View
231
Download
0
Category
Preview:
Citation preview
8/12/2019 Theory of Chemical Bonding
1/36
Theories of chemical bonding 1
Theories of Chemical Bonding
Theories of bonding: explanations for chemical bond, Lewis dotstructures 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
8/12/2019 Theory of Chemical Bonding
2/36
8/12/2019 Theory of Chemical Bonding
3/36
Theories of chemical bonding 3
The Valence-bond Method
Valence bondmethod considers the covalent bond as a result ofoverlap 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 aboutatomic orbitalshybridization.
How does valence-bond approach explain
the formation of chemical bonds?
8/12/2019 Theory of Chemical Bonding
4/36
Theories of chemical bonding 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
formnew set of orbitalsthis method is called hybridization.
During the lecture, these hybridized orbitals will be explained:
sp 2sp hybrid orbitals from mixing of a s and ap orbital
sp 2 3sp2 hybrid orbitals from mixing of a s and 2p orbital
sp3 fill in you explanation please
sp3d 5 sp3d hybrid orbitals from mixing of a s and 3p and ad orbital
sp3d 2 ____________
Provide a description for hybrid orbitals sp, sp2, sp3, sp3d, and sp3d2
8/12/2019 Theory of Chemical Bonding
5/36
Theories of chemical bonding 5
The spHybrid Orbitals
The sp hybrid orbitals: formation of two sp hybrid orbitals+ + + - = + -
+ + - = - +
hybridization of s andporbitals = 2 sphybrid orbitals
_ _ __ __
__ __ __ Two sp hybrid orbitlas =>
Two states of Be
8/12/2019 Theory of Chemical Bonding
6/36
Theories of chemical bonding 6
Bonds with spHybrid Orbitals
Formations of bonds in these molecules are discussed during thelecture. Be prepared to do the same by yourself.
ClBeCl HCCH HCN : O=C=O
Double and triple bonds involve pi pbonding, and the the application
of valence bond method to pbonds will be discussed.
You are expected to be able to draw pictures to show the pbonding.
8/12/2019 Theory of Chemical Bonding
7/36
Theories of chemical bonding 7
A pBond
Overlap of 2 2porbitals forthe formation of pbond
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 - lessoverlap.
Nodal plane
Bonding of C2H4
C
2s 2p 2p2p
sp2 sp2sp22p
How are pi bonds formed?
8/12/2019 Theory of Chemical Bonding
8/36
Theories of chemical bonding 8
Triple Bonds in H-CC-H
H-C-C-H: threes
bonds due to overlapping of 1sHspC; spCspC;and spC1sH.
Two pbonds in HCCH and HCN triple bonds are due to
overlapping ofporbitals results.
Draw and describe how atomic orbitals overlap
to form all bonds in acetylene, HCCH
pyover lap
pxover lap
H H
sp hybrid orbitals
Two nodal planesof p
bonds are perpendicular
to each other.
in pbond
in pbond
C
2s2p2p2p
spsp2p2p
8/12/2019 Theory of Chemical Bonding
9/36
Theories of chemical bonding 9
Two pBonds in HCCH
A triple bond consists of a sigma and two pi bonds. Overlaps
of two sets ofporbitals form of two pbonds.
8/12/2019 Theory of Chemical Bonding
10/36
Theories of chemical bonding 10
Bonding of CO2For CO2, the C atom forms a sbond and a pbond with each of two O
atoms. The two nodal planes of the two pbonds are also perpendicular.
During the lecture, I draw diagrams and explain the two stwo pbonds
in CO2. You are expected to be able to do the same, in a test.
pyover lap in pbond
Overlap pp in sbonds
pxover lap in pbond
O=C=O or H2C=C=CH2Discuss the bonding of allene H2C=C=CH2
See extra problems B17 in the handout
Resonance structures
: OC O :
: O CO :. .
8/12/2019 Theory of Chemical Bonding
11/36
Theories of chemical bonding 11
09_174
O C O
sigma bond(1 pair of electrons) pi bond
(1 pair ofelectrons)
pi bond(1 pair ofelectrons)
(a)
O C O
Bonding in CO2another view
Compare with H2C=C=CH2
8/12/2019 Theory of Chemical Bonding
12/36
Theories of chemical bonding 12
The sp2Hybrid Orbitals
Ground state and excited stateelectronic configuration of B
_ _ _ __
_ __ __
The hybridization of a sand two
porbitals led to 3 sp2hybrid
orbitalsfor bonding.
