1. Theories of Bonding and Structure CHAPTER 10 Chemistry: The Molecular Nature of Matter, 6 th edition By Jesperson, Brady, & Hyslop

2. CHAPTER 10: Bonding & Structure 2 Learning Objectives  VESPR theory:  Determine molecular geometry based on molecular formula and/or lewis dot structures.  Effect of bonded atoms & non-bonded electrons on geometry  Molecular polarity & overall dipole moment  Assess overall dipole moment of a molecule  Identify polar and non-polar molecules  Valence Bond Theory  Hybridized orbitals  Multiple bonds  Sigma vs pi orbitals  Molecular Orbital Theory  Draw & label molecular orbital energy diagrams  Bonding & antibonding orbitals  Predict relative stability of molecules based on MO diagrams

3. Molecular Geometry Basic Molecular Geometries Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 3 Linear 3 atoms Trigonal Planar or Planar Triangular 4 atoms Tetrahedral: 5 atoms Trigonal Bipyramidal 6 atoms Octahedral: 7 atoms

4. VESPR Definition Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E http://chemistry-desk.blogspot.com/2011/05/prediction-of-shape-of-molecules-by.html 4 Valence Shell Electron Pair Repulsion Model Electron pairs (or groups of electron pairs) in the valence shell of an atom repel each other and will position themselves so that they are far apart as possible, thereby minimizing the repulsions. Electron pairs can either be lone pairs or bonding pairs. Tetrahedral arrangement of electron pairs Bent geometry

5. VESPR Definition Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 5 Valence Shell Electron Pair Repulsion Model Electron pairs (or groups of electron pairs) in the valence shell of an atom repel each other and will position themselves so that they are far apart as possible, thereby minimizing the repulsions. Text uses “electron domain” to describe electron pairs: Bonding domain: contains electrons that are shared between two atoms. So electrons involved in single, double, or triple are part of the same bonding domain. Nonbonding Domain: Valence electrons associated with one atom, such as a lone pair, or a unpaired electron.

6. VESPR Basic Examples Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 6 2 bonding domains 3 bonding domains 4 bonding domains 5 bonding domains 6 bonding bonding domains

7. VESPR When Lone Pairs or Multiple Bonds Present Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 7 Including lone pairs: Take up more space around central atom Effect overall geometry Counted as nonbonded electron domains Including multiple bonds (double and triple) For purposes of determining geometry focus on the number of atoms bonded together rather then the number of bonds in between them: ie, treat like a single bond. Treat as single electron bonding domain

8. VESPR Electrons that are Bonding & Not Bonding Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 8 Bonding Electrons – More oval in shape – Electron density focused between two positive nuclei. Nonbonding Electrons – More bell or balloon shaped – Take up more space – Electron density only has positive nuclei at one end

9. VESPR 3 atoms or lone pairs Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 9 Number of Bonding Domains 3 2 Number of Nonbonding Domains 0 1 Molecular Shape Planar Triangular (e.g. BCl 3 ) All bond angles 120  Nonlinear Bent or V-shaped (e.g. SnCl 2) Bond <120  Structure

10. VESPR 4 atoms or lone pairs Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 10 Number of Bonding Domains 4 3 2 Number of Nonbonding Domains 0 1 2 Molecular Shape Tetrahedron (e.g. CH 4 ) All bond angles 109.5  Trigonal pyramidal (e.g. NH 3 ) Bond angle less than 109.5  Nonlinear, bent (e.g. H 2 O) Bond angle less than109.5  Structure

11. VESPR 5 atoms or lone pairs Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 11 90  120  Trigonal Bipyramidal Two atoms in axial position – 90  to atoms in equatorial plane Three atoms in equatorial position – 120  bond angle to atoms in axial position – More room here – Substitute here first

12. VESPR 5 atoms or lone pairs Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 12 Number of Bonding Domains 5 4 Number of Nonbonding Domains 0 1 Molecular Shape Trigonal bipyramid (e.g. PF 5 ) Ax-eq bond angles 90  Eq-eq 120  Distorted Tetrahedron, or Seesaw (e.g. SF 4 ) Ax-eq bond angles < 90  Structure

