Theories of Chemical Bonding

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 explanation Theories of chemical bonding

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

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 s-p s-d p-p p-d d-d H-H H-C H-Pd C-C Se-F Fe-Fe (?) Li-H H-N in Pd P-P H-F hydride 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 of chemical bonds? Theories of chemical bonding

Hybridization of Atomic Orbitals The solutions of Schrodinger equation led to these atomic orbitals. 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 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 23 sp2 hybrid orbitals from mixing of a s and 2 p orbital sp3 fill in you explanation pleasesp3d 5 sp3d hybrid orbitals from mixing of a s and 3 p and a d orbitalsp3d 2 ____________ Provide a description for hybrid orbitals sp, sp2, sp3, sp3d, and sp3d2 Theories of chemical bonding

The sp Hybrid Orbitals The sp hybrid orbitals: formation of two 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

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 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

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. Nodal plane Overlap of 2 2p orbitals for the formation of p bond C2s 2p 2p2psp2 sp2sp22p Bonding of C2H4 Theories of chemical bonding How are pi bonds formed?

Triple Bonds in H-CC-H H-C-C-H: three s bonds due to overlapping of 1sH – spC; spC – spC; and spC – 1sH. Two p bonds in HCCH and HCN triple bonds are due to overlapping of p orbitals results. sp hybrid orbitals py over lap C2s2p 2p2pspsp2p2p in p bond H H px over lap Two nodal planes of p bonds are perpendicular to each other. in p bond Draw and describe how atomic orbitals overlap to form all bonds in acetylene, H–CC–H Theories of chemical bonding

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

Bonding of CO2 For CO2, the C atom forms a s bond and 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 CO2. You are expected to be able to do the same, in a test. Resonance structures : O – C  O : : O  C – O : . . py over lap in p bond px over lap in p bond Overlap p–p in s bonds Discuss the bonding of allene H2C=C=CH2 See extra problems B17 in the handout O=C=O or H2C=C=CH2 Theories of chemical bonding

Bonding in CO2 – another view Compare with H2C=C=CH2 Theories of chemical bonding

The sp2 Hybrid Orbitals Ground state and excited state electronic configuration of B ___ __ _ __ __ The hybridization of a s and two p orbitals led to 3 sp2 hybrid orbitals for bonding. Compounds involving sp2 hybrid orbitals: BF3, CO32–, H2CO, H2C=CH2, NO3–, etc Theories of chemical bonding Nov. 25

An example of using sp2 hybrid orbitals __ orbitals for bonding? Dipole moment = ____? Theories of chemical bonding

Bonding of H2C=CH2 molecules Utilizing the sp2 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. C2s 2p 2p2psp2 sp2sp22p Hybrid orbitals (sp2) for H–C and C–C s bond Overlap of p orbital for C–C p bond Theories of chemical bonding

The sp3 Hybridized Orbitals Ground state and excited state electronic configuration of C ___ _ _ _ __ The hybridization of a s and three p orbitals led to 4 sp3 hybrid orbitals for bonding. Compounds involving sp3 hybrid orbitals: CF4, CH4, : NH3, H2O::, SiO44–, SO42–, ClO4–, etc Theories of chemical bonding

C2s 2p 2p2psp3 sp3sp3sp3 Theories of chemical bonding

The sp3d Hybrid Orbitals Hybridization of one s, three p, and a d orbitals results in 5 sp3d hybrid orbitals. The arrangement of these orbitals is a trigonal pyramid. Some structures due to these type of orbitals are PClF4, TeCl4E, and BrF3E2. How many unshared electron pairs are present in TeCl4 and BrF3? What are their shapes? Theories of chemical bonding

The sp3d2 Hybrid Orbitals Hybridization of one s, three p, and two d orbitals results in 6 sp3d2 hybrid orbitals. The arrangement of these orbitals is an octahedron. Compounds using these type of orbitals are shown here. AX6, AX5E, AX4E2AX3E3 and AX2E4IOF5, IF5E, XeF4E2 No known compounds of AX3E3 and AX2E4 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

