Theories of Chemical Bonding

1
Theories of Chemical Bonding
Theories of bonding explanations for chemical
bond, Lewis dot structures and the
following. Valance-bond (VB) theory Hybridization
of atomic orbitals Multiple covalent
bonds Molecular orbital (MO) theory Delocalized
electrons Bonding in metals
Practice mental reasoning and verbal explanation
2
Energy of Interaction Between Two H Atoms
Energies of attraction and repulsion as functions
of distance between two H atoms are shown here.
The minimum of the attraction force occur at
HH bond length of 74 pm, at which, the
antibonding orbital is 346 kJ mole1 above 0,
energy when H atoms are far apart.
Potential energy
346 kJ mol 1 antibonding
distance
346 kJ mol 1 H H bond
How does energy affect the two-atom system?
3
The Valence-bond Method
Valence bond method considers the covalent bond
as a result of overlap of atomic orbitals.
Electrons stay in regions between the two atoms.
Some bond examples s-s s-p s-d p-p p-d d-d
H-H H-C H-Pd C-C Se-F Fe-Fe (?) Li-H H-N
in Pd P-P H-F hydride But overlapping of
simple atomic orbitals does not explain all the
features. Thus, we have to take another look, or
do something about atomic orbitals
hybridization.
How does valence-bond approach explain the
formation of chemical bonds?
4
Hybridization of Atomic Orbitals
The solutions of Schrodinger equation led to
these atomic orbitals. 1s, 2s, 2p, 3s, 3p, 3d,
4s, 4p, 4d, 4f, etc. However, overlap of these
orbitals does not give a satisfactory
explanation. In order to explain bonding, these
orbitals are combined to form new set of orbitals
this method is called hybridization. During the
lecture, these hybridized orbitals will be
explained sp 2 sp hybrid orbitals from mixing
of a s and a p orbital sp 2 3 sp2 hybrid
orbitals from mixing of a s and 2 p orbital sp3
fill in you explanation please sp3d 5 sp3d
hybrid orbitals from mixing of a s and 3 p and a
d orbital sp3d 2 ____________
Provide a description for hybrid orbitals sp,
sp2, sp3, sp3d, and sp3d2
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The sp Hybrid Orbitals
The sp hybrid orbitals formation of two sp
hybrid orbitals - -
- - hybridization of s and p
orbitals 2 sp hybrid orbitals ?_ ?_ __ __ ??
__ __ __ Two sp hybrid orbitlas gt Two
states of Be
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Bonds with sp Hybrid Orbitals
Formations of bonds in these molecules are
discussed during the lecture. Be prepared to do
the same by yourself. ClBeCl HC?CH HC?N
OCO
Double and triple bonds involve pi p bonding, and
the the application of valence bond method to p
bonds will be discussed.
You are expected to be able to draw pictures to
show the p bonding.
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A p Bond
Sigma (s) bond is symmetric about axis. Pi (p)
electron distribution above and below axis with a
nodal plane, on which probability of finding
electron is zero p bond is not as strong as
sigma - less overlap.
Nodal plane
Overlap of 2 2p orbitals for the formation of p
bond
C2s 2p 2p 2psp2 sp2 sp2 2p
Bonding of C2H4
How are pi bonds formed?
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Triple Bonds in H-C?C-H
H-C-C-H three s bonds due to overlapping of 1sH
spC spC spC and spC 1sH. Two p bonds in
HC?CH and HC?N triple bonds are due to
overlapping of p orbitals results.
sp hybrid orbitals
py over lap
C2s 2p 2p 2psp sp 2p 2p
in p bond
H
H
px over lap
Two nodal planes of p bonds are perpendicular to
each other.
in p bond
Draw and describe how atomic orbitals overlap to
form all bonds in acetylene, HC?CH
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Two p Bonds in HC?CH
A triple bond consists of a sigma and two pi
bonds. Overlaps of two sets of p orbitals form of
two p bonds.
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Bonding of CO2
For CO2, the C atom forms a s bond and a p bond
with each of two O atoms. The two nodal planes of
the two p bonds are also perpendicular. During
the lecture, I draw diagrams and explain the two
s two p bonds in CO2. You are expected to be able
to do the same, in a test.
