Transcript Chapter 5 Bonding in polyatomic molecules Dr. Said M. El-Kurdi 1

Chapter 5
Bonding in polyatomic molecules
Dr. Said M. El-Kurdi
1
5.1 Introduction
In Chapter 2, we considered three approaches to the
bonding in diatomic molecules:
 Lewis structures;
 valence bond (VB) theory;
 molecular orbital (MO) theory.
A polyatomic species contains three or more atoms.
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Consider H2O
there is a problem in trying to derive a localized bonding
scheme in terms of an atomic orbital basis set
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5.2 Valence bond theory: hybridization
of atomic orbitals
The word ‘hybridization’ means ‘mixing’ and when used in
the context of atomic orbitals, it describes a way of deriving
spatially directed orbitals which may be used within VB
theory.
Like all bonding theories, orbital hybridization is a model,
and should not be taken to be a real phenomenon.
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Hybrid orbitals are generated by mixing the characters of
atomic orbitals ( close in energy).
A set of hybrid orbitals provides a bonding picture for a
molecule in terms of localized -bonds.
sp Hybridization: a scheme for linear species
if we begin with n atomic orbitals, we must end up with n
orbitals after hybridization.
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sp hybrid orbital which possesses 50% s and 50% p
character.
represent two wavefunctions which are equivalent in every
respect except for their directionalities with respect to the x
axis.
The model of sp hybridization can be used to describe the
-bonding in a linear molecule such as BeCl2
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BeCl2
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sp2 Hybridization: a scheme for trigonal planar species
The notation sp2 means that one s and two p atomic
orbitals mix to form a set of three hybrid orbitals with
different directional properties.
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The model of sp2 hybridization can be used to describe the bonding in trigonal planar molecules such as BH3.
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sp3 Hybridization: a scheme for tetrahedral
and related species
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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The directions of the orbitals that make up a set of four sp3 hybrid orbitals
correspond to a tetrahedral array
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CH4
The relationship between a tetrahedron and a cube; in CH4
NH3
sp3d (dz2 ) trigonal bipyramidal
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Sp3d (dx2 y2) square-based pyramidal
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Hybridization of s, px, py, pz, dz2 and dx2 y2 atomic orbitals
gives six sp3d2 hybrid orbitals corresponding to an octahedral
arrangement.
hybridize only the s, px, py and dx2 y2 atomic orbitals, the
resultant set of four sp2d hybrid orbitals corresponds to a
square planar arrangement
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5.3 Valence bond theory: multiple
bonding in polyatomic molecules
C2H4
117.4o
121.3o
each C centre is approximately trigonal planar and the bonding framework within C2H4 can be described in terms
of an sp2 hybridization scheme
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This leaves a 2p atomic orbital on each C atom; overlap
between them gives a CC -interaction.
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HCN
An sp hybridization scheme is appropriate for both C and N
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The -character in the CN bond arises from 2p–2p overlap.
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BF3
Boron trifluoride is trigonal planar
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BF3
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[NO3]
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D3h symmetry
planar
Of the 24 valence electrons, 18 are accommodated either in
-bonds or as oxygen lone pairs.
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5.4 Molecular orbital theory: the ligand group orbital
approach and application to triatomic molecules
MO approach to bonding in linear XH2:
symmetry matching by inspection
ligand group orbital (LGO) approach.
 consider the bonding in a linear triatomic XH2 in which
the valence orbitals of X are the 2s and 2p atomic orbitals.
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Each 1s atomic orbital has two possible phases and, when the
two 1s orbitals are taken as a group, there are two possible
phase combinations.
These are called ligand group orbitals (LGOs)
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The number of ligand group orbitals formed = the number of
atomic orbitals used.
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The -bonding character in orbitals 1 and 2 is spread
over all three atoms, indicating that the bonding character
is delocalized over the HXH framework.
Delocalized bonding is a general result within MO theory.
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5.4 Molecular orbital theory: the ligand group orbital
approach and application to triatomic molecules
Character tables: an introduction
The H2O molecule
Each point group has an associated
character table
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The left-hand column in a character table gives a list of
symmetry labels.
Symmetry labels give us information about degeneracies
as follows:
. A and B (or a and b) indicate non-degenerate;
. E (or e) refers to doubly degenerate;
. T (or t) means triply degenerate.
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A bent triatomic: H2O
The labels in the first column
(under the point group symbol)
tell us the symmetry types of
orbitals that are permitted within
the specified point group.
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The numbers in the column headed E (the identity operator) indicate
the degeneracy of each type of orbital; in the C2v point group, all
orbitals have a degeneracy of 1, i.e. they are non-degenerate.
Each row of numbers following a given symmetry label indicates how a
particular orbital behaves when operated upon by each symmetry
operation.
1 means that the orbital is unchanged by the operation,
1 means the orbital changes sign,
0 means that the orbital changes in some other way.
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Apply each symmetry operation of the C2v point group in turn.
