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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.