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 1 Energy of Interaction Between Two H Atoms
Energies of attraction and Potential repulsion as functions of energy distance between two H atoms are shown here. +346 kJ mol –1 The minimum of the antibonding attraction force occur at H–H bond length of 74 pm, at which, the antibonding distance orbital is +346 kJ mole–1 above 0, energy when H –346 kJ mol –1 atoms are far apart. H – H bond
How does energy affectTheories the two of chemical-atom bonding system? 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 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 Theoriesbonds? of chemical bonding 3 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 2 3 sp2 hybrid orbitals from mixing of a s and 2 p orbital sp3 fill in you explanation please sp3d 5 sp3d hybrid orbitals from mixing of a s and 3 p and a d orbital sp3d 2 ______
Provide a description for hybridTheories orbitals of chemical sp, bonding sp2, sp3, sp3d, and sp3d2 4 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 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–CC–H H–CN : 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 6 A p Bond
Sigma (s) bond is symmetric about axis. Nodal plane 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 of 2 2p orbitals for overlap. the formation of p bond C 2s 2p 2p 2p sp2 sp2 sp2 2p
Bonding of C2H4
Theories of chemical bonding 7 How are pi bonds formed? Triple Bonds in H-CC-H
H-C-C-H: three s bonds due to overlapping of 1sH – spC; spC – spC; and spC – 1sH. Two p bonds in HCCH and HCN triple bonds are due to overlapping of p orbitals results. sp hybrid orbitals
py over lap C in p bond 2s 2p 2p 2p H H sp sp 2p 2p px over lap Two nodal planes of p in p bond bonds are perpendicular to each other. Draw and describe how atomic orbitals overlap Theories of chemical bonding 8 to form all bonds in acetylene, H–CC–H Two p Bonds in H–CC–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 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 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 H2C=C=CH2 Theories of chemical bonding 10 O=C=O or H2C=C=CH2 See extra problems B17 in the handout Bonding in CO2 – another view 09_174 sigma bond (1 pair of electrons) pi bond (1 pair of electrons)
O C O
pi bond (1 pair of electrons) (a)
Compare with H2C=C=CH2 O C O
Theories of chemical bonding 11 (b) 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 2– orbitals: BF3, CO3 , H2CO, – H2C=CH2, NO3 , etc
Theories of chemical bonding 12 Nov. 25 An example of using sp2 hybrid orbitals
__ orbitals for bonding? Dipole moment = ____?
Theories of chemical bonding 13 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. C 2s 2p 2p 2p sp2 sp2 sp2 2p
Hybrid orbitals (sp2) for H–C and C–C s bond Overlap of p orbital for C–C p bond
Theories of chemical bonding 14 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::, 4– 2– – SiO4 , SO4 , ClO4 , etc
Theories of chemical bonding 15 C 2s 2p 2p 2p sp3 sp3 sp3 sp3
Theories of chemical bonding 16 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 17 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, AX4E2 AX3E3 and AX2E4 IOF5, 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 18 Molecules with more than one central atom
Describe the structure of CH3NCO. Take a new look at slide 22 in Bonding Basics Draw the skeleton and add all valence electrons
H3C – N – C – O Which Lewis dot structure is the most important (stable)?
o NC–O 180 N–CO N = C = O Which 109o H–C 120o H–C structure is H–C more stable, H H H H H H and why?
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 Theoriesatoms of in chemical all structures. bonding 19 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 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 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 1sA 1sB The energy levels of these AO and MO are represented by the diagram here, with the math hidden. s Theories of chemical bonding AO MO AO21 + – + MO for H2–type molecules: H2 , H2, H2 , He2
Generalize the technique of LCAO
Theories of chemical bonding 22 2 Electronic configuration, s , for H2 molecules
Generalize the technique of LCAO Theories of chemical bonding 23 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 + 1 H2 1s (1s ) ½ 106 pm 2+ 2 H2, He2 1s 1 74, ~75 – + 2 H2 , He2 1s 1s* ½ ~106, 108 2– 2 2 H2 , He2 1s 1s* 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 (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 25 Generalize the technique of LCAO Pi p MOs from p AOs
GeneralizeTheories the of chemicaltechnique bonding of LCAO 26 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, – – Theories of chemical bondingO2 , F2, F2 & Ne2. 27 + – + – 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 Theories of chemical bonding 28 energy-level diagrams 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, B – , C +, 2 2 C2, – + C2 , N2 , N2, – N2 . See p. 457 for two MO energy-level diagrams Theories of chemical bonding 29 A more realistic energy level diagram for
Be2 – N2 involving sp mixing, not in text __ s* 2p __ __ p* This diagram from my CaCt 2p website accounts for the sp ______
2p 2p mixing of the AO for the bonding
consideration. This sp mixing
effect is more detailed than that __ s 2p __ __p required for freshman chemistry 2p (not to be tested).
__ s* The sp mixing of AO gives 2s stronger s bond and a weaker _ _ 2s
2s 2s s*2s bond. Thus, the split from 2s
is not even. Effects on other __ s 2s bonds are also shown, but
Atomic Atomic qualitatively. orbital orbital Molecular orbitals Theories of chemical bonding 30 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 31 Chem120 students may ignore More About Benzene this slide.
The p and p* of C6H6 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 the structures formula contribute most to their structures. 2– CO3 & have delocalized electrons 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 the Order of the Brittish Empire in 1956.
Theories of chemical bonding 34 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 35 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.
+ 2+ Give the electronic configurations and bond orders for N2, N2 , N2 , – 2– N2 , N2 , 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 36