Molecular structure and bonding

How can we describe the bonding between atoms forming molecules ? We start from valence bond theory and come to molecular orbital theory.

1. Molecular Structure and Bonding
Dr.Christoph
Phayao University June 2014

3. Part 1
What is a chemical bond ?

4. Ionic Bond
Normally between a metal and a non-metal:

They exchange electrons and become ions
(charged atoms) which attract each other by
electrostatic force.
A pair of ions does not stay alone but form crystals

5. Covalent Bond
Two non-metals share
(valence) electrons:
(Remark: Transition metals can form covalent bonds also !)

6. Polar Covalent Bond
Two non-metals share electrons unevenly because
of electronegativity difference.
Electrons are closer to one atom than the other.
This results on partially negative and positive charges on the atoms

7. Metallic Bond
Metal atoms share all
their valence electrons,
which freely move
between all atoms which
form a network.
Therefore all metals can conduct electricity and look shiny

8. Bond Polarity

9. Polar Bonds
Uneven sharing of electrons due to
differences in Electronegativity
The “pull”
an atom has
for electrons

11. Electronegativity Trends

12. Common Electronegativites
Highest value,
set to 4

14. Polar Molecules
Electrons are not equally shared in a bond,
which can lead to a dipolmoment of the whole molecule

15. Polar Bonds and Geometry

16. Which bond type ?
(exception:
Transition
metals !)

17. Electron counting

18. Formal Charge

Split all bonds in the middle
=> “real” charge on atoms
(2) Octet Rule

Count all bonding electrons for
one atom
=> 8 is most stable
(3) Oxidation Number

Give all bonding electrons to
the more electronegative atom

19. Special Cases
“Extended
octet”

Especially P and
S can use d-
orbitals to make
more than 3
resp. 2 bonds !
6 VE:

Especially
common for
B and Al !

20. Part 2: Valence Bond Theory
(VB)
“Valence Electrons are located in
bonds and lone pairs”

22. Sigma bonds

23. Pi Bonds

24. “Resonance”
Write the resonance formula for OZONE !
Does the molecule have a charge ?

26. Important exception: Carbon Monoxide !

27. Homework (
2
)

Draw Lewis structure(s) and find
formal charges (all atoms)
and hybridization (central atom) in:

1.NO3 (-)
2.PO4 (3-)
3.CH3 Cl
4.CH2Cl2
5.SO2
6.SO3
7.CO3 (2-)
8.H2O2
9.N2O
10.Cl O2

28. ***** Break *****

29. Prentice Hall © 2003 Chapter 9

•Atomic orbitals can mix or hybridize in order to adopt an
appropriate geometry for bonding.
•Hybridization is determined by the electron domain
geometry.
sp Hybrid Orbitals
•Consider the BeF
2 molecule (experimentally known to
exist):
Hybrid Orbitals

30. Figure 11.2 The sp hybrid orbitals in gaseous BeCl
2.
atomic
orbitals
hybrid
orbitals
orbital box diagrams

31. Figure 11.2 The sp hybrid orbitals in gaseous BeCl
2
(continued).
orbital box diagrams with orbital contours

33. Figure 11.3 The sp
2
hybrid orbitals in BF
3.

34. sp
2
and sp
3
Hybrid Orbitals

35. Figure 11.4 The sp
3
hybrid orbitals in CH
4.

36. Figure 11.5 The sp
3
hybrid orbitals in NH
3.

37. Figure 11.5 continued The sp
3
hybrid orbitals in H
2O.

38. Including d-orbitals
3d orbitals can be filled as well
=> Al acts as Lewis acid
=> P and S have “hypervalence”

39. Figure 11.6 The sp
3
d hybrid orbitals in PCl
5.

40. Figure 11.7 The sp
3
d
2
hybrid orbitals in SF
6.

41. Acid or Base ?
Compare AlCl
3 and PCl
3 ?
Which acts as acid and which as base – and why ?

Why is FeCl
3 a strong Lewis acid ?