Compounds involving sp2 hybridorbitals: BF3, CO3
2, H2CO,
H2C=CH2, NO3, etc
Nov. 25
8/12/2019 Theory of Chemical Bonding
13/36
Theories of chemical bonding 13
An example of using sp2hybrid orbitals
__ orbitals for bonding?
Dipole moment = ____?
8/12/2019 Theory of Chemical Bonding
14/36
Theories of chemical bonding 14
Bonding of H2C=CH2molecules
Utilizing the sp2hybrid orbitals, each C atom form
two HC sbonds for a total of 4 sHC bonds.
The CC sbond is common to both C atoms.
A CC pbond is formed due to overlap ofp
orbitals from each of the C atoms.
Hybrid orbitals (sp2) for
HC and CC sbond
Overlap ofporbital
for CC pbond
C
2s 2p 2p2p
sp2 sp2sp22p
8/12/2019 Theory of Chemical Bonding
15/36
Theories of chemical bonding 15
The sp3Hybridized Orbitals
Ground state and excited state
electronic configuration of C
_ _ _ _
_ _ __
The hybridization of a sand three
porbitals led to 4 sp3hybrid
orbitalsfor bonding.
Compounds involving sp3
hybridorbitals: CF4, CH4, :NH3, H2O::,
SiO44, SO4
2, ClO4, etc
8/12/2019 Theory of Chemical Bonding
16/36
Theories of chemical bonding 16
C
2s 2p 2p2p
sp3 sp3sp3sp3
8/12/2019 Theory of Chemical Bonding
17/36
Theories of chemical bonding 17
The sp3dHybrid OrbitalsHybridization of one s, threep, and a d
orbitals results in 5 sp3dhybrid
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 presentin TeCl4and BrF3?
What are their shapes?
8/12/2019 Theory of Chemical Bonding
18/36
Theories of chemical bonding 18
The sp3d2Hybrid OrbitalsHybridization of one s, threep, and
two dorbitals results in 6 sp3d2hybrid 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 ofAX3E3andAX2E4are known or recognized,
because they are predicted to have a T shape and linear shape
respectively when the lone pairs of electrons are ignored.
8/12/2019 Theory of Chemical Bonding
19/36
Theories of chemical bonding 19
Molecules with more than one central atomDescribe the structure of CH3NCO.
Draw the skeleton and add all valence electrons
H3CNCO
Which Lewis dot structure is the most important (stable)?
N = C = O
HC
H H
120o109o
180o
What hybridized orbitals are used for bonding in N and C? Why are the
bond angles as indicated? No of sand pbonds = __, __?
Give formal charges to all atoms in all structures.
Take a new look at slide
22 in Bonding Basics
Which
structure is
more stable,
and why?
NCO
HC
H H
NCO
HC
H H
8/12/2019 Theory of Chemical Bonding
20/36
Theories of chemical bonding 20
Why Molecular Orbital (MO) TheoryLewis dotand valence bondtheories do not always give satisfactory
account for various properties of molecules.
For example, the dot and VB theory does not explain the fact that O2is
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-
atomic-orbitals (LCAO) is usually used.
There are other reasons, but its human nature to theorize. The theory
is beautiful, and worth learning or teaching.
8/12/2019 Theory of Chemical Bonding
21/36
Theories of chemical bonding 21
The Molecular Orbital (MO) Theory
The two atoms in the H2molecule may be
represented by A and B. Their s orbitals 1sA
and 1sBrespectively, are used for two MOs:
s* = 1sA1sB
s= 1sA+ 1sB
The energy levels of these AO and MO are
represented by the diagram here, with the math
hidden.
