Theories of chemical bonding1 Theories of Chemical Bonding Theories of bonding: explanations for...

Theories of chemical bonding 1

Theories of Chemical BondingTheories 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


Energy of Interaction Between Two H Atoms

Potential energy

distance

–346 kJ mol –1 H – H bond

+346 kJ mol –1 antibonding

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.

How does energy affect the two-atom system?


The Valence-bond MethodValence 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?


Hybridization of Atomic OrbitalsThe 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 orbitalsp3 fill in you explanation please

sp3d 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


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


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 bonding, and the the application of valence bond method to bonds will be discussed.

You are expected to be able to draw pictures to show the bonding.


A Bond

Overlap of 2 2p orbitals for the formation of bond

Sigma () bond is symmetric about axis.

Pi () electron distribution above and below axis with a nodal plane, on which probability of finding electron is zero; bond is