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Theories of Chemical Bonding

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					        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
                           Theories of chemical bonding          1
Energy of Interaction Between Two H Atoms
Energies of attraction and          Potential
repulsion as functions of            energy
distance between two H
atoms are shown here.                                +346 kJ mol –1
The minimum of the                                   antibonding
attraction force occur at
H–H bond length of 74 pm,
at which, the antibonding
                                                                 distance
orbital is +346 kJ mole–1
above 0, energy when H
                                                     –346 kJ mol –1
atoms are far apart.                                 H – H bond


                       the two-atom system?
How does energy affectTheories of chemical bonding                    2
            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
                          bonds?
the formation of chemicalTheories of chemical bonding                   3
         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
                             Theories of chemical bonding                 4
                       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

                               Theories of chemical bonding    5
        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.


                          Theories of chemical bonding                  6
                               A p Bond
  Sigma (s) bond is symmetric about axis.                      Nodal plane
  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 of 2 2p orbitals for
  overlap.
                                                        the formation of p bond
                                                                   C
                                                              2s 2p 2p 2p
                                                             sp2 sp2 sp2 2p


           Bonding of C2H4
                              Theories of chemical bonding                        7
How are pi bonds formed?
          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                                C
                                    in p bond
                                                                   2s 2p 2p 2p
           H                                            H          sp sp 2p 2p
                        px over lap
                                                          Two nodal planes of p
                             in p bond                    bonds are perpendicular
                                                          to each other.
Draw and describe how atomic orbitals overlap
                               Theories of chemical bonding                      8
to form all bonds in acetylene, H–CC–H
       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.

                    Theories of chemical bonding               9
                           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

          ..

                                                 Overlap p–p in s bonds
Discuss the bonding of allene H2C=C=CH2
                                                 O=C=O or H2C=C=CH2
                                Theories of chemical bonding                       10
 See extra problems B17 in the handout
               Bonding in CO2 – another view
09_174              sigma bond
                    (1 pair of electrons)     pi bond
                                              (1 pair of
                                              electrons)



                O                      C                   O


                    pi bond
                    (1 pair of
                    electrons)
         (a)

                Compare with H2C=C=CH2

                O             C              O
                     Theories of chemical bonding              11
         (b)
                   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

                             Theories of chemical bonding   12
Nov. 25
    An example of using sp2 hybrid orbitals




__ orbitals for bonding?
Dipole moment = ____?
                           Theories of chemical bonding   13
              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
                            Theories of chemical bonding                          14
     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

                           Theories of chemical bonding   15
                                      C
                                2s 2p 2p 2p
                               sp3 sp3 sp3 sp3




Theories of chemical bonding         16
              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?
                            Theories of chemical bonding                        17
              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.
                            Theories of chemical bonding         18
      Molecules with more than one central atom
  Describe the structure of CH3NCO.                  Take a new look at slide
                                                     22 in Bonding Basics
  Draw the skeleton and add all valence electrons
          H3C – N – C – O
  Which Lewis dot structure is the most important (stable)?

  NC–O                       180o            N–CO
                          N=C=O                               Which
H–C                109o
                          120o           H–C                  structure is
                 H–C                                          more stable,
H H                                        H H                and why?
                  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 = __, __?
                           atoms chemical bonding
Give formal charges to all Theories ofin all structures.           19
   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-
atomic-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.
                          Theories of chemical bonding                20
     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                                    1sA        1sB
The energy levels of these AO and MO are
represented by the diagram here, with the math
hidden.
                                                                 s
                           Theories of chemical bonding   AO    MO   AO
                                                                     21
MO for H2–type molecules: H2+, H2, H2–, He2+




       Generalize the technique of LCAO
                   Theories of chemical bonding   22
Electronic configuration, s2, for H2 molecules




          Generalize the technique of LCAO
                 Theories of chemical bonding   23
 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 1s*2            0                    not formed


    Describe the relationships of bondlength & bondorder
    and e-configurations; learn to reason


                          Theories of chemical bonding                  24
             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              25
Generalize the technique of LCAO
Pi p MOs from p AOs




      Theories of technique of
Generalize the chemical bonding LCAO   26
                  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–,
Theories of chemical bonding F2, F2 & Ne2.
                                   –
                                                27
       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     Theories of chemical bonding                 28
energy-level diagrams
      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
                            Theories of chemical bonding                29
energy-level diagrams
               A more realistic energy level diagram for
                 Be2 – N2 involving sp mixing, not in text
               __ s*2p
              __ __ p*2p                            This diagram from my CaCt
  ___                                               website accounts for the sp
                               ___
   2p                          2p                   mixing of the AO for the bonding
                                                    consideration. This sp mixing
               __ s2p
                                                    effect is more detailed than that
              __ __p2p                              required for freshman chemistry
                                                    (not to be tested).

               __ s*2s                              The sp mixing of AO gives
     _                                              stronger s2s bond and a weaker
                               _
    2s                         2s                   s*2s bond. Thus, the split from 2s
                                                    is not even. Effects on other
               __ s2s
                                                    bonds are also shown, but
Atomic                         Atomic               qualitatively.
orbital                        orbital
          Molecular orbitals         Theories of chemical bonding                   30
                               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               31
                                         Chem120 students may ignore
More About Benzene                       this slide.
                                         The p and p* of C6H6 are shown
                                         here; the symmetry is also
                                         interesting.




          Theories of chemical bonding                         32
  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




                           Theories of chemical bonding                33
     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.
                           Theories of chemical bonding               34
         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   35
Chem120 students may ignore this slide.
                              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.



                          Theories of chemical bonding                  36

				
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