Theories of Chemical Bonding Theories of bonding explanations

Theories of Chemical Bonding Theories of bonding: explanations for chemical bond, Lewis dot structures and the following. Valance-bond (VB) theory Hybridization of atomic orbitals Multiple covalent bonds Molecular orbital (MO) theory Delocalized electrons Bonding in metals Practice mental reasoning and verbal Theories of chemical bonding explanation 1

Energy of Interaction Between Two H Atoms Energies of attraction and repulsion as functions of distance between two H atoms are shown here. Potenti al energy The minimum of the attraction force occur at H–H bond length of 74 pm, at which, the antibonding orbital is +346 k. J mole– 1 above 0, energy when H atoms are far apart. Theories of chemical bonding How does energy affect the two-atom +346 k. J mol – 1 antibonding distanc e – 346 k. J mol – 1 H – H bond 2

The Valence-bond Method Valence bond method considers the covalent bond as a result of overlap of atomic orbitals. Electrons stay in regions between the two atoms. Some bond examples s-s H-H Li-H s-p H-C H-N H-F s-d p-p H-Pd C-C in Pd P-P hydride p-d Se-F d-d Fe-Fe (? ) But overlapping of simple atomic orbitals does not explain all the features. Thus, we have to take another look, or do something about atomic orbitals – hybridization. How does valence-bond approach explain the formation. Theories of chemical bonding bonds? 3

Hybridization of Atomic Orbitals The solutions of Schrodinger equation led to these atomic orbitals. 1 s, 2 p, 3 s, 3 p, 3 d, 4 s, 4 p, 4 d, 4 f, etc. However, overlap of these orbitals does not give a satisfactory explanation. In order to explain bonding, these orbitals are combined to form new set of orbitals – this method is called hybridization. During the lecture, these hybridized orbitals will be explained: sp 2 sp hybrid orbitals from mixing of a s and a p orbital sp 2 3 sp 2 hybrid orbitals from mixing of a s and 2 p orbital sp 3 fill in you explanation please 3 d hybrid orbitals 3, 3 p and Theories of from chemical bonding 4 sp 3 d 5 a sp mixing a 2 s, sp and Provide description for hybrid orbitals sp, ofsp 3 3 2

The sp Hybrid Orbitals The sp hybrid orbitals: formation oftwo sp hybrid orbitals + + + - = + - + – + - = - + hybridization of s and p orbitals = 2 sp hybrid orbitals _ _ __ __ __ Two sp hybrid orbitlas => Two states of Be Theories of chemical bonding 5

Bonds with sp Hybrid Orbitals Formations of bonds in these molecules are discussed during the lecture. Be prepared to do the same by yourself. Cl–Be–Cl H–C C–H H–C N : O=C=O Double and triple bonds involve pi p bonding, and the application of valence bond method to p bonds will be discussed. You are expected to be able to draw pictures to show the p bonding. Theories of chemical bonding 6

A p Bond Sigma (s) bond is symmetric about axis. Pi (p) electron distribution above and below axis with a nodal plane, on which probability of finding electron is zero; p bond is not as strong as sigma - less overlap. Bonding of C 2 H 4 How are pi bonds Nodal plane Overlap of 2 2 p orbitals for the formation. Cof p bond Theories of chemical bonding 2 s 2 p 2 p 2 p sp 2 2 p 7

Triple Bonds in H-C CH H-C-C-H: three s bonds due to overlapping of 1 s. H – sp. C; sp. C – sp. C; and sp. C – 1 s. H. Two p bonds in HC CH and HC N triple bonds are due to overlapping of p orbitals results. py over in p bond lap H sp hybrid orbitals H C 2 s 2 p 2 p 2 p sp sp 2 p 2 p px over lap Two nodal planes of in p p bonds are bond perpendicular to Draw and describe how atomic orbitals each other. Theories of chemical bonding 8 overlap to form all bonds in acetylene,

Two p Bonds in H–C C–H A triple bond consists of a sigma and two pi bonds. Overlaps of two sets of p orbitals form of two p bonds. Theories of chemical bonding 9

Bonding of CO 2 For CO 2, the C atom forms a s bond a p bond with each of two O atoms. The two nodal planes of the two p bonds are also perpendicular. During the lecture, I draw diagrams and explain the two s two p bonds in CO 2. You are expected to be able to do the same, in a test. Resonance structures py over lap in p bond : O–C O: : O C–O: . . px over lap in p bond Overlap p–p in s bonds Discuss the bonding of allene Theories of chemical bonding O=C=O H 2 C=C=CH 2 or 10

Bonding in CO 2 – another view Compare with H 2 C=C=CH 2 Theories of chemical bonding 11

The sp 2 Hybrid Orbitals Ground state and excited state electronic configuration of B _ __ __ The hybridization of a s and two p orbitals led to 3 sp 2 hybrid orbitals for bonding. Compounds involving sp 2 hybrid orbitals: BF 3, CO 32–, –, H 2 CO, H 2 C=CH 2, NO 3 Theories of chemical bonding etc Nov. 25 12

An example of using sp 2 hybrid orbitals __ orbitals for bonding? Dipole moment = ____? Theories of chemical bonding 13

Bonding of H 2 C=CH 2 molecules Utilizing the sp 2 hybrid orbitals, each C atom form two H–C s bonds for a total of 4 s H–C bonds. The C–C s bond is common to both C atoms. A C–C p bond is formed due to overlap of p orbitals from each of the C atoms. Overlap of p orbital for C–C p Theories of chemical bonding bond C 2 s 2 p 2 p 2 p sp 2 2 p Hybrid orbitals (sp 2) for H–C and C–C s bond 14

The 3 sp Hybridized Orbitals Ground state and excited state electronic configuration of C _ _ _ __ The hybridization of a s and three p orbitals led to 4 sp 3 hybrid orbitals for bonding. Compounds involving sp 3 hybrid orbitals: CF 4, CH 4, : NH 3, H 2 O: : , Si. O 44–, SO 42–, Cl. O 4–, etc Theories of chemical bonding 15

C 2 s 2 p 2 p 2 p sp 3 sp Theories of chemical bonding 16

The sp 3 d Hybrid Orbitals Hybridization of one s, three p, and a d orbitals results in 5 sp 3 d hybrid orbitals. The arrangement of these orbitals is a trigonal pyramid. Some structures due to these type of orbitals are PCl. F 4, Te. Cl 4 E, and Br. F 3 E 2. How many unshared electron pairs are present in Te. Cl 4 and Br. F 3? Theories of chemical bonding What are their shapes? 17

The sp 3 d 2 Hybrid Orbitals Hybridization of one s, three p, and two d orbitals results in 6 sp 3 d 2 hybrid orbitals. The arrangement of these orbitals is an octahedron. Compounds using these type of orbitals are shown here. AX 6, AX 5 E, AX 4 E 2 AX 3 E 3 and AX 2 E 4 IOF 5, IF 5 E, Xe. F 4 E 2 No known compounds of AX 3 E 3 and AX 2 E 4 are known or recognized, because they are predicted to have a T shape and linear shape respectively when the lone pairs of electrons are ignored. Theories of chemical bonding 18

Molecules with more than one central atom Describe the structure of CH 3 NCO. Draw the skeleton and add all valence Take a new look at slide 22 in Bonding electrons Basics H 3 C – N – C – O Which Lewis dot structure is the most important (stable)? 180 N C– N– o Which O C O 109 structure is o 120 N = C = O H– o more H–C C H H stable, and H H why? What hybridized orbitals are used for bonding in N and C? Why are the bond angles as indicated? No of s and p Theories of chemical bonding 19 bonds = __, __? Give formal charges to all atoms in all structures.

Why Molecular Orbital (MO) Theory Lewis dot and valence bond theories do not always give satisfactory account for various properties of molecules. For example, the dot and VB theory does not explain the fact that O 2 is paramagnetic and has a double bond. Dot and VB structures : O are unsatisfactory. O: • O O • MO theory, different from VB in that MO theory considers the orbitals of the whole molecules. However the approach of linear-combination-of-atomic-orbitals (LCAO) is usually used. There are other reasons, but it’s human nature to theorize. The theory is beautiful, and worth learning or teaching. Theories of chemical bonding 20

The Molecular Orbital (MO) Theory For a molecule, there are certain orbitals each of which accommodates two electrons of opposite spin. The MO theory combines atomic orbitals (AO) to form MOs, & this method is called LCAO s* The two atoms in the H 2 molecule may be represented by A and B. Their s orbitals 1 s. A and 1 s. B respectively, are used for two MOs: s* = 1 s. A – 1 s. B s = 1 s. A + 1 s. B The energy levels of these AO and MO are represented by the diagram Theories of chemical bonding here, with the math hidden. 1 s 1 s A B AO s MO 21 AO

MO for H 2–type molecules: H 2+, H 2–, He 2+ Generalize the technique of LCAO Theories of chemical bonding 22

Electronic configuration, s 2, for H 2 molecules Generalize the technique of LCAO Theories of chemical bonding 23

Electronic Configuration of H 2 -type Molecules From the previous theory, we can fill the M Os with electrons for the H 2 -type molecule: Molecule e-configuration Bond order bondlength H 2+ 1 s (1 s 1) ½ 106 pm H 2, He 22+ 1 s 2 1 74, ~75 H 2–, He 2+ 1 s 2 1 s* ½ ~106, 108 H 22–, He 2 1 s*2 0 not formed Describe the relationships of bondlength & bondorder and e-configurations; learn to reason Theories of chemical bonding 24