13. 5 atoms or lone pairs Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 13 Lone pair takes up more space Goes in equatorial plane Pushes bonding pairs out of way Result: distorted tetrahedron VESPR

14. 5 atoms or lone pairs Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 14 Number of Bonding Domains 3 2 Number of Nonbonding Domains 2 3 Molecular Shape T-shape (e.g. ClF 3 ) Bond angles 90  Linear (e.g. I 3 – ) Bond angles 180  Structure

15. VESPR 6 atoms or lone pairs Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 15 Number of Bonding Domains 6 5 Molecular Shape Octahedron (e.g. SF 6 ) Square Pyramid (e.g. BrF 5 ) StructureNumber of Nonbonding Domains 0 1

16. VESPR 6 atoms or lone pairs Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 16 Number of Bonding Domains 4 Number of Nonbonding Domains 2 Molecular Shape Square planar (e.g. XeF 4 ) Structure

17. VESPR Determining 3-D Structures Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 17 1. Draw Lewis Structure of Molecule – Don't need to compute formal charge – If several resonance structures exist, pick only one 2. Count electron pair domains – Lone pairs and bond pairs around central atom – Multiple bonds count as one set (or one effective pair) 3. Arrange electron pair domains to minimize repulsions Lone pairs – Require more space than bonding pairs – May slightly distort bond angles from those predicted. – In trigonal bipyramid lone pairs are equatorial – In octahedron lone pairs are axial 4.Name molecular structure by position of atoms—only bonding electrons

18. Molecular Polarity Polar Molecules Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 18 Have net dipole moment – Negative end – Positive end Polar molecules attract each other. – Positive end of polar molecule attracted to negative end of next molecule. – Strength of this attraction depends on molecule's dipole moment – Dipole moment can be determined experimentally Polarity of molecule can be predicted by taking vector sum of bond dipoles Bond dipoles are usually shown as crossed arrows, where arrowhead indicates negative end

19. Molecular Polarity Molecular Shape & Polarity Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E http://wps.prenhall.com/wps/media/objects/3081/3155729/blb0903.html 19 Many physical properties (melting and boiling points) affected by molecular polarity For molecule to be polar: – Must have polar bonds Many molecules with polar bonds are nonpolar -Possible because certain arrangements of bond dipoles cancel -For molecules with more than two atoms, must consider the combined effects of all polar bonds

20. Molecular Polarity Symmetrical Nonpolar Molecules Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 20 Symmetrical molecules – Nonpolar because bond dipoles cancel All five shapes are symmetrical when all domains attached to them are composed of identical atoms

21. Molecular Polarity Symmetrical Nonpolar Molecules Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 21 Cancellation of Bond Dipoles In Symmetrical Trigonal Bipyramidal and Octahedral Molecules All electron pairs around central atom are bonding pairs and All terminal groups (atoms) are same The individual bond dipoles cancel

22. Molecular Polarity Polar Molecules Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 22 Molecule is usually polar if – All atoms attached to central atom are NOT same Or, – There are one or more lone pairs on central atom

23. Molecular Polarity Polar Molecules Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 23  Water and ammonia both have non-bonding domains  Bond dipoles do not cancel  Molecules are polar

24. Molecular Polarity Polar Molecules: Exception Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 24 Exception to these general rules for identifying polar molecules: Nonbonding domains (lone pairs) are symmetrically placed around central atom

25. 25 Problem Set A 1.For the following molecules: a.Draw a lewis dot structure. b.Determine the molecular geometry at each central atom. c.Identify the bond angles. d.Identify all polar bonds: δ+ / δ- e.Assess the polarity of the molecule & indicate the overall dipole moment if one exists AsF 5 AsF 3 SeO 2 GaH 3 ICl2 - SiO 4 - 4 TeF 6

