Theories of Covalent Bonding

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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.

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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.
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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
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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.)

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Figure 11.1
Orbital overlap and spin pairing in three
diatomic molecules
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The sp hybrid orbitals in gaseous BeCl2
Figure 11.2
Promote to create two half filled orbitals that
participate in bond formation
Promotion
Filled 2s orbital cant 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 es 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.
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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
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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
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The three sp2 hybrid orbitals in BF3
Figure 11.3
Promotion
Note the single left over Unhybridized p orbital
on B
Region of overlap
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The four sp3 hybrid orbitals in CH4
Figure 11.4
Promotion
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Carbon-diamond structure
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The four sp3 hybrid orbitals in NH3
Figure 11.5
Promotion not required!
Lone pair
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The sp3 hybrid orbitals in H2O
Figure 11.5
Lone pairs
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The five sp3d hybrid orbitals in PCl5
Figure 11.6
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Figure 11.7
The six sp3d2 hybrid orbitals in SF6
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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
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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.
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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.
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The s bonds in ethane.
Figure 11.9
Rotation about C-C bond allowed.

• (Greek sigma) bonds
• have axial symmetry and
• good overlap