Sigma MOs Formed Using p AOs Sigma MOs (s 2 p* ) can be formed using p AOs, similar to VB theory. The gain in bonding orbital s 2 p (lower energy) is at the expense of the anti-bonding orbital s 2 p* (higher energy) Theories of chemical bonding Generalize the technique of 25

Pi p MOs from p AOs Theories ofthe chemical bonding Generalize technique of 26

MO Diagrams for O 2 and F 2 A full diagram of the energy level of molecular orbitals of O 2 and F 2 is shown here. The relative (approximate) height of these energies will be explained verbally during the lecture, and you are suppose to be able to do the same. Write the electronic configurations for Theories of chemical bonding 27 O , O –, F – &

The O 2+, O 2– , & F 2+, F 2– Molecules For O=O _ _ O=O Paramagnetic , bond length indicates double bond, electronic configuration agrees F–F See p. 457 for two MO energy-level Electronic configuration agree with single bond. Theories of chemical bonding 28

MO Energy Level Diagram for Be 2 – of. N 2 s 2 and 2 p, the MO energy Due to close energy levels level diagram for Be 2 to N 2 differs from those of O 2 to F 2. Reasons and explanation are given during the lecture. Hope you can do the same. Give electronic configuratio ns for Be 2+, Be 2–, B 2+, B 2–, C 2+ , C 2–, N 2+ , N 2, See p. 457 for two Theories of chemical bonding 29 N 2–. MO energy-level

A more realistic energy level diagram for Be 2 – N 2 involving sp mixing, not in text __ s*2 p __ __ p*2 p ___ 2 p __ s 2 p __ __p 2 p __ s*2 s _ 2 s Atomi c orbital The sp mixing of AO gives stronger s 2 s bond a weaker s*2 s bond. Thus, the split from 2 s is not even. Atomi Effects on other bonds are c also shown, but qualitatively. Theories of chemical bonding 30 _ 2 s __ s 2 s Molecular orbitals This diagram from my Ca. Ct website accounts for the sp mixing of the AO for the bonding consideration. This sp mixing effect is more detailed than that required for freshman chemistry (not to be tested). orbita

Benzene The benzene structure has fascinated scientists for centuries. It’s bonding is particularly interesting. The C atom utilizes sp 2 hybrid AO in the sigma bonds, and the remaining p AO overlap forming a ring of p bonds. Sigma s bonds are represented by + + lines, and the p + + orbitals for the p bonds are shown + + by balloon-shape – – blobs. Note the + – – and – signs of the p orbitals. Thus, we represent it by Theories of chemical bonding 31

More About Benzene Chem 120 students may ignore this slide. The p and p* of C 6 H 6 are shown here; the symmetry is also interesting. Theories of chemical bonding 32

Delocalized electrons in Benzene and Ozone When p bonds are adjacent to each other or separated by on single C-C bonds, the p bonding electrons are delocalized. The delocalized electron path for benzene and ozone are shown here. These pictures represent p electron of the 2– & have delocalized electrons CO structures formula contribute most to their structures. 3 O O O Theories of chemical bonding 33

Joy on structure of benzene – a story Kathleen Londsdale (1903 -1971) came from a very poor family in Kildare who moved to Essex when she was five. She studied physics, math and chemistry at school and went to college in London when she was 16, where she did extremely well. She was offered a place in the research team of William Bragg, so starting her life's work on X-ray crystallography. In 1929 she showed, by her analysis of hexamethyl benzene, that the benzene ring was flat. Benzene has fascinated scientists, and its precise structure was a matter of controversy till then. In 1945 she was elected the first ever woman Fellow of the Royal Society. She was also created a Dame Commander of of chemical bonding 34 the Order of the Brittish. Theories Empire in 1956.

MO for Heteronuclear Diatomic Molecules For heteronuclear diatomic molecules, the atomic orbitals are at different energy levels. Thus, the MO shifts are different from those of homonuclear diatomic molecules. The interactions of AO for MO for Theories of chemical bonding Chem 120 students may ignore this 35

Review Explain the bonding and anti-bonding orbitals, with a picture if possible Describe a s and a p bond, picture may be used. Give the electronic configurations and bond orders for N 2, N 2+, N 2–, N 22–, O 2, Draw the Lewis dot structure for ozone. Describe the molecular shape and justify for it. Explain the delocalized electrons of ozone. Use the MO theory to explain the fact that O 2 is paramagnetic, and has a double bond. A diagram of the MO energy levels will help. Give a few compounds that have the same number of electrons as O 3. Theories of chemical bonding 36