26. VB Theory Review: Modern Atomic Theory of Bonding Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 26 Modern Atomic Theory of Bonding is based on wave mechanics and gave us: – Electrons and shapes of orbitals – Four quantum numbers – Heisenberg uncertainty principle Electron probabilities – Pauli Exclusion Principle

27. VB Theory Valence Bond Theory & Molecular Orbital Theory Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 27 Valence Bond Theory Individual atoms, each have their own orbitals and orbitals overlap to form bonds Extent of overlap of atomic orbitals is related to bond strength Molecular Orbital Theory Views molecule as collection of positively charged nuclei having a set of molecular orbitals that are filled with electrons (similar to filling atomic orbitals with electrons) Doesn't worry about how atoms come together to form molecule

28. VB Theory Valence Bond Theory & Molecular Orbital Theory Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 28 Both Theories: Try to explain structure of molecules, strengths of chemical bonds, bond orders, etc. Can be extended and refined and often give same results Valence Bond Theory Bond between two atoms formed when pair of electrons with paired (opposite) spins is shared by two overlapping atomic orbitals

29. VB Theory H2H2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 29 H 2 bonds form because 1s atomic valence orbital from each H atom overlaps

30. VB Theory F2F2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 30 F 2 bonds form because atomic valence orbitals overlap Here 2p overlaps with 2p Same for all halogens, but different np orbitals

31. VB Theory HF Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 31 HF involves overlaps between 1s orbital on H and 2p orbital of F 1s1s2p2p

32. VB Theory H2SH2S Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 32  Predicted 90˚ bond angle is very close to experimental value of 92˚. Assume that unpaired electrons in S and H are free to form paired bond We may assume that H—S bond forms between s and p orbital

33. VB Theory Need to Change Approach to Explain Bonding in CH 4 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 33 Example: CH 4 C 1s 2 2s 2 2p 2 and H 1s 1 In methane, CH 4 – All four bonds are the same – Bond angles are all 109.5° Carbon atoms have – All paired electrons except two unpaired 2p – p orbitals are 90° apart – Atomic orbitals predict CH 2 with 90° angles

34. VB Theory Hybridization Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 34 Mixing of atomic orbitals to allow formation of bonds that have realistic bond angles. – Realistic description of bonds often requires combining or blending two or more atomic orbitals Hybridization just rearranging of electron probabilities Why do it? To get maximum possible overlap Best (strongest) bond formed

35. VB Theory Hybrid Orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 35 Blended orbitals result from hybridization process Hybrid orbitals have – New shapes – New directional properties – Each hybrid orbital combines properties of parent atomic orbitals

36. VB Theory Hybrid Orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 36 Symbols for hybrid orbitals combine the symbols of the orbitals used to form them – Use s + p form two sp hybrid orbitals – Use s + p + p form three sp 2 hybrid orbitals One atomic orbital is used for each hybrid orbital formed Sum of exponents in hybrid orbital notation must add up to number of atomic orbitals used

37. VB Theory Hybrid Orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 37  Mixing or hybridizing s and p orbital of same atom results in two sp hybrid orbitals  Two sp hybrid orbitals point in opposite directions

38. VB Theory Ex: sp Hybridized Orbitals: BeH 2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 38 Now have two sp hybrid orbitals Oriented in correct direction for bonding 180  bond angles – As VSEPR predicts and – Experiment verifies Bonding = – Overlap of H 1s atomic orbitals with sp hybrid orbitals on Be

39. VB Theory Hybrid Orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 39 HybridAtomic Orbitals UsedElectron Geometry sps + pLinear Bond angles 180° sp 2 s + p + pTrigonal planar Bond angles 120° sp 3 s + p + p + pTetrahedral Bond angles 109.5° sp 3 ds + p + p + p + dTrigonal Bipyramidal Bond angles 90° and 120° sp 3 d 2 s + p + p + p + d + dOctahedral Bond angles 90°

40. VB Theory Bonding in BCl 3 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 40 Overlap of each half- filled 3p orbital on Cl with each half-filled sp 2 hybrid on B  Forms three equivalent bonds  Trigonal planar shape  120  bond angle