Resonance structures O C ? O O ? C O
. .
py over lap in p bond
px over lap in p bond
Overlap pp in s bonds
Discuss the bonding of allene H2CCCH2 See extra
problems B17 in the handout
OCO or H2CCCH2
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Bonding in CO2 another view
Compare with H2CCCH2
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The sp2 Hybrid Orbitals
Ground state and excited state electronic
configuration of B ?_ ?_ ?_ __ ?? ?_ __ __ The
hybridization of a s and two p orbitals led to 3
sp2 hybrid orbitals for bonding. Compounds
involving sp2 hybrid orbitals BF3, CO32, H2CO,
H2CCH2, NO3, etc
Nov. 25
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An example of using sp2 hybrid orbitals
__ orbitals for bonding? Dipole moment ____?
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Bonding of H2CCH2 molecules
Utilizing the sp2 hybrid orbitals, each C atom
form two HC s bonds for a total of 4 s HC
bonds. The CC s bond is common to both C atoms.
A CC p bond is formed due to overlap of p
orbitals from each of the C atoms.
C2s 2p 2p 2psp2 sp2 sp2 2p
Hybrid orbitals (sp2) for HC and CC s bond
Overlap of p orbital for CC p bond
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The sp3 Hybridized Orbitals
Ground state and excited state electronic
configuration of C ?_ ?_ ?_ ?_ ?? ?_ ?_ __ The
hybridization of a s and three p orbitals led to
4 sp3 hybrid orbitals for bonding. Compounds
involving sp3 hybrid orbitals CF4, CH4, NH3,
H2O, SiO44, SO42, ClO4, etc
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C2s 2p 2p 2psp3 sp3 sp3 sp3
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The sp3d Hybrid Orbitals
Hybridization of one s, three p, and a d orbitals
results in 5 sp3d hybrid orbitals. The
arrangement of these orbitals is a trigonal
pyramid. Some structures due to these type of
orbitals are PClF4, TeCl4E, and BrF3E2.
How many unshared electron pairs are present in
TeCl4 and BrF3? What are their shapes?
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The sp3d2 Hybrid Orbitals
Hybridization of one s, three p, and two d
orbitals results in 6 sp3d2 hybrid orbitals. The
arrangement of these orbitals is an octahedron.
Compounds using these type of orbitals are shown
here.
AX6,   AX5E,   AX4E2   AX3E3   and   AX2E4IOF5,
  IF5E,   XeF4E2
No known compounds of AX3E3 and AX2E4 are known
or recognized, because they are predicted to have
a T shape and linear shape respectively when the
lone pairs of electrons are ignored.
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Molecules with more than one central atom
Describe the structure of CH3NCO. Draw the
skeleton and add all valence electrons H3C N
C O Which Lewis dot structure is the most
important (stable)? N C O
Take a new look at slide 22 in Bonding Basics
180o
N?CO HC
NC?O HC
Which structure is more stable, and why?
109o
120o
HC
H H
H H
H H
What hybridized orbitals are used for bonding in
N and C? Why are the bond angles as indicated? No
of s and p bonds __, __? Give formal charges
to all atoms in all structures.
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Why Molecular Orbital (MO) Theory
Lewis dot and valence bond theories do not always
give satisfactory account for various properties
of molecules. For example, the dot and VB theory
does not explain the fact that O2 is paramagnetic
and has a double bond. Dot and VB structures O
O O O are unsatisfactory. MO
theory, different from VB in that MO theory
considers the orbitals of the whole molecules.
However the approach of linear-combination-of-atom
ic-orbitals (LCAO) is usually used. There are
other reasons, but its human nature to theorize.
The theory is beautiful, and worth learning or
teaching.
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The Molecular Orbital (MO) Theory
For a molecule, there are certain orbitals each
of which accommodates two electrons of opposite
spin. The MO theory combines atomic orbitals (AO)
to form MOs, this method is called LCAO
s
The two atoms in the H2 molecule may be
represented by A and B. Their s orbitals 1sA and
1sB respectively, are used for two MOs s
1sA 1sB s 1sA 1sB The energy levels of
these AO and MO are represented by the diagram
here, with the math hidden.
