1 © 2009 Brooks/Cole - Cengage Advanced Theories of Chemical Bonding Chapter 9 Atomic Orbitals Molecules

2 © 2009 Brooks/Cole - Cengage MOLECULAR ORBITAL THEORY — Robert Mullikan ( )MOLECULAR ORBITAL THEORY — Robert Mullikan ( ) valence electrons are delocalizedvalence electrons are delocalized valence electrons are in orbitals (called molecular orbitals) spread over entire molecule.valence electrons are in orbitals (called molecular orbitals) spread over entire molecule. Two Theories of Bonding

3 © 2009 Brooks/Cole - Cengage Two Theories of Bonding VALENCE BOND THEORY — Linus PaulingVALENCE BOND THEORY — Linus Pauling valence electrons are localized between atoms (or are lone pairs).valence electrons are localized between atoms (or are lone pairs). half-filled atomic orbitals overlap to form bonds.half-filled atomic orbitals overlap to form bonds. Linus Pauling,

4 © 2009 Brooks/Cole - Cengage Sigma Bond Formation by Orbital Overlap Two s orbitals overlap

5 © 2009 Brooks/Cole - Cengage Sigma Bond Formation Two s orbitals overlap Two p orbitals overlap

6 © 2009 Brooks/Cole - Cengage Using VB Theory Bonding in BF 3 planar triangle angle = 120 o

7 © 2009 Brooks/Cole - Cengage Bonding in BF 3 How to account for 3 bonds 120 o apart using a spherical s orbital and p orbitals that are 90 o apart?How to account for 3 bonds 120 o apart using a spherical s orbital and p orbitals that are 90 o apart? Pauling said to modify VB approach with ORBITAL HYBRIDIZATIONPauling said to modify VB approach with ORBITAL HYBRIDIZATION — mix available orbitals to form a new set of orbitals — HYBRID ORBITALS — that will give the maximum overlap in the correct geometry.— mix available orbitals to form a new set of orbitals — HYBRID ORBITALS — that will give the maximum overlap in the correct geometry.

8 © 2009 Brooks/Cole - Cengage Why Hybridize? Just looking at valence electrons: Be should form no covalent bonds B should form one covalent bond C should form 2 covalent bonds But… BeF 2, BF 3 and CF 4 Exist! HOW?

9 © 2009 Brooks/Cole - Cengage Hybrid Orbitals: Why? To explain the bonding in molecules like BeF 2, BF 3 and CF 4, Linus Pauling proposed that orbitals become ‘hybridized’ –Hybrid orbitals are orbitals created by mixing the s, p or d orbitals of an atom.

10 © 2009 Brooks/Cole - Cengage Hybrid Orbitals: The Rules 1.The number or hybrid orbitals is ALWAYS equal to the number of atomic orbitals that are combined to make the hybrid set 2.Hybrid orbital sets are always built by combining an s orbital with as many p or d orbitals necessary to accommodate the bonding and lone pairs on the central atom (Remember Electron Pair Geometry?) 3.The Hybrid Orbitals are directed TOWARDS the terminal atoms This results in a better orbital overlap AND stronger bonds between the central and terminal atoms

11 © 2009 Brooks/Cole - Cengage sp Hybrid Orbitals Mix an s orbital with a p orbital to create two sp orbitals

12 © 2009 Brooks/Cole - Cengage sp 2 Hybrid Orbitals Mix an s orbital with 2 p orbitals to create three sp 2 orbitals

13 © 2009 Brooks/Cole - Cengage sp 3 Hybrid Orbitals Mix an s orbital with 3 p orbitals to create four sp 3 orbitals

14 © 2009 Brooks/Cole - Cengage sp 3 Hybrid Orbitals: Examples

15 © 2009 Brooks/Cole - Cengage Bonding in BF 3 rearrange electronshydridize orbs. unused p orbital three sp 2 hybrid orbitals 2p 2s

16 © 2009 Brooks/Cole - Cengage The three hybrid orbitals are made from 1 s orbital and 2 p orbitals  3 sp 2 hybrids. The three hybrid orbitals are made from 1 s orbital and 2 p orbitals  3 sp 2 hybrids. Bonding in BF 3 Now we have 3, half-filled HYBRID orbitals that can be used to form B-F sigma bonds.Now we have 3, half-filled HYBRID orbitals that can be used to form B-F sigma bonds.

17 © 2009 Brooks/Cole - Cengage An orbital from each F overlaps one of the sp 2 hybrids to form a B-F  bond. Bonding in BF 3

18 © 2009 Brooks/Cole - Cengage BF 3, Planar Trigonal

19 © 2009 Brooks/Cole - Cengage Bonding in CH 4 How do we account for 4 C—H sigma bonds 109 o apart? Need to use 4 atomic orbitals — s, p x, p y, and p z — to form 4 new hybrid orbitals pointing in the correct direction.

20 © 2009 Brooks/Cole - Cengage 4 C atom orbitals hybridize to form four equivalent sp 3 hybrid atomic orbitals. Bonding in a Tetrahedron Formation of Hybrid Atomic Orbitals

21 © 2009 Brooks/Cole - Cengage Bonding in a Tetrahedron — Formation of Hybrid Atomic Orbitals 4 C atom orbitals hybridize to form four equivalent sp 3 hybrid atomic orbitals.

22 © 2009 Brooks/Cole - Cengage Bonding in CH 4

23 © 2009 Brooks/Cole - Cengage

24 Bonding in Glycine

25 © 2009 Brooks/Cole - Cengage Bonding in Glycine

26 © 2009 Brooks/Cole - Cengage Bonding in Glycine

27 © 2009 Brooks/Cole - Cengage Bonding in Glycine

28 © 2009 Brooks/Cole - Cengage Bonding in Glycine

29 © 2009 Brooks/Cole - Cengage

30 Orbital Hybridization 2 e - clouds 3 e- clouds 4 e- clouds 5 e- clouds 6 e- clouds

31 © 2009 Brooks/Cole - Cengage Multiple Bonds Consider ethylene, C 2 H 4 C H H H H sp 2 120° C

32 © 2009 Brooks/Cole - Cengage Sigma Bonds in C 2 H 4 C H H H H sp 2 120° C

33 © 2009 Brooks/Cole - Cengage π Bonding in C 2 H 4 The unused p orbital on each C atom contains an electron and this p orbital overlaps the p orbital on the neighboring atom to form the π bond.

34 © 2009 Brooks/Cole - Cengage π Bonding in C 2 H 4 The unused p orbital on each C atom contains an electron and this p orbital overlaps the p orbital on the neighboring atom to form the π bond.

35 © 2009 Brooks/Cole - Cengage Multiple Bonding in C 2 H 4

36 © 2009 Brooks/Cole - Cengage  and π Bonding in C 2 H 4

37 © 2009 Brooks/Cole - Cengage  and π Bonding in CH 2 O

38 © 2009 Brooks/Cole - Cengage  and π Bonding in C 2 H 2

39 © 2009 Brooks/Cole - Cengage  and π Bonding in C 2 H 2

40 © 2009 Brooks/Cole - Cengage Consequences of Multiple Bonding There is restricted rotation around C=C bond.

41 © 2009 Brooks/Cole - Cengage Consequences of Multiple Bonding Restricted rotation around C=C bond.

42 © 2009 Brooks/Cole - Cengage Consequences of Multiple Bonding Formation of Isomers One isomer may have biological activity while the other may not

43 © 2009 Brooks/Cole - Cengage Double Bonds and Vision