3 hybridizeds-sp3 overlaps to bonds

sp3-sp3 overlap to form a bondrelatively even

distribution of electron density over all bonds (Greek sigma) bonds

have axial symmetry andgood overlap

Rotation about C-Cbond allowed.