1sA
1sB
sMO
AO
AO
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MO for H2type molecules H2, H2, H2, He2
Generalize the technique of LCAO
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Electronic configuration, s2, for H2 molecules
Generalize the technique of LCAO
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Electronic Configuration of H2-type Molecules
From the previous theory, we can fill the M Os
with electrons for the H2-type
molecule Molecule e-configuration Bond
order bondlength H2 1s (1s1) ½ 106 pm H2,
He22 1s2 1 74, 75 H2, He2 1s2 1s ½
106, 108 H22, He2 1s2 1s2 0 not formed
Describe the relationships of bondlength
bondorder and e-configurations learn to reason
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Sigma MOs Formed Using p AOs
Sigma MOs (s2p s2p ) can be formed using p AOs,
similar to VB theory. The gain in bonding orbital
s2p (lower energy) is at the expense of the
anti-bonding orbital s2p (higher energy)
Generalize the technique of LCAO
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Pi p MOs from p AOs
Generalize the technique of LCAO
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MO Diagrams for O2 and F2
A full diagram of the energy level of molecular
orbitals of O2 and F2 is shown here. The
relative (approximate) height of these energies
will be explained verbally during the lecture,
and you are suppose to be able to do the same.
Write the electronic configurations for O2, O2,
F2, F2 Ne2.
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The O2, O2, O2 , F2, F2, F2 Molecules
?OO? Paramagnetic , bond length indicates double
bond, electronic configuration agrees FF
Electronic configuration agree with single bond.
For ?OO??_ ?_
See p. 457 for two MO energy-level diagrams
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MO Energy Level Diagram for Be2 N2
Due to close energy levels of 2s and 2p, the MO
energy level diagram for Be2 to N2 differs from
those of O2 to F2. Reasons and explanation are
given during the lecture. Hope you can do the
same.
Give electronic configurations for Be2,
Be2, B2, B2,B2, C2, C2, C2, N2,N2,
N2.
See p. 457 for two MO energy-level diagrams
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A more realistic energy level diagram for Be2
N2 involving sp mixing, not in text
_ _ _2p _2sAtomicorbital
__ s2p__ __ p2p__ s2p__ __p2p__
s2s__ s2sMolecular orbitals
_ _ _2p _2s Atomicorbital
This diagram from my CaCt website accounts for
the sp mixing of the AO for the bonding
consideration. This sp mixing effect is more
detailed than that required for freshman
chemistry (not to be tested). The sp mixing of
AO gives stronger s2s bond and a weaker s2s
bond. Thus, the split from 2s is not even.
Effects on other bonds are also shown, but
qualitatively.
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Benzene
The benzene structure has fascinated scientists
for centuries. Its bonding is particularly
interesting. The C atom utilizes sp2 hybrid AO in
the sigma bonds, and the remaining p AO overlap
forming a ring of p bonds.
Sigma s bonds are represented by lines, and the p
orbitals for the p bonds are shown by
balloon-shape blobs. Note the and signs of
the p orbitals. Thus, we represent it by










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More About Benzene
Chem120 students may ignore this slide. The p and
p of C6H6 are shown here the symmetry is also
interesting.
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Delocalized electrons in Benzene and Ozone
When p bonds are adjacent to each other or
separated by on single C-C bonds, the p bonding
electrons are delocalized. The delocalized
electron path for benzene and ozone are shown
here. These pictures represent p electron of the
the structures formula contribute most to their
structures.
CO32 have delocalized electrons
O O O
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Joy on structure of benzene a story
Kathleen Londsdale (1903-1971) came from a very
poor family in Kildare who moved to Essex when
she was five. She studied physics, math and
chemistry at school and went to college in London
when she was 16, where she did extremely well.
She was offered a place in the research team of
William Bragg, so starting her life's work on
X-ray crystallography. In 1929 she showed, by
her analysis of hexamethyl benzene, that the
benzene ring was flat. Benzene has fascinated
scientists, and its precise structure was a
matter of controversy till then. In 1945 she was
elected the first ever woman Fellow of the Royal
Society. She was also created a Dame Commander of
the Order of the Brittish Empire in 1956.
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MO for Heteronuclear Diatomic Molecules
For heteronuclear diatomic molecules, the atomic
orbitals are at different energy levels. Thus,
the MO shifts are different from those of
homonuclear diatomic molecules. The interactions
of AO for MO for HF, LiF are similar, and
explained in lectures.
Chem120 students may ignore this slide.
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Review
Explain the bonding and anti-bonding orbitals,
with a picture if possible Describe a s and a p
bond, picture may be used. Give the electronic
configurations and bond orders for N2, N2, N22,
N2, N22, O2, Draw the Lewis dot structure for
ozone. Describe the molecular shape and justify
for it. Explain the delocalized electrons of
ozone. Use the MO theory to explain the fact that
O2 is paramagnetic, and has a double bond. A
diagram of the MO energy levels will help. Give a
few compounds that have the same number of
electrons as O3.