Chapter 3:
Bonding in polyatomic molecules
3.1. Introduction
Polyatomic species: contains three or more atoms
Within the valence bond model, treatment of a molecule XYn (n ≥ 2) raises the
question of compatibility (or not) between the positions of the Y atoms and the
directionalities of the atomic orbitals on the central atom X.
Example:
H2O
if the atoms of the H2O molecule lie in (for example) the yz plane,
The directionalities of the 2py and 2pz atomic orbital of oxygen are not compatible
with the directionalities of the two OH bonds.
In the next section we describe a bonding model within valence bond (VB) theory
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that overcomes this problem.
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3.2. Valence bond theory:
hybridization of atomic orbitals
Hybrid orbitals are generated by mixing the characters of atomic orbitals.
The labels given to hybrid orbitals reflect the contributing atomic orbitals, e.g.
an sp hybrid possesses equal amounts of s and p orbital character
A set of hybrid orbitals provides a bonding picture for a molecule in terms of
localized -bonds.
sp Hybridization: a scheme for linear species
The formation of two
sp hybrid orbitals
from one 2s atomic
orbital and one 2p
atomic orbital.
Ψsphybrid =
1
(ψ 2 s +ψ 2 p )
2
Ψsphybrid =
1
(ψ 2 s −ψ 2 p )
2
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Example:
beryllium atom Be (Z=4)
formation of the sp
hybridized valence state
of a beryllium atom
from its ground state.
Note :
The notation sp means that one s atomic orbital and one p atomic
orbital mix to form a set of two hybrid orbitals with different
directional properties.
This is a formalism and is not a ‘real’ observation
the valence state cannot be observed by spectroscopic techniques.
The choice of using the 2px orbital for hybridization is arbitrary.
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sp2 Hybridization: a scheme for trigonal planar species
The formation of three sp2 hybrid orbitals from one 2s atomic orbital and two
2p atomic orbitals
Ψsp2hybrid =
1
2
ψ 2 s + ψ 2 px
3
3
Ψsp2hybrid =
1
1
1
ψ 2 s − ψ 2 px + ψ 2 p y
3
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2
Ψsp2hybrid =
1
1
1
ψ 2 s − ψ 2 px − ψ 2 p y
3
6
2
Note :
The choice of px and py is arbitrary
If we started with 2px and 2pz atomic orbitals, the hybrids would lie in
the xz plane
using the 2py and 2pz atomic orbitals gives hybrid orbitals in the yz plane
The directionalities of the hybrid orbitals follow from the relative
5 contributions of the atomic orbitals
Example:
BH3
The bonding in trigonal planar
BH3 can be conveniently
described in terms of the
interactions between a set of
sp2 hybrid orbitals centred on
the B atom and three H 1s
atomic orbitals. Three pairs of
electrons are available (three
electrons from B and one from
each H) to give three σ-bonds.
sp3 Hybridization: a scheme for
tetrahedral and related species
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The notation sp3 means that one s and
three p atomic orbitals mix to form a
set of four hybrid orbitals with
different directional properties.
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Example:
CH4
4 valence electrons
2 unpaired electrons
Problem:
4 nonequivalent orbitals,
who get e- first?
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4 nonequivalent orbitals
4 equivalent orbitals
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sp3 Hybridized Atoms
Orbital Diagrams
• Place electrons into hybrid and unhybridized valence orbitals as if all the
orbitals have equal energy
• Lone pairs generally occupy hybrid orbitals
sp3 hybridized atom
Unhybridized atom
↑
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2s
C
↑
↑
↑
↑
↑
↑
↑
↑
2sp3
2p
2s
↑↓
↑
↑
N
2p
↑
↑
↑↓
↑
2sp3
Other hybridization schemes
Example: sp3d
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Hybridization Scheme from Electron Geometry
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3.3. Valence bond theory:
multiple bonding in polyatomic molecules
• Many molecules have larger structures with many interior atoms
• We can think of them as having multiple central atoms
• When this occurs, we describe the shape around each central atom
in sequence
Example:
CH3COOH
Lewis structure:
H O
H C C O H
H
shape around left C is tetrahedral (type EX4)
shape around center C is trigonal planar (type EX3)
shape around right O is tetrahedral-bent (type EX2L2)
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Example:
Describing the Geometry of Methanol, CH3OH
Lewis structure For CH3OH
Local shapes:
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Example:
Describing the Geometry of Glycine, H2N-CH2-COOH
Lewis structure For Glycine
Local shapes:
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Example:
Predict the molecular geometries in H3BO3
Lewis structure For H3BO3
oxyacid, so H attached to O
B atom is less electronegative then O atom
→ B Is Central Atom
B = 3e─
O3 = 3(6e─) = 18e─
H3 = 3(1e─) = 3e─
Total = 24e─
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Example:
Predict the molecular geometries in H3BO3 (continued)
Lewis structure For H3BO3
Local shapes:
3 electron groups on B
B
has
3 Bonding groups
0 Lone pairs
(Structure type EX3)
4 electron groups on O
O has
2 Bonding groups
2 Lone pairs
(Structure type EX2L2)
Shape on O = tetrahedral bent
Shape on B = trigonal planar
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