41. VB Theory Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 41  Overlap of each half- filled 1s orbital on H with each half- filled sp 3 hybrid on carbon  Forms four equivalent bonds  Tetrahedral geometry  109.5  bond angle Bonding in CH 4

42. VB Theory Hybrid sp Orbitals 42 Two sp hybrids Three sp 2 hybrids Four sp 3 hybrids Linear Planar Triangular Tetrahedral All angles 120  All angles 109.5  Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

43. VB Theory Expanded Octet Hybridization Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 43 Hybridization When Central Atom has More Than Octet If there are more than four equivalent bonds on central atom, then must add d orbitals to make hybrid orbitals Why? One s and three p orbitals means that four equivalent orbitals is the most you can get using s and p orbitals alone So, only atoms in third row of the periodic table and below can exceed their octet These are the only atoms that have empty d orbitals of same n level as s and p that can be used to form hybrid orbitals One d orbital is added for each pair of electrons in excess of standard octet

44. VB Theory Expanded Octet Hybridization Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 44

45. VB Theory Hybridization in Molecules with Lone Pairs Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 45 CH 4 sp 3 tetrahedral geometry109.5° bond angle NH 3 107° bond angle H 2 O104.5° bond angle Angles suggest that NH 3 and H 2 O both use sp 3 hybrid orbitals in bonding Not all hybrid orbitals used for bonding e – – Lone pairs can occupy hybrid orbitals Lone pairs must always be counted to determine geometry

46. VB Theory Ex: H 2 O Hybridization Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 46

47. VB Theory Multiple Bonds Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 47 So where do extra electron pairs in multiple bonds go? – Not in hybrid orbitals – Remember VSEPR, multiple bonds have no effect on geometry Why don’t they effect geometry? Two types of bond result from orbital overlap Sigma (  ) bond – Accounts for first bond Pi (  ) bond – Accounts for second and third bonds

48. VB Theory Sigma (  ) Bonds Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 48 Head on overlap of orbitals Concentrate electron density concentrated most heavily between nuclei of two atoms Lie along imaginary line joining their nuclei s + s p + p sp + sp

49. VB Theory Pi (  ) Bonds Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 49 Sideways overlap of unhybridized p orbitals Electron density divided into two regions – Lie on opposite sides of imaginary line connecting two atoms Electron density above and below  bond.  No electron density along  bond axis   bond consists of both regions  Both regions = one  bond

50. VB Theory Pi (  ) Bonds Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 50 Can never occur alone – Must have  bond Can form from unhybridized p orbitals on adjacent atoms after forming  bonds  bonds allow atoms to form double and triple bonds

51. VB Theory Multiple Bonds Ex: Ethene (C 2 H 4 ) Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 51 Each carbon is – sp 2 hybridized (violet) – has one unhybridized p orbital (red) C=C double bond is – one  bond (sp 2 – sp 2 ) – one  bond (p – p) p—p overlap forms a C— C  bond

52. VB Theory Conformations Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 52 C—C single bond has free rotation around the C—C bond Conformations – Different relative orientations on molecule upon rotation

53. VB Theory Conformations Ex: Pentane, C 5 H 12 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 53

54. VB Theory Properties of  -Bonds Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 54 Can’t rotate about double bond  bond must first be broken before rotation can occur

55. Ex: Bonding in Formaldehyde Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 55 C and O each – sp 2 hybridized (violet) – Has one unhybridized p orbital (red)  C=O double bond is  one  bond (sp 2 – sp 2 )  one  bond (p – p) Unshared pairs of electrons on oxygen in sp 2 orbitals sp 2 —sp 2 overlap to form C—O  bond VB Theory

56. Bonding in Ethyne, C 2 H 2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 56  Each carbon  is sp hybridized (violet)  Has two unhybridized p orbitals, p x and p y (red)  C  C triple bond  one  bond  sp – sp  two  bonds  p x – p x  p y – p y VB Theory