Applying the E operator leaves the 2s atomic orbital unchanged.
this matches those for the symmetry type A1 in the C2v
character table. We therefore label the 2s atomic orbital on
the oxygen atom in water as an a1 orbital.
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The same test is now carried out on each atomic orbital of the O atom.
The oxygen 2px orbital
This matches the row of characters
for symmetry type B1 in the C2v
character table, and the 2px orbital
therefore possesses b1 symmetry.
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The oxygen 2py orbital
This corresponds to symmetry type
B2 in the C2v character table, and
the 2py orbital is labelled b2.
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The 2pz orbital
the 2pz orbital therefore has a1
symmetry.
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The next step is to work out the nature of the H-----H ligand
group orbitals that are allowed within the C2v point group.
Since we start with two H 1s orbitals, only two LGOs can be
constructed.
what happens to each of the two H 1s orbitals when each
symmetry operation is performed?
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in which a ‘2’ shows that ‘two orbitals are unchanged by the operation’,
and a ‘0’ means that ‘no orbitals are unchanged by the operation’
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(i) we can construct only two ligand group orbitals,
(ii) the symmetry of each LGO must correspond to one of
the symmetry types in the character table.
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We now compare the row of characters above with the sums
of two rows of characters in the C2v character table.
A match is found with the sum of the characters for the A1
and B2 representations. As a result, we can deduce that the
two LGOs must possess a1 and b2 symmetries, respectively.
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5.5 Molecular orbital theory applied to
the polyatomic molecules BH3, NH3 and CH4
BH3
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By using the same approach as we did for the orbitals of the O
atom in H2O, we can assign symmetry labels to the orbitals of
the B atom in BH3:
 the 2s orbital has a1’ symmetry;
 the 2pz orbital has a2’’ symmetry;
 the 2px and 2py orbitals are degenerate and the orbital set
has e’ symmetry.
We now consider the nature of the three ligand group orbitals that are
formed from linear combinations of the three H 1s orbitals.
how many H 1s orbitals are left unchanged by each symmetry
operation in the D3h point group
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This same row of characters can be obtained by summing
the rows of characters for the A1’ and E’ representations in
the D3h character table. Thus, the three LGOs have a1’ and e’
symmetries
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The MO diagram for BH3 can now be constructed by allowing
orbitals of the same symmetry to interact.
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 The MO approach describes the bonding in BH3 in terms of
three MOs of a1’ and e’ symmetries.
 The a1’ orbital possesses -bonding character which is
delocalized over all four atoms.
 The e’ orbitals also exhibit delocalized character, and the
bonding in BH3 is described by considering a combination of
all three bonding MOs.
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Building a MO diagram for NH3
1. The point group is C3v.
2. XN  3 and XH  2.2 so the energy levels of the AO’s on N will be lower than those
on the H atoms.
3. From the C3v character table, the symmetry of the AO’s on N are: A1(2s), A1(2pz),
and E(2px,2py). Each of these orbitals can interact with the LGOs from the H3
framework.
4. Fill the MO’s with the 8 valence electrons.
In NH3, the HOMO is a mostly nitrogen-based orbital that corresponds to the lone pair
of electrons from VBT. This is why ammonia acts as a Lewis base at the N atom. The
LUMO is the 2e level that has more H character - this shows why NH3 can also act as a
Lewis acid through the H atoms.
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Building a MO diagram for NH3
By seeing how each symmetry operation affects each orbital
of the N atom in NH3, the orbital symmetries are assigned as
follows:
 each of the 2s and 2pz orbitals has a1 symmetry;
 the 2px and 2py orbitals are degenerate and the orbital set
has e symmetry.
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To determine the nature of the ligand group orbitals, we
consider how many H 1s orbitals are left unchanged by each
symmetry operation in the C3v point group
It follows that the three ligand group orbitals have a1 and e
symmetries.
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CH4
the 2s orbital has a1 symmetry;
the 2px, 2py and 2pz orbitals are degenerate and the
orbital set has t2 symmetry.
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In order to construct the LGOs of the H4 fragment in CH4, we
begin by working out the number of H 1s orbitals left
unchanged by each symmetry operation of the Td point
group.
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This same row of characters results by summing the rows of
characters for the A1 and T2 representations in the Td
character table
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5.6 Molecular orbital theory: bonding
analyses soon become complicated
The BF3 molecule has D3h symmetry.
the atomic orbitals of the B
atom in BF3 are assigned the following symmetries:
 the 2s orbital has a1’ symmetry;
 the 2pz orbital has a2’’ symmetry;
 the 2px and 2py orbitals are degenerate and the orbital
set has e’ symmetry.
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Ligand group orbitals involving the F 2s orbitals in BF3 and
having a1’ and e’ symmetries
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5.7 Molecular orbital theory: learning
to use the theory objectively
drawing a partial MO diagram for the molecule in question
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[NO3]
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