43. SOLUTION:
PROBLEM: Describe the types of bonds and orbitals in acetone, (CH
3)
2CO.
PLAN: Use the Lewis structures to ascertain the arrangement of groups and
shape at each central atom. Postulate the hybrid orbitals taking note of
the multiple bonds and their orbital overlaps. H
3C
C
CH
3
O
sp
3
hybridized
sp
3
hybridized C
C
C
O
H
H
HH
H
H
sp
2
hybridized
bonds
bond C
C
C
O
sp
3
sp
3
sp
3
sp
3
sp
3
sp
3
sp
3
sp
3
sp
2
sp
2
sp
2
sp
2
sp
2
sp
2
H
H
H
H
H
H

44. Tasks
•Draw the Lewis Structures and the Hybrid
Orbitals for Ethane, Ethene and Ethyne
(mark the hybrid orbitals)
•Which hybridization has the central atom in:
H
2O, O
2, NH
3, NH
4+, N in pyridine, O in THF,
S in SOCl
2, C in HCHO compared to CO

45. Chemical Reactivity
From the hybrid orbitals we can estimate if a
molecule acts as Lewis acid or base
(if there is an electrophilic or nucleophilic center)

Consider the “empty” pz orbital of C in HCHO vs. the
“filled” sp orbital of C in CO
-> in the first case, it acts as Lewis acid, in the second
as base !

46. ***** Break *****

47. VSEPR

48. VSEPR Theory
Clip:
http://www.youtube.com/watch?v=nxebQZUVvTg

49. MO Theory

50. The Central Themes of MO
Theory
A molecule is viewed on a quantum mechanical level as a collection of nuclei
surrounded by delocalized molecular orbitals.
Atomic wave functions are summed to obtain molecular wave functions.
If wave functions reinforce each other, a bonding MO is formed (region of
high electron density exists between the nuclei).
If wave functions cancel each other, an antibonding MO is formed (a node of
zero electron density occurs between the nuclei).

51. Amplitudes of wave
functions added
Figure 11.14
An analogy between light waves and atomic wave functions.
Amplitudes of
wave functions
subtracted.

52. Prentice Hall © 2003 Chapter 9
Molecular Orbitals
•Molecular orbitals:
•each contain a maximum of two electrons
•have definite energies
•can be visualized with contour diagrams
•are distributed over the whole molecule
(not only in between 2 atoms)

•When two AOs overlap, two MOs form.

53. Prentice Hall © 2003 Chapter 9
Molecular Orbitals
The Hydrogen Molecule

54. Prentice Hall © 2003 Chapter 9
Figure 11.15 The MO diagram for H
2.
Energy

MO
of H
2
*
1s

1s
AO
of H
1s
AO
of H
1s
H
2 bond order
= 1/2(2-0) = 1
Filling molecular orbitals with electrons follows the
same concept as filling atomic orbitals.

56. Prentice Hall © 2003 Chapter 9
Electron Configurations and Molecular
Properties
•Two types of magnetic behavior:
•paramagnetism (unpaired electrons in molecule):
strong attraction between magnetic field and
molecule;
•diamagnetism (no unpaired electrons in molecule):
weak repulsion between magnetic field and
molecule.
•Magnetic behavior is detected by determining the mass
of a sample in the presence and absence of magnetic
field:

57. Diatomic molecules

58. The energy level
is the lower, the
higher the EN of
the atom is !

60. Naming of MO’s: example O
2 molecule
“g” = symmetric to C axis
“u” = anti-symmetric

61. Diatomic molecules
Consider the EN of each atom – the higher the EN,
the lower is the energy of the orbitals !
The highest filled MO is called “HOMO”, the lowest
unoccupied MO “LUMO”

-> check example CO
http://firstyear.chem.usyd.edu.au/calculators/
mo_diagrams.shtml

62. Example CO
HOMO
LUMO
“lone
pair” on C

63. Chemical Reactivity
Important are the HOMO and LUMO (“frontier orbitals”)
http://www.meta-synthesis.com/webbook/12_lab/lab.html

64. Group
Orbitals

65. Construction of Group Orbitals – example H
2O

66. Interaction 1: in-phase H orbitals

67. Interaction 2: out-of-phase H orbitals

68. Indicate different MO types: (bonding, non-bonding. anti-bonding)

69. Combination of 3 H
orbitals to
3 group orbitals
BH
3 molecule

70. Compare HOMO/LUMO to BH3 !
=> what is an acid / base ?

71. Homework (3)
Number Molecule
1 CN
2 CN(-)
3 BC
4 BN
5 BO
6 BF
7 CF
8 NO
9 NO (+)
10 NO (-)
Number Molecule
11 NF
12 OF
13 CH
4
14 BH
3
15 SbF
6
16 XeF
2
17 XeF
4
18 XeF
6
http://firstyear.chem.usyd.edu.au/calculators/mo_diagrams.shtml