Molecules with more than one central atom Describe the structure of CH3NCO. Draw the skeleton and add all valence electrons H3C – N – C – O Which Lewis dot structure is the most important (stable)? N = C = O Take a new look at slide 22 in Bonding Basics 180o NC–O H–C N–CO H–C Which structure is more stable, and why? 109o 120o H–C H H H H H H What hybridized orbitals are used for bonding in N and C? Why are the bond angles as indicated? No of s and p bonds = __, __? Give formal charges to all atoms in all structures. Theories of chemical bonding

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 O2 is paramagnetic and has a double bond. Dot and VB structures : O O :• O O •are unsatisfactory. 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

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 H2 molecule may be represented by A and B. Their s orbitals 1sA and 1sB respectively, are used for two MOs: s* = 1sA – 1sB s = 1sA + 1sB The energy levels of these AO and MO are represented by the diagram here, with the math hidden. 1sA 1sB sMO AO AO Theories of chemical bonding

MO for H2–type molecules: H2+, H2, H2–, He2+ Generalize the technique of LCAO Theories of chemical bonding

Electronic configuration, s2, for H2 molecules Generalize the technique of LCAO Theories of chemical bonding

Electronic Configuration of H2-type Molecules From the previous theory, we can fill the M Os with electrons for the H2-type molecule: Molecule e-configuration Bond order bondlength H2+ 1s(1s1) ½ 106 pm H2, He22+ 1s2 1 74, ~75 H2–, He2+ 1s2 1s* ½ ~106, 108H22–, He2 1s2 1s*2 0 not formed Describe the relationships of bondlength & bondorder and e-configurations; learn to reason Theories of chemical bonding

Sigma MOs Formed Using p AOs Sigma MOs (s2p s2p* ) can be formed using p AOs, similar to VB theory. The gain in bonding orbital s2p (lower energy) is at the expense of the anti-bonding orbital s2p* (higher energy) Theories of chemical bonding Generalize the technique of LCAO

Pi p MOs from p AOs Generalize the technique of LCAO Theories of chemical bonding

MO Diagrams for O2 and F2 A full diagram of the energy level of molecular orbitals of O2 and F2 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 O2, O2–, F2, F2– & Ne2. Theories of chemical bonding

The O2+, O2, O2– , & F2+, F2, F2– Molecules For O=O_ _ O=O Paramagnetic , bond length indicates double bond, electronic configuration agrees F–F Electronic configuration agree with single bond. See p. 457 for two MO energy-level diagrams Theories of chemical bonding

MO Energy Level Diagram for Be2 – N2 Due to close energy levels of 2s and 2p, the MO energy level diagram for Be2 to N2 differs from those of O2 to F2. Reasons and explanation are given during the lecture. Hope you can do the same. Give electronic configurations for Be2+, Be2–, B2+, B2,B2–, C2+, C2, C2–, N2+,N2, N2–. See p. 457 for two MO energy-level diagrams Theories of chemical bonding

A more realistic energy level diagram for Be2 – N2 involving sp mixing, not in text _ _ _2p_2sAtomicorbital __ s*2p__ __ p*2p__ s2p__ __p2p__ s*2s__ s2sMolecular orbitals _ _ _2p_2sAtomicorbital This diagram from my CaCt website accounts for the spmixing of the AO for the bonding consideration. This spmixing effect is more detailed than that required for freshman chemistry (not to be tested). The sp mixing of AO gives stronger s2sbond and a weaker s*2sbond. Thus, the split from 2s is not even. Effects on other bonds are also shown, but qualitatively. Theories of chemical bonding

Benzene The benzene structure has fascinated scientists for centuries. It’s bonding is particularly interesting. The C atom utilizes sp2 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

More About Benzene Chem120 students may ignore this slide. The p and p* of C6H6 are shown here; the symmetry is also interesting. Theories of chemical bonding

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 the structures formula contribute most to their structures. CO32– & have delocalized electrons O O O Theories of chemical bonding

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 the Order of the Brittish Empire in 1956. Theories of chemical bonding

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 HF, LiF are similar, and explained in lectures. Theories of chemical bonding Chem120 students may ignore this slide.

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 N2, N2+, N22+, N2–, N22–, O2, 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 O2 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 O3. Theories of chemical bonding

Theories of Chemical Bonding