Transcript Theories of Chemical Bonding

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
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Energy of Interaction Between Two H Atoms
Energies of attraction and
repulsion as functions of
distance between two H
atoms are shown here.
Potential
energy
The minimum of the
attraction force occur at
H–H bond length of 74 pm,
at which, the antibonding
orbital is +346 kJ mole–1
above 0, energy when H
atoms are far apart.
of chemical bonding
How does energy affectTheories
the two-atom
system?
+346 kJ mol –1
antibonding
distance
–346 kJ mol –1
H – H bond
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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
H-H
Li-H
s-p
H-C
H-N
H-F
s-d
p-p
H-Pd C-C
in Pd P-P
hydride
p-d
Se-F
d-d
Fe-Fe (?)
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 chemicalTheories
bonds?
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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 ____________
Theories
of chemical
Provide a description for hybrid
orbitals
sp,bonding
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 =>
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.
Cl–Be–Cl
H–CC–H
H–CN :
O=C=O
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
C
2s 2p 2p 2p
sp2 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
in p bond
H
px over lap
in p bond
H
C
2s 2p 2p 2p
sp sp 2p 2p
Two nodal planes of p
bonds are perpendicular
to each other.
Draw and describe how atomic orbitals overlap
Theories of chemical bonding
to form all bonds in acetylene, H–CC–H
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Two p Bonds in H–CC–H
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
py over lap in p bond
:O–CO:
:OC–O:
px over lap in p bond
..
Discuss the bonding of allene H2C=C=CH2
Overlap p–p in s bonds
O=C=O or H2C=C=CH2
Theories of chemical bonding
See extra problems B17 in the handout
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Bonding in CO2 – another view
sigma bond
(1 pair of electrons) pi bond
(1 pair of
electrons)
09_174
O
C
O
pi bond
(1 pair of
electrons)
(a)
Compare with H2C=C=CH2
O
(b)
C
O
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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,
H2C=CH2, 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 H2C=CH2 molecules
Utilizing the sp2 hybrid orbitals, each C atom form
two H–C s bonds for a total of 4 s H–C bonds.
The C–C s bond is common to both C atoms.
A C–C p bond is formed due to overlap of p
orbitals from each of the C atoms.
C
2s 2p 2p 2p
sp2 sp2 sp2 2p
Hybrid orbitals (sp2) for
H–C and C–C s bond
Overlap of p orbital
for C–C 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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C
2s 2p 2p 2p
sp3 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 AX2E4
IOF5, 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
Take a new look at slide
22 in Bonding Basics
H3C – N – C – O
Which Lewis dot structure is the most important (stable)?
180o
NC–O
H–C
H H
109o
H–C
H
N=C=O
120o
H
N–CO
H–C
H H
Which
structure is
more stable,
and why?
What hybridized orbitals are used for bonding in N and C? Why are the
bond angles as indicated? No of s and p bonds = __, __?
bonding
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Give formal charges to all Theories
atomsofinchemical
all structures.
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
are unsatisfactory.
:O
O:
•O
O•
MO theory, different from VB in that MO theory considers the orbitals of
the whole molecules. However the approach of linear-combination-ofatomic-orbitals (LCAO) is usually used.
There are other reasons, but it’s 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.
Theories of chemical bonding
1sA
AO
1sB
s
MO
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AO
MO for H2–type 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
H2+
H2, He22+
H2–, He2+
H22–, He2
e-configuration
1s (1s1)
1s2
1s2 1s*
1s2 1s*2
Bond order
½
1
½
0
bondlength
106 pm
74, ~75
~106, 108
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)
Theories of chemical bonding
Generalize the technique of LCAO
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Pi p MOs from p AOs
Theories
bonding
Generalize
theof chemical
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
For O=O
_ _
O=O
Paramagnetic ,
bond length
indicates double
bond, electronic
configuration
agrees
F–F
Electronic
configuration agree
with single bond.
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
__ s*2p
__ __ p*2p
___
2p
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).
___
2p
__ s2p
__ __p2p
__ s*2s
_
2s
_
2s
__ s2s
Atomic
orbital
Molecular orbitals
Atomic
orbital
The sp mixing of AO gives
stronger s2s bond and a weaker
s*2s 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. It’s 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
+
+
+
+
+
+
–
–
–
Theories of chemical bonding
–
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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
Theories of chemical bonding
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.
Theories of chemical bonding
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.
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