1. 18.2 Valence bond theory: hybridized orbitals and polyatomic molecules
Lecture 16 C1403 October 31, 2005
18.1 Molecular orbital theory: molecular orbitals and diatomic molecules
18.2 Valence bond theory: hybridized orbitals and polyatomic molecules
Bond order, bond lengths, connections of MO theory and VB theory with Lewis structures

2. + _

3. Making of a z and z* orbital from overlap of two 2pz orbitals
Making of a x and x* orbital from overlap of two 2px orbitals
Making of a y and y* orbital from overlap of two 2py orbitals

4. Distance dependence of the energy of a  and * orbital
Potential energy curves for the and * orbitals of a diatomic molecule
Distance dependence of the energy of a  and * orbital

5. The reason for the “switch” in the s and p MOs
Larger gap between 2s and 2p with increasing Z
Switch

6. Bond order?
O2 Bond length = 1.21Å
O2+ Bond length = 1.12 Å
O2- Bond length = 1.26 Å
O22- Bond length = 1.49 Å

7. Compare the Lewis and MO structures of diatomic molecules

8. What is the bond order of NO in Lewis terms and MO theory?

9. 18.2 Polyatomic molecules
Valence bond versus molecular orbital theory
Hybridization of atomic orbitals to form molecular orbitals
sp, sp2 and sp3 hybridized orbitals
Hybridized orbitals and Lewis structures and molecular geometries
Double bonds and triple bonds

10. 18.2 Bonding in Methane and Orbital Hybridization

11. Structure of Methane
tetrahedral
bond angles = 109.5°
bond distances = 110 pm
but structure seems inconsistent with electron configuration of carbon

12. Electron configuration of carbon
only two unpaired electrons
should form s bonds to only two hydrogen atoms
bonds should be at right angles to one another 2p 2s

13. sp3 Orbital Hybridization
Promote an electron from the 2s to the 2p orbital 2s

14. sp3 Orbital Hybridization

15. sp3 Orbital Hybridization
Mix together (hybridize) the 2s orbital and the three 2p orbitals 2s

16. sp3 Orbital Hybridization
4 equivalent half-filled orbitals are consistent with four bonds and tetrahedral geometry 2s

17. Shapes of orbitals p s

18. Nodal properties of orbitals+ – + s

19. Shape of sp3 hybrid orbitals+ –
take the s orbital and place it on top of the p orbital + s

20. Shape of sp3 hybrid orbitals+
s + p + –
reinforcement of electron wave in regions where sign is the same
destructive interference in regions of opposite sign

21. Shape of sp3 hybrid orbitals–
sp hybrid +
orbital shown is sp hybrid
analogous procedure using three s orbitals and one p orbital gives sp3 hybrid
shape of sp3 hybrid is similar

22. Shape of sp3 hybrid orbitals+ –
sp hybrid
hybrid orbital is not symmetrical
higher probability of finding an electron on one side of the nucleus than the other
leads to stronger bonds

23. The C—H s Bond in Methane
In-phase overlap of a half-filled 1s orbital of hydrogen with a half-filled sp3 hybrid orbital of carbon: + – +
sp3 s H C
gives a s bond. + –
H—C s H C

24. Justification for Orbital Hybridization
consistent with structure of methane
allows for formation of 4 bonds rather than 2
bonds involving sp3 hybrid orbitals are stronger than those involving s-s overlap or p-p overlap

25. 18.2 sp3 Hybridization and Bonding in Ethane

26. tetrahedral geometry at each carbon
Structure of Ethane
C2H6
CH3CH3
tetrahedral geometry at each carbon
C—H bond distance = 110 pm
C—C bond distance = 153 pm

27. The C—C s Bond in Ethane
In-phase overlap of half-filled sp3 hybrid orbital of one carbon with half-filled sp3 hybrid orbital of another.
Overlap is along internuclear axis to give a s bond.

28. The C—C s Bond in Ethane
In-phase overlap of half-filled sp3 hybrid orbital of one carbon with half-filled sp3 hybrid orbital of another.
Overlap is along internuclear axis to give a s bond.

29. 18.2 sp2 Hybridization and Bonding in Ethylene

30. Structure of Ethylene C2H4 H2C=CH2 planar bond angles: close to 120°
bond distances: C—H = 110 pm C=C = 134 pm

31. sp2 Orbital Hybridization
Promote an electron from the 2s to the 2p orbital 2s

32. sp2 Orbital Hybridization

33. sp2 Orbital Hybridization
Mix together (hybridize) the 2s orbital and two of the three 2p orbitals 2s

34. sp2 Orbital Hybridization
3 equivalent half-filled sp2 hybrid orbitals plus 1 p orbital left unhybridized 2s

35. sp2 Orbital Hybridization
2 of the 3 sp2 orbitals are involved in s bonds to hydrogens; the other is involved in a s bond to carbon
2 sp2

36. sp2 Orbital Hybridization

37. p Bonding in Ethylene p 2 sp2
the unhybridized p orbital of carbon is involved in p bonding to the other carbon p
2 sp2

38. p Bonding in Ethylene p 2 sp2
each carbon has an unhybridized 2p orbital axis of orbital is perpendicular to the plane of the s bonds

39. p Bonding in Ethylene p 2 sp2
side-by-side overlap of half-filled p orbitals gives a p bond
double bond in ethylene has a s component and a p component

40. Hybridization and methane: CH4

41. sp3 hybridization and ethylene: H2C=CH2

43. Ground state > excite one electron > mix orbitals:
one s orbital and one p orbital = two sp orbitals
Acetylene

45. Other examples of sp2 and sp hybridized carbon
Formaldehyde: H2C=O
Carbon dioxide: O=C=O

46. d2sp3 hybridization
dsp3 hybridization

47. SN = 2
SN = 3
SN = 4
SN = 5
SN = 6
Hybrid orbitals are constructed on an atom to reproduce the electronic arrangement characteristics that will yield the experimental shape of a molecule

48. BeF2: SN = 2 = sp
BF3: SN = 3 = sp2
CH4: SN = 4 = sp3
PF5: SN = 5 = sp3d
SF6: SN = 6 = sp3d2

49. Describe the bonding for ethane, ethene and acetylene in terms of overlap of hybridized orbitals
Ethylene:
Acetylene: