Molecular Geometry and Chemical Bonding Theory

1. Molecular Geometry andChemical Bonding Theory Page 232 # 2 – 5 Page 235 # 10, 12 Page 238 # 18, 20 - 24

2. What is the “shape” of the molecule? • How are valence electrons in a molecule distributed among the orbitals? • What are the shapes of these orbitals? • What order are they occupied?

3. Molecular Geometry VSEPR Theory Valence Shell Electron Pair Repulsion Theory • draw Lewis electron dot structure • count the number of bonding electron pairs about central atom (double and triple bonds count as one pair for shape prediction) • count the number of lone pairs of electrons • match electron pair information to shapes

8. Shape: PCl3 .. .. .. :Cl :P : Cl: .. .. .. :Cl: .. Lewis Electron Dot Structure 3 bond pairs 1 lone pair => AB3E trigonal pyramidal shape

9. Shape: IF5 Lewis Electron Dot Structure .. .. : F : : F : .. .. .. : I : F : .. .. .. : F : :F : .. .. 5 bonds pairs 1 lone pair => AB5E Square pyramidal shaped

10. Shape: IF4-1 Lewis Electron Dot Structure .. .. -1 : F : : F : .. .. .. : I : F : .. .... : F : .. 4 bond pairs 2 lone pairs => AB4E2 square planar shape

11. Shape: SO3 Lewis Electron Dot Structure .. : O : S : : O : .. .. .. : O : .. 3 bond pairs 0 lone pairs => AB3 trigonal planar shape

12. What would be expected to be the shape of chloroform, CHCl3? “see-saw” square planar tetrahedral

14. Central Themes of Valence Bond Theory 3) Hybridization of atomic orbitals. To explain the bonding in simple diatomic molecules such as HF it is sufficient to propose the direct overlap of the s and p orbitals of isolated ground state atoms. In cases such as methane CH4 where 4 hydrogen atoms are bonded to a central carbon atom it is impossible to obtain the correct bond angles. Pauling proposed that the valence atomic orbitals in the molecule are different from those in the isolated atoms.We call this Hybridization!

16. Hybrid Orbitals http://www.colby.edu/chemistry/OChem/DEMOS/Orbitals.html

17. Hybrid Orbital Types - Periodic Groups Hybrid Orbital Types Groups in the Periodic Table Associated SP Group IIA Alkaline Earth Elements SP2 Group IIIA Boron Family SP3 Group IVA Carbon Family** SP3d Group VA Nitrogen Family SP3d2 Group VIA Oxygen Family ** The exception is carbon which can have: SP, SP 2, SP 3 hybrid orbitals

19. WHY DOESN’T THE ATOMIC ORBITAL APPROACH WORK ? After bonding (overlap) we get a totally new solution for the new molecule. During bonding …. new orbitals form that are more suitable for making bonds. These orbitals are for the atom - we can’t expect that they are suitable for the molecule. 2s 2px,2py,2pz sp,sp 2py,2pz s, p, p ,n atomic orbitals hybrid atomic orbitals molecular orbitals overlap LCAO HYPOTHETICAL BONDING PROCESS NOTE. Formally LCAO theory and Molecular Orbital theory are two completely different approaches. You do not need to use hybid orbitals to derive the molecular orbitals, combinations of any type of function will do. Nevertheless, the abstraction presented above is quite useful, as we will see quite soon.

20. HYBRID ORBITALS PRECURSORS TO THE FINAL RESULT We will concentrate first on hybrid orbitals. ADVANTAGE HYBRID ORBITALS allow us to correctly predict the final shape of the molecule. DISADVANTAGE Hybrid orbitals are an abstraction and are not the final MOLECULAR ORBITALS that result after bonds form. The types of hybrids common to organic molecules follow ……

21. sp LINEAR HYBRIDIZATION

22. FORMATION OF LINEAR HYBRID ORBITALS 2 pair unused unused 2p orbitals 2p 2p y 2 orbitals filled 2s sp(1) C hybridization x sp(2) 2p (1) (2) linear sp hybrid orbitals z

23. SP HYBRID ORBITAL more density in the bonding lobe than in an sp2 orbital sp smaller tail than in an sp2 orbital Courtesy of Professor George Gerhold

24. The sp Hybrid Orbitals in Gaseous BeCl2 Fig. 11.2 A&B

25. Fig. 11.2 C&D

26. sp2 TRIGONAL PLANAR HYBRIDIZATION 3 pair in the valence shell and either an incomplete octet or a double bond

27. FORMATION OF TRIGONAL PLANAR HYBRID ORBITALS 3 pair unused unused 2p orbital sp2(2) 2p 2p 3 orbitals filled 2s B hybridization x 120o sp2(1) 2p sp2(3) trigonal planar (1) (2) (3) sp2 hybrid orbitals z

28. SP2 HYBRID ORBITAL cusp more density in the bonding lobe than in an sp3 orbital sp2 smaller tail than in an sp3 orbital Courtesy of Professor George Gerhold

29. Fig. 11.3

30. sp3 TETRAHEDRAL HYBRIDIZATION 4 pair in the valence shell (no double or triple bonds)

31. FORMATION OF TETRAHEDRAL HYBRID ORBITALS New orbitals point to the corners of a tetrahedron. 4 pair sp3(1) 2p FILLED VALENCE SHELL 109o28’ 2s O hybridization occurs when orbitals are full and have finished bonding sp3(3) sp3(4) sp3(2) (1) (2) (3) (4) tetrahedral geometry sp3 hybrid orbitals (cartoon)

32. FORMATION OF SP3 HYBRID ORBITALS (1) (2) (3) (4) X X sp3 hybridized atom These orbital shapes are cartoons - actual shapes are shown on the next slide. FORMATION OF SP3 HYBRID ORBITALS 2p 2pz 2s 2s 2px 2py unhybridized atom [animation]

33. SP3 HYBRID ORBITAL … and its cartoon ( cross section ) The hybrid orbital has more density in the bonding lobe than a p orbital and forms stronger bonds. sp3 To avoid confusion the back lobe is omitted from the cartoons, already shown, and the front lobe is elongated to show its direction. The shape shown is calculated from quantum theory. Courtesy of Professor George Gerhold omitted

34. ORIGIN OF THE SP3 DESIGNATION add together, divide in four hybridization 2s 2p (1) (2) (3) (4) sp3 hybrid orbitals each new orbital is 1/4 s + 3/4 p (25% s, 75% p) S1P3 = SP3 ( 1+3 ) = 4 parts total

35. sp3 hybrid orbital - + x HYBRIDIZATION ORIGIN OF THE SP3 ORBITAL SHAPE 2s orbital 2p orbital + - + x RECALL: signs are mathematical coordinates, not electronic charge [animation] HYBRIDIZATION

36. Fig. 11.4

37. Bonding in Water

38. The sp3 Hybrid Orbitals in NH3 and H2O Fig. 11.5

39. COMPARISON OF THE HYBRIDS

40. COMPARISON OF SPx HYBRID ORBITALS bigger “tail” “cusp” more “p” character sp3 sp2 sp more “s” character more electron density in the bonding lobe Orbital plots courtesy of Professor George Gerhold

41. more p-like sp3 HYBRID ORBITALS COMPARISONS OF BONDING DISTANCE SIZE OF CUSP SIZE OF TAIL sp2 sp more s-like Orbital plots courtesy of Professor George Gerhold makes shorter, stronger bonds

42. The sp3d Hybrid Orbitals in PCl5 Fig. 11.6

43. The sp3d2 Hybrid Orbitals in SF6 Sulfur Hexafluoride -- SF6 Fig. 11.7

45. Fig. 11.8

46. WHY DO HYBRIDS FORM ?

47. WHY DO HYBRIDS FORM? 1. Electron pair repulsions are minimized(= lower energy) 2. Stronger bonds(= lower energy) are formed 3. Hybrids have better directionality for forming bonds 4. Since promotion usually occurs, hybrids allow more bonds to form (= lower energy)

48. CONSTRUCTION BLOCKS THE HYBRIDS ARE “MOLECULAR LEGOS” EACH IS USED IN A SPECIFIC BONDING SITUATION

49. HYBRID CONSTRUCTION BLOCKS 4 PAIR 3 PAIR 2 PAIR TRIGONAL PLANAR TETRAHEDRAL LINEAR sp3 sp2 sp X Y Z 109o28’ 120o 180o

50. Postulating the Hybrid Orbitals in a Molecule Problem: Describe how mixing of atomic orbitals on the central atoms leads to the hybrid orbitals in the following: a) Methyl amine, CH3NH2b) Xenon tetrafluoride, XeF4 Plan: From the Lewis structure and molecular shape, we know the number and arrangement of electron groups around the central atoms, from which we postulate the type of hybrid orbitals involved. Then we write the partial orbital diagram for each central atom before and after the orbitals are hybridized.