57. Ex: Bonding in N 2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 57  Each nitrogen  sp hybridized (violet)  Has two unhybridized p orbitals, p x and p y (red) N  N triple bond  one  bond  sp – sp  two  bonds  p x – p x  p y – p y

58. 58 Problem Set B 2.What is the hybridization of oxygen in OCl 2 ? 3.For the species and XeF 4 O, determine the following: a.electron domain geometry (geometry including non-bonding pairs) b.molecular geometry c.Hybridization around central atom d.Polarity 4.How many  and  bonds are there in CH 2 CHCHCH 2, and what is the hybridization around the carbon atoms? 5.Draw & list the bonding orbitals for HCN.

59. 59 Problem Set B 2.sp 3 3.XeF 4 O: octahedral, square pyramid, sp 3 d 2, polar 3.9, 2, sp 2 3.HCN: C will be create a σ bond to H and N with sp 2 hybridized orbitals and use 2 p orbitals to participate in 2 π bonds with N. N will participate in the σ bond with C with an sp 2 hybridized orbital, the other will hold the N lone pair, and then N will use 2 p orbitals to π bond with C.

60. MO Theory Molecular Orbital Theory Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 60 1.Molecular orbitals are associated with entire molecule as opposed to one atom 2.Allows us to accurately predict magnetic properties of molecules 3.Energies of molecular orbitals determined by combining electron waves of atomic orbitals Molecular Orbital Theory Views molecule as collection of positively charged nuclei having a set of molecular orbitals that are filled with electrons (similar to filling atomic orbitals with electrons) Doesn't worry about how atoms come together to form molecule

61. MO Theory  Bonding Molecular Orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 61 Come from various combinations of atomic orbital wave functions For H 2, two 1s wave functions, one from each atom, combine to make two molecular orbital wave functions 1s A + 1s B Combined  Bonding MO  Constructive interference of waves  Energy of bonding MO lower than atomic orbitals

62. MO Theory  * Antibonding Molecular Orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 62 Destructive interference of the 1s waves Energy of the bonding molecular orbital is higher than energy of parent atomic orbitals Number of atomic orbitals used must equal number of molecular orbitals Other possible combination of two 1s orbitals: 1s A – 1s B

63. Summary of MO from 1s Atomic Orbital Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 63 Bonding molecular orbital – Electron density builds up between nuclei – Electrons in bonding MOs tend to stabilize molecule Antibonding molecular orbital – Cancellation of electron waves reduces electron density between nuclei – Electrons in antibonding MOs tend to destabilize molecule MO Theory

64. MO Diagram for H 2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 64

65. MO Theory Rules for Filling in MO Energy Diagrams Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 65 1.Electrons fill lowest-energy orbitals that are available – Aufbau principle applies 2.No more than two electrons, with spin paired, can occupy any orbital – Pauli exclusion principle applies 3.Electrons spread out as much as possible, with spins unpaired, over orbitals of same energy – Hund’s rules apply

66. MO Theory Bond Order Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 66 Measure of number of electron pairs shared between two atoms H 2 bond order = 1 A bond order of 1 corresponds to a single bond

67. MO Theory MO Diagram for He 2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 67 Four electrons, so both  and  * molecular orbitals are filled Bond order There is no net bonding He 2 does not form

68. 2p Molecular Orbitals Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 68 MO Theory

69. 2 nd Row Periodic Table MO Diagrams Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 69 Li 2  N 2  2p Lower in energy than  2p O 2, F 2 and Higher  2p Lower in energy than  2p Can ignore filled 1s bonding & antibonding and focus on valence electrons

70. MO Theory MO Diagram for Li 2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 70  2p Lower in Energy than  2p Li Li 2 Diamagnetic as no unpaired spins Bond order = (2 – 0)/2 = 1

71. MO Theory MO Energy Diagram for F 2 Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 71 F electron configuration = [He]2s 2 2p 5 Bond order = (8 – 6)/2 = 1 F – F single bond  stable molecule Diamagnetic as no unpaired spins FF F2F2  2p Lower in Energy than  2p

