Bonding and Molecular Structure: Orbital Hybridization and Molecular Orbitals

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Chapter 10

• Bonding and Molecular StructureOrbital
  Hybridization and Molecular Orbitals

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Advanced Theories of Chemical Bonding
Atomic Orbitals
Molecules
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Two Theories of Bonding

• MOLECULAR ORBITAL THEORY
• Robert Mullikan (1896-1986)
• valence electrons are delocalized
• valence electrons are in orbitals (called
  molecular orbitals) spread over entire molecule.

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Two Theories of Bonding

• VALENCE BOND THEORY Linus Pauling
• valence electrons are localized between atoms (or
  are lone pairs).
• half-filled atomic orbitals overlap to form bonds.

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Sigma Bond Formation by Orbital Overlap
Two s orbitals overlap
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Sigma Bond Formation
Two s orbitals overlap
One s and one p orbital overlap
Two p orbitals overlap
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Using VB Theory

• Bonding in BF3

planar triangle angle 120o
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Bonding in BF3

• How to account for 3 bonds 120o apart using a
  spherical s orbital and p orbitals that are 90o
  apart?
• Pauling said to modify VB approach with ORBITAL
  HYBRIDIZATION
• mix available orbitals to form a new set of
  orbitals HYBRID ORBITALS that will give the
  maximum overlap, in the correct geometry.

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Bonding in BF3
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Bonding in BF3

• The three hybrid orbitals are made from 1 s
  orbital and 2 p orbitals ? 3 sp2 hybrids.

• Now we have 3, half-filled HYBRID orbitals that
  can be used to form B-F sigma bonds.

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Bonding in BF3
An orbital from each F overlaps one of the sp2
hybrids to form a B-F ? bond.
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BF3, Planar Trigonal
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Bonding in CH4

• How do we account for 4 CH sigma bonds 109o
  apart?
• Need to use 4 atomic orbitals s, px, py, and pz
  to form 4 new hybrid orbitals pointing in the
  correct direction.

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Bonding in CH4
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Bonding in a Tetrahedron Formation of Hybrid
Atomic Orbitals

• 4 C atomic orbitals hybridize to form four
  equivalent sp3 hybrid atomic orbitals.

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Bonding in Glycine

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Orbital Hybridization

• BONDS SHAPE HYBRID REMAIN
• 2 linear sp 2 ps
• 3 trigonal sp2 1 p planar
• 4 tetrahedral sp3 none

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Multiple Bonds

• Consider ethylene, C2H4

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Sigma Bonds in C2H4
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p Bonding in C2H4

• The unused p orbital on each C atom contains an
  electron and this p orbital overlaps the p
  orbital on the neighboring atom to form the p
  bond.

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p Bonding in C2H4

• The unused p orbital on each C atom contains an
  electron and this p orbital overlaps the p
  orbital on the neighboring atom to form the p
  bond.

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Multiple Bondingin C2H4
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s and p Bonding in C2H4
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s and p Bonding in CH2O
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s and p Bonding in C2H2
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s and p Bonding in C2H2
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Consequences of Multiple Bonding
There is restricted rotation around CC bond.
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Consequences of Multiple Bonding
Restricted rotation around CC bond.
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Diatomic Molecules

• AO MO AO
• H H2 H

1s
1s
ENERGY
B gt bonding electrons
B - AB 2 - 0 B.O.
------------ -------- 1
2 2
AB gt antibonding electrons
all electrons are paired, thus diamagnetic
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Diatomic Molecules

• AO MO AO
• He He2 He

1s
1s
ENERGY
Zero bond order means it does not exist
B - AB 2 - 2 B.O.
------------ -------- 0
2 2
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Diatomic Molecules

• AO MO AO
• He He2 He

1s
1s
ENERGY
B - AB 2 - 1 B.O.
------------ -------- 1/2
2 2
One unpaired electron, thus, paramagnetic
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AO B
AO B
MO B2
2p
2p
2s
2s
1s
1s
nonbonding
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Molecular Orbital Diagram
antibonding MO
end-to-end
bonding MO
antibonding MO
side-by-side
bonding MO
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AO C
AO C
MO C2
2p
2p
2s
2s
1s
1s
nonbonding
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AO O
AO O
MO O2
2p
2p
2s
2s
1s
1s
nonbonding
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Energy