Bonding and Molecular Structure:

1
Chapter 10

• Bonding and Molecular Structure
• Orbital Hybridization and
• Molecular Orbitals

2
Goals

• Understand the differences between valence bond
  theory and molecular orbital theory.
• Identify the hybridization of an atom in a
  molecule or ion.
• Understand the differences between bonding and
  antibonding molecular orbitals.
• Write the molecular orbital configuration for
  simple diatomic molecules.

3
Orbitals and Bonding Theories

• VSEPR Theory only explains molecular shapes.
• It says nothing about bonding in molecules
• In Valence Bond (VB) Theory (Linus Pauling)
• atoms share electron pairs by allowing their
• atomic orbitals to overlap.
• Another approach to rationalize chemical
• bonding is the Molecular Orbital (MO) Theory
• (Robert Mulliken) molecular orbitals are spread
• out or delocalized over the molecule.

4
Valence Bond (VB) Theory

• Covalent bonds are formed by the overlap of
• atomic orbitals.
• Atomic orbitals on the central atom can mix and
• exchange their character with other atoms in a
• molecule.
• Process is called hybridization.
• Hybrids are common
• Pink flowers
• Mules
• Hybrid Orbitals have the same shapes as
• predicted by VSEPR.

5
1s
1s

H
H
? bond
6
1s
1s

H
H
E
1s
H
? bond
7

H
H
H
E
1s
H
? bond
8
2p
2p

F
F
? bond
F2
9
2p
E
2s
1s
F
10
F
2p
E
2s
1s
F
11
Methane
CH4
2p
E
2s
1s
C
12
Methane
CH4
H
H
2p
E
2s
1s
C
13
Methane
CH4
H
H
H
2p
E
2s
1s
C
14
Methane
CH4
H
H
H
H
2p
E
2s
1s
C
15
Methane
CH4
H
H
H
H
2p
H
E
90
2s
H
C
H
90
H
1s
C
16
Methane
CH4
H
109.5
C
H
H
H
Tetrahedral Geometry 4 Identical Bonds
17
Problem and Solution

• C must have 4 identical orbitals in valence shell
  for bonding
• solution hybridization

18
Methane
CH4
2p
E
2s
1s
19
Methane
CH4
2s
2p
2p
E
E
2s
1s
1s
20
Methane
CH4
2s
2p
2p
E
E
2s
1s
1s
21
Methane
CH4
2s
2p
2p
E
E
2s
1s
1s
22
Methane
CH4
2p
sp3
E
E
2s
1s
1s
23




2p
2s
24




2p
Three

2s
four sp3 hybrid orbitals
25
4 identical sp3 hybrid orbitals they are four
because there was the combination of one s and
three p atomic orbitals (25 s, 75 p)
tetrahedral geometry
26
Methane
CH4
H
H
H
H
2p
sp3
E
E
2s
1s
1s
27
Valence Bond (VB) Theory
28
Predict the Hybridization of the Central Atom
in aluminum bromide
?
?
?
?
Br
?
?
Electron-pair shape trigonal planar
3 regions
Al
?
?
?
?
Br
Br
?
?
?
?
?
?
?
?
Hybridization sp2
29
Trigonal Planar Electronic Geometry, sp2

• Electronic Structures BF3

1s 2s 2p B ??????????? ?
1s sp2 hybrid ??? ?????? ??? ??? ?
1s 2s 2p 2p B ?????? ????
? ?
2s 2p F He ????????????
30
Trigonal Planar Electronic Geometry, sp2

• BF3

31
Predict the Hybridization of the Central Atom
in carbon dioxide
CO2
?
?
?
?
O
C
O
?
?
?
?
2 regions
Electron-pair shape, linear
Hybridization sp (50 s, 50 p)
32
Linear Electronic Geometry, sp

• Electronic Structures BeCl2

1s 2s 2p Be ???????????
1s sp hybrid ? ?? ? ?
3s 3p Cl Ne ????????????
33
Predict the Hybridization of the Central Atom in
Beryllium Chloride

• Two regions electron-pair shape
• sp hybridization

34
Predict the Hybridization of the Central Atom in
PF5

• Five regions Trigonal Bipyramidal Electronic
• Geometry - sp3d hybridization,
• five sp3d hybrid orbitals

35
Predict the Hybridization of the Central Atom
in xenon tetrafluoride
36
Predict the Hybridization of the Central Atom
in xenon tetrafluoride
?
?
?
?
?
?
F
F
?
?
?
?
6 regions electron-pair shape octahedral
?
?
?
?
Xe
?
?
?
?
?
?
?
?
F
F
?
?
?
?
?
?
37
Predict the Hybridization of the Central Atom
in xenon tetrafluoride
?
?
?
?
?
?
F
F
?
?
?
?
6 regions electron-pair shape octahedral
?
?
?
?
Xe
?
?
?
?
?
?
?
?
F
F
?
?
?
?
?
?
sp3d2
38
Predict the Hybridization of the Central Atom in
SF6