72. MO Theory Heteronuclear Diatomic Molecules Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 72 If Li through N  2p below  2p If O, F and higher atomic number, then  2p below  2p Example – BC both are to left of N so  2p below  2p – OF both are to right of N so  2p below  2p – What about NF? Each one away from O so average is O and  2p below  2p

73. MO Theory B-C and N-F Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 73  2p lower  2p lower BCNF Number of valence e  = 3 + 4 = 7 Number of valence e  = 5 + 7 = 12 Bond Order = (5 – 2)/2 = 1.5 Bond Order = (8 – 4)/2 = 2

74. MO Theory N-O Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 74 Bond Order for NO tricky N predicts  2p lower O predicts  2p lower Have to look at experiment Shows that  2p is lower  2p lower Number of valence e  = 5 + 6 = 11 Bond Order = (8 – 3)/2 = 2.5

75. MO Theory N-O + and N-O – Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 75 NO + NO  Same diagram Different number of e – NO + has 11 – 1 = 10 valence e  Bond order = (8 – 2)/2 = 3 NO  has 11 + 1 = 12 valence e  Bond order = (8 – 4)/2 = 2

76. MO Theory Relative Stability of N-O, N-O + and N-O – Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 76 Recall that as bond order increases, bond length decreases, and bond energy increases Molecule or ion Bond Order Bond Length (pm) Bond Energy (kJ/mol) NO + 31061025 NO2.5115630 NO  2130400  So NO + is most stable form  Highest bond order, shortest and strongest bond

77. 77 Problem Set C 6.What is the MO Energy Diagram for B 2 ? How many unpaired electrons does B 2 have? 7.What is the bond order & number of unpaired electrons in 8.Draw the MO Energy Diagram for BN.

78. Bonding VB vs MO Theory Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 78 Neither VB or MO theory is entirely correct – Neither explains all aspects of bonding – Each has its strengths and weaknesses MO theory correctly predicts unpaired electrons in O 2 while Lewis structures do not MO theory is a difficult because even simple molecules have complex energy level diagrams MO theory is a difficult because molecules with three or more atoms require extensive calculations

79. Bonding VB vs MO Theory Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 79 Successes of VB Theory Based on simple Lewis structures and related geometric figures Three dimensional structures based on electron domains without massive calculations Simple hybrid orbitals invoked where experimental evidence shows the need Integer bond orders are often correct Successes of MO Theory MO theory is particularly successful in explaining paramagnetism of B 2 and O 2 – One electron each in  2px and  2py (for B 2 ) – One electron each in  * 2px and  * 2py (for O 2 )

80. Resonance VB Theory Treatment of Resonance Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 80 Formate anion, HCOO – C has three electron domains (all bonding pairs) so – sp 2 hybridized; trigonal planar Each O has three electron domains (one bonding pair and two lone pairs) – so sp 2 hybridized; trigonal planar

81. Resonance VB Theory Treatment of Resonance Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 81 Have two resonance structures Have lone pair on each O atom in unhybridized p orbitals as well as empty p orbital on C Lewis theory says – Lone pair on one O – Use lone pair of other O to form  (pi) bond – Must have two Lewis structures

82. Resonance MO Theory Treatment of Resonance Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 82 Bonding MO delocalized over all three atoms  This is also our resonance hybrid picture  This is the best view of what actually occurs and can be obtained from both VB and MO theory

83. Resonance MO / VB Theory Treatment of Resonance: Benzene Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 83 Six C atoms, each sp 2 hybridized (3  bonds) Each C also have one unhybridized p orbital (6 total) So six  MOs, 3 bonding and three antibonding So three  bonds

84. Resonance MO / VB Theory Treatment of Resonance: Benzene Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E 84 Can write benzene as two resonance structures But actual structure is composite of these two Electrons are delocalized Have three pairs of electrons delocalized over six C atoms Extra stability is resonance energy Functionally, resonance and delocalization energy are the same thing