Chapter5 Bonding in Polyatomic Molecules

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Polyatomic species: contains three or more atoms

Three approaches to bonding in diatomicmolecules

1.Lewis structures

2.Valence bond theory

3.Molecular orbital theory

Chapter 5

Bonding in polyatomic molecules


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Orbital hybridization - sp

Hybrid orbitals generated by mixingthe characters of atomic orbitals

)(2

122_ xpshybridsp

+=

)(2

122_ xpshybridsp

=


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Orbital hybridization sp2

xpshybridsp 22_2

3

2

3

1 +=

yx ppshybridsp 222_22

1

6

1

3

1 +=

yx ppshybridsp 222_2

2

1

6

1

3

1 =


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sp3 hybrid orbitals one s and threep atomic orbitals mix toform a set of four orbitals with different directional properties

( )zyx pppshybridsp 2222_3 2

1 +++=

( )zyxpppshybridsp

2222_3 2

1 +=

( )zyx pppshybridsp 2222_3 2

1 +=

( )zyxpppshybridsp

2222_3 2

1 =

sp3d hybrid orbitals one s, threep, and one datomic orbitalsmix to form a set of five orbitals with different directional properties

[Ni(CN)5]3-


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Valence bond theory multiple bonding in polyatomic molecules



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Molecular orbital theory:

l ig a n d g r o u p o r b i t a l approach in triatomic molecules


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Molecular orbital theory: BF3


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S3

S3

1

2

3

S3

3

1

2

S3

2

3

1

1

2

3

S3

3

1

2

2

3

1

Consider the S3 operation (=C3h) on the pz orbitals in the F3 fragment.

S3

C32

hC3

Unique, S3

Unique, S32

The resulting wavefunction contributions from the S3 and S32

operations are 3 and 2, respectively.


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BF3 Resonance StructuresBF3 Resonance Structures

The presence of the

resonance

contributions account

for the partial double

bond character in BF3


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SF6


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3

1

2

4

6

5 Find number of unchanged radial 2p

orbitals that are unchanged under each Ohsymmetry operation.

C2 Note the C2 axis bisect the planescontaining 4 p orbitals. The C2 axiscontains no 2p orbitals.

0

S6

0

S4

24022006

dhiC2(C4

2)

C4C2C3E

C2

Use the reduction formula to find the resulting symmetries: a1g, t1u, eg

Could derive the equations for the LGOs for the F6 fragment.


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( )65432116

1)( +++++=ga

( )6111

2

1)( =ut

( )4221

2

1)( =ut

( )5331

2

1)( =ut

( )6543211 22

12

1)( +=ge

( )54322

2

1)( +=ge

Three-center two-electron interactions


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Chapter5 Bonding in Polyatomic Molecules

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