Molecular Structure Hybridization

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Molecular StructureHybridization

• Edward A. Mottel
• Department of Chemistry
• Rose-Hulman Institute of Technology

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Walsh Diagram
Estimates the energy stabilization that
occurs when atomic orbital overlap varies between
limiting structures.
The overlap of nonspherical orbitals varies with
bond angle.
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Walsh Diagram for AH2
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Walsh Diagram for AH2
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What angles would be expected for
BeH2
BeH2
CH2(singlet)
CH2(triplet)
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Walsh Diagram for AH2
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Hybridization and Structure
For simple s-p hybrids with two or
more equivalent orbitals
for ? 109.5 ?? S 0.25, P 0.75
180 ?? S 0.50, P 0.50
104.5 ?? S 0.20, P 0.80
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Walsh Diagram for H2O
better overlap of p orbitals favors the bent
structure
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Hybridization and Structure
Five coordinate structures use s, p and d
orbitals and range from trigonal bipyramidal to
square pyramidal.
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sp3d Hybridization



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Trigonal BipyramidalEquatorial Orbitals
s

3 equal lobes sitting in the xy-plane (like sp2)
px
py
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Trigonal BipyramidalAxial Orbitals
dp hybridization 2 equal lobes lying primarily
along the z-axis. (similar to sp hybridization)
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Trigonal BipyramidalAxial Equatorial
Differences
1.577 Å
Equatorial bonds have more s character and are
shorter and stronger than axial bonds.
1.534 Å
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Bents Rule Axial Equatorial Differences
More electronegative substituents prefer hybrid
orbitals having less s character.
More electropositive substituents prefer hybrid
orbitals having more s character.
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Factors Affecting Molecular Geometry
Electronegativity differences lead to less
covalent bonding. (energy match)
Significant differences in orbital size or poor
geometric position lead to less covalent
bonding. (overlap)
Physical size can affect the location of
substituents in the molecule. (steric)
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Energy Match Orbitals of the same
electronegativity form better covalent bonds.
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Overlap Orbitals of the same size form
better covalent bonds.
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Steric Big substituents can affect geometry.
CH3 rvdw 200 pm
I rvdw 195-212 pm
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Structural Comparisons
ammonia
?H-N-H 106.8
NF3
?F-N-F 102.3
PF3
?F-P-F 96.3
What factors can account for the variation in
bond angles?
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Bent Bonds
Geometric constraints, especially caused by ring
structure, can require orbital overlap in unusual
positions.
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e- density map
differential e- density
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e- density map
difference e- density
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Bond Length
Improved orbital overlap results in shorter,
stronger bonds.
Rank the following compounds by C-C bond length
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Bond Length Hybridization Effects
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