1 Polyatomic species: contains three or more atoms Three approaches to bonding in diatomic molecules 1.Lewis structures 2.Valence bond theory 3.Molecular orbital theory Chapter 5 Bonding in polyatomic molecules Directionalities of atomic orbitals are not compatible with the H-O-H bond angle.
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1
Polyatomic species: contains three or more atoms
Three approaches to bonding in diatomic molecules
1.Lewis structures
2.Valence bond theory
3.Molecular orbital theory
Chapter 5
Bonding in polyatomic molecules
Directionalities of atomic orbitals are not compatible with the H-O-H bond angle.
2
Orbital hybridization - sp
Hybrid orbitals – generated by mixing the characters of atomic orbitals
)(2
122_ xpshybridsp
)(2
122_ xpshybridsp
sp hybridized valence state is a formalism and is not a ‘real’ observation
3
Orbital hybridization – sp2
xpshybridsp 22_2 32
31
yx ppshybridsp 222_2 21
61
31
yx ppshybridsp 222_2 21
61
31
Trigonal planar molecule, BH3
Each B-H interaction is formed by the overlap of one B sp2
hybrid orbital with the 1s atomic orbital of an H atom
4
sp3 hybrid orbitals – one s and three p atomic orbitals mix to form a set of four orbitals with different directional properties
zyx pppshybridsp 2222_3 2
1
zyx pppshybridsp 2222_3 2
1
zyx pppshybridsp 2222_3 2
1
zyx pppshybridsp 2222_3 2
1
sp3d hybrid orbitals – one s, three p, and one d atomic orbitals mix to form a set of five orbitals with different directional properties
[Ni(CN)5]3-
5
Valence bond theory – multiple bonding in polyatomic molecules
Valence bond theory – multiple bonding in polyatomic molecules
6
Valence bond theory – multiple bonding in polyatomic molecules
Molecular orbital theory:
ligand group orbital approach in triatomic molecules
7
MO diagram is constructed by allowing interactions between orbitals of the same symmetry.
8
9
The bonding is described by a consideration of all three bonding MOs
10
NH3
11
CH4
Td
12
Derive the symmetries of the valence orbitals and symmetries for the ligand group orbitals (LGOs) to derive a qualitative MO diagram for BF3. Determine the composition of each LGO in terms of the individual wavefunctions ψ1, ψ2, … and sketch the resulting LGO.
D3h
26 m E 2C3 3C2 h 2S3 3v
1A 1 1 1 1 1 1
2A 1 1 –1 1 1 –1 E 2 –1 0 2 –1 0
1A 1 1 1 –1 –1 –1
2A 1 1 –1 –1 –1 1
E 2 –1 0 –2 1 0
Molecular orbital theory: BF3
13
S3
S3
ψ1
ψ2
ψ3
S3
ψ3
ψ1
ψ2
S3
ψ2
ψ3
ψ1
ψ1
ψ2
ψ3
S3
ψ3
ψ1
ψ2ψ2
ψ3
ψ1
Consider the S3 operation (=C3·σh) on the pz orbitals in the F3 fragment.
S3
C32
σhC3
Unique, ‘S3’
Unique, ‘S32’
The resulting wavefunction contributions from the S3 and S32
operations are –ψ3 and –ψ2, respectively.
14
Partial MO diagram that illustrates the formation of delocalized CO -bonds
15
SF6
16
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 planes containing 4 p orbitals. The C2 axis contains no 2p orbitals.
E C3 C2 C4 C2(C4
2)i S4 S6 h d
6 0 0 2 2 0 0 0 4 2
C2
Use the reduction formula to find the resulting symmetries: a1g, t1u, eg
Could derive the equations for the LGOs for the F6 fragment.
6543211 61)( ga
6111 21)( ut
4221 21)( ut
5331 21)( ut
6543211 22121)( ge
54322 21)( ge
17
Three-center two-electron interactions
18
B2H6
19
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