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
1sA 1sB
s
MO
s*
AO AO
8/12/2019 Theory of Chemical Bonding
22/36
Theories of chemical bonding 22
MO for H2type molecules: H2+, H2, H2
, He2+
Generalize the technique of LCAO
8/12/2019 Theory of Chemical Bonding
23/36
Theories of chemical bonding 23
Electronic configuration, s2, for H2molecules
Generalize the technique of LCAO
8/12/2019 Theory of Chemical Bonding
24/36
Theories of chemical bonding 24
Electronic Configuration of H2-type Molecules
From the previous theory, we can fill the M Os with electrons for theH2-type molecule:
Molecule e-configuration Bond order bondlength
H2+ 1s(1s1) 106 pm
H2, He22+ 1s2 1 74, ~75H2
, He2+ 1s2 1s* ~106, 108
H22, He2
1s2 1s*2 0 not formed
Describe the relationships of bondlength & bondorderand e-configurations; learn to reason
8/12/2019 Theory of Chemical Bonding
25/36
Theories of chemical bonding 25
Sigma MOs Formed UsingpAOs
Sigma MOs (s2ps
2p
* ) can be formed usingpAOs, 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
8/12/2019 Theory of Chemical Bonding
26/36
Theories of chemical bonding 26
Pi pMOs frompAOs
Generalize the technique of LCAO
8/12/2019 Theory of Chemical Bonding
27/36
Theories of chemical bonding 27
MO Diagrams for O2and F2
A full diagram of the energylevel of molecular orbitals of
O2and F2is shown here.
The relative (approximate)
height of these energies willbe 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.
8/12/2019 Theory of Chemical Bonding
28/36
Theories of chemical bonding 28
The O2+, O2, O2
, & F2+, F2, F2
Molecules
O=O
Paramagnetic ,
bond length
indicates double
bond, electronicconfiguration
agrees
FF
Electronic
configuration agree
with single bond.
For
O=O
_ _
See p. 457 for two MO
energy-level diagrams
8/12/2019 Theory of Chemical Bonding
29/36
Theories of chemical bonding 29
MO Energy Level Diagram for Be2N2Due to close energy levels of 2sand 2p, the MO energy level diagram
for Be2to N2differs from those of O2to F2. Reasons and explanationare 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
A more realistic energy level diagram for
8/12/2019 Theory of Chemical Bonding
30/36
Theories of chemical bonding 30
A more realistic energy level diagram for
Be2N2involving spmixing, not in text
_ _ _
2p
_
2s
Atomic
orbital
__ s*2p__ __ *
2p
__ s2p__ __2p
__ s*2s
__ s2s
Molecular orbitals
_ _ _
2p
_
2s
Atomic
orbital
This diagram from my CaCt
website accounts for the spmixingof the AO for the bonding
consideration. This spmixing
effect is more detailed than that
required for freshman chemistry(not to be tested).
The spmixingof AO gives
stronger s2sbond and a weakers 2sbond. Thus, the split from 2sis not even. Effects on other
bonds are also shown, but
qualitatively.
8/12/2019 Theory of Chemical Bonding
31/36
Theories of chemical bonding 31
BenzeneThe benzene structure has fascinated scientists for centuries. Its bonding is
particularly interesting. The C atom utilizes sp2 hybrid AO in the sigmabonds, and the remaining p AO overlap forming a ring of p bonds.
Sigma sbonds are
represented by lines,and the porbitals for
the pbonds are shown
by balloon-shape
blobs. Note the + and
signs of the porbitals.
Thus, we represent it
by
+
+ +
+
++
Ch 120 t d t i
8/12/2019 Theory of Chemical Bonding
32/36
Theories of chemical bonding 32
More About BenzeneChem120 students may ignore
this slide.
The pand p* of C6H6are shown
here; the symmetry is also
interesting.
8/12/2019 Theory of Chemical Bonding
33/36
Theories of chemical bonding 33
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 electronpath for benzene and ozone are shown here. These pictures represent p
electron of the the structures formula contribute most to their structures.
O
O O
CO32& have delocalized electrons
http://chem.ufl.edu/~chm2040/cgi-bin/image_eval.cgi?Op=display&Chapter=12&File=theory&Image=c10f35.gifhttp://chem.ufl.edu/~chm2040/cgi-bin/image_eval.cgi?Op=display&Chapter=12&File=theory&Image=c10f35.gif8/12/2019 Theory of Chemical Bonding
34/36
Theories of chemical bonding 34
Joy on structure of benzene a storyKathleen 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.
8/12/2019 Theory of Chemical Bonding
35/36
Theories of chemical bonding 35
MO for Heteronuclear Diatomic Molecules
For heteronuclear
diatomic molecules,
the atomic orbitals
are at different
energy levels. Thus,
the MO shifts aredifferent from those
of homonuclear
diatomic molecules.
The interactions ofAO for MO for HF,
LiF are similar, and
explained in lectures.
Chem120 students may ignore this slide.
8/12/2019 Theory of Chemical Bonding
36/36
Recommended