• Six regions Octahedral Electronic Geometry
• - sp3d2 hybridization,
• six sp3d2 hybrid orbitals

39
Consider Ethylene, C2H4
40
Consider Ethylene, C2H4
H
H
C
C
H
H
41
Consider Ethylene, C2H4
H
H
C
C
H
H
3 regions
trigonal planar
42
Consider Ethylene, C2H4
H
H
C
C
H
H
3 regions
trigonal planar
sp2
43
Consider Ethylene, C2H4
H
H
C
C
H
H
3 regions
trigonal planar
sp2
44
2p
E
2s
1s
45
2s
2p
2p
E
E
2s
1s
1s
46
2p
2p
sp2
E
E
2s
1s
1s
47
sp2
2p
sp2
sp2
48
2p
sp2
sp2
sp2
49
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50
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51
? bond framework
52
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53
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54
? bond
55
? bond
56
Compounds Containing Double Bonds

• Thus a CC bond looks like this and is made
• of two parts, one ? and one ? bond.

57
Consider Acetylene, C2H2
C
H
H
C
58
Consider Acetylene, C2H2
C
H
H
C
2 regions
linear
59
Consider Acetylene, C2H2
C
H
H
C
2 regions
linear
sp hybridization
60
Consider Acetylene, C2H2
C
H
H
C
2 regions
linear
sp hybridization
61
2s
2p
2p
E
E
2s
1s
1s
62
2p
2p
sp
E
E
2s
1s
1s
63
2p
sp
sp
2p
64
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65
? bond framework
66
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67
? bonds
68
Compounds Containing Triple Bonds

• A ? bond results from the head-on overlap of
• two sp hybrid orbitals.
• The unhybridized p orbitals form two p bonds
  (side-on overlap of atomic orbitals.)
• Note that a triple bond consists of one ? and
• two p bonds.

69
? bonds
70
Generally

• single bond is a ? bond
• double bond consists of 1 ? and 1 ? bond
• triple bond consists of 1 ? and 2 ? bonds

71
Molecular Orbital (MO) Theory

• when atoms combine to form molecules, atomic
  orbitals overlap and are then combined to form
  molecular orbitals
• of orbitals are conserved
• a molecular orbital is an orbital associated with
  more than 1 nucleus
• like any other orbital, an MO can hold 2
  electrons
• consider hydrogen atoms bonding to form H2

72
Molecular Orbital Theory

• Combination of atomic orbitals on different atoms
  forms molecular orbitals (MOs) so that electrons
  in MOs belong to the molecule as a whole.
• Waves that describe atomic orbitals have both
  positive and negative phases or amplitudes.
• As MOs are formed the phases can interact
  constructively or destructively.

73
Molecular Orbitals

• There are two simple types of molecular
• orbitals that can be produced by the overlap
• of atomic orbitals.
• Head-on overlap of atomic orbitals
• produces ? (sigma) orbitals.
• Side-on overlap of atomic orbitals
• produces ? (pi) orbitals.
• Two 1s atomic orbitals that overlap produce
• two molecular orbitals designated as
• ?1s or bonding molecular orbital
• ?1s or antibonding molecular orbital.

74

H
H
75
subtract
add
76
subtract
antibonding
add
bonding
77
subtract
antibonding
??1s
add
?1s
bonding
78
Molecular Orbital Energy Level Diagram

• Now that we have seen what these MOs look
• like and a little of their energetics, how are
• the orbitals filled with electrons?
• Order of filling of MOs obeys same rules as
• for atomic orbitals.
• Including
• Aufbau principle increasing energy
• Hunds Rule two unaligned e- per orbital
• Thus the following energy level diagram
• results for the homonuclear diatomic
• molecules H2 and He2.

79
??1s
E
E
1s
1s
?1s
H
H2
H
80
??1s
E
E
1s
1s
?1s
H
H2
H
81
??1s
E
E
1s
1s
?1s
H
H2
H
82
??1s
E
E
1s
1s
?1s
H
H2
H
83
(?1s ) 2
??1s
E
E
1s
1s
?1s
H
H2
H
84
(?1s ) 2
total spin 0
??1s
E
E
1s
1s
?1s
H
H2
H
85

• Diamagnetic slightly repelled by a magnetic
  field
• total spin 0
• paramagnetic attracted to a magnetic field
• total spin not 0
• (bonding e
  antibonding e)
• Bond Order --------------------
• 
  2

86
Bond Order and Bond Stability

• The larger the bond order, the more stable the
• molecule or ion is.
• Bond order 0 implies there are equal numbers of
  electrons in bonding and antibonding orbitals,
• same stability as separate atoms no bond
  formed
• Bond order gt 0 implies there are more electrons
  in bonding than antibonding orbitals.
• Molecule is more stable than separate atoms.
• The greater the bond order, the shorter the bond
• length and the greater the bond energy.

87
(?1s ) 2
total spin 0
diamagnetic
??1s
E
E
1s
1s
?1s
H
H2
H
88
BO 1/2 ( 2 0) 1
??1s
E
E
1s
1s
?1s
H
H2
H
89
Consider He2
90
??1s
E
E
1s
1s
?1s
He
He2
He
91
??1s
E
E
1s
1s
?1s
He
He2
He
92
(?1s ) 2
(? 1s ) 2
??1s
E
E
1s
1s
?1s
He
He2
He
93
diamagnetic
??1s
E
E
1s
1s
?1s
He
He2
He
94
BO 1/2 ( 2 2 ) 0 He2 does not exist
??1s
E
E
1s
1s
?1s
He
He2
He
95
Combination of p Atomic Orbitals
96
Molecular Orbitals

• The head-on overlap of two corresponding p
• atomic orbitals on different atoms, say 2px
• with 2px produces
• ?2px bonding orbital
• ?2px antibonding orbital

97
2p
2p
98
subtract
add
99
antibonding MO
subtract
add
bonding MO
100
antibonding MO
? 2p
subtract
add
bonding MO
? 2p
101
Molecular Orbitals

• Side-on overlap of two corresponding p
• atomic orbitals on different atoms (say 2py
• with 2py or 2pz with 2pz) produces
• or (both are bonding orbitals)
• or (both are nonbonding orbitals)

102
2p
2p
103
subtract
add
104
subtract
antibonding MO
add
bonding MO
105
subtract
??2p
add
?2p
106
subtract
??2p
add
?2p
107
Consider Li2
108
??2p
?2p
2p
2p
?2p
E
E
?2p
??2s
2s
2s
?2s
Li
Li
Li2
109
??2p
?2p
2p
2p
?2p
E
E
?2p
??2s
2s
2s
?2s
Li
Li
Li2
110
??2p
?2p
2p
2p
?2p
E
E
?2p
??2s
2s
2s
?2s
Be
Be
Be2
111
??2p
?2p
2p
2p
?2p
E
E
?2p
??2s
2s
2s
?2s
Be
Be
Be2
112
??2p
?2p
2p
2p
?2p
E
E
?2p
??2s
2s
2s
?2s
B
B
B2
113
??2p
?2p
2p
2p
?2p
E
E
?2p
??2s
2s
2s
?2s
B
B
B2
114
??2p
?2p
2p
2p
?2p
E
E
?2p
??2s
2s
2s
?2s
C
C
C2
115
??2p
?2p
2p
2p
?2p
E
E
?2p
??2s
2s
2s
?2s
N
N
N2
116
Homonuclear Diatomic Molecules

• In shorthand notation we represent the
• configuration of N2 as

117
Bond Order of N2

• The greater the bond order of a bond the
• more stable we predict it to be.
• For N2 the bond order is

118
??2p
?2p
2p
2p
E
E
?2p
??2s
2s
2s
?2s
O
O
O2
119
Homonuclear Diatomic Molecules

• In shorthand notation we represent the
• configuration of O2 as
• We can see that O2 is a paramagnetic
• molecule (two unpaired electrons).

120
??2p
?2p
2p
2p
E
E
?2p
??2s
2s
2s
?2s
F
F
F2
121
??2p
?2p
2p
2p
E
E
?2p
??2s
2s
2s
?2s
Ne
Ne
Ne2
122
Bond Order for Ne2

• 2?4 - 2?4
• BO ------- 0
• 2
• We can see that Ne2 is not stable. It does not
• exist

123
Delocalization and Shapes of Molecular Orbitals

• Molecular orbital theory describes
• shapes in terms of delocalization of
• electrons.
• Carbonate ion (CO32-) is a good example.
• VB Theory MO Theory

124
Delocalization and Shapes of Molecular Orbitals

• Benzene, C6H6, Resonance structure - VB theory

125
Delocalization and Shapes of Molecular Orbitals

• This is the picture of the valence bond
• (VB) theory

126
Delocalization and Shapes of Molecular Orbitals

• The structure of benzene is described
• well by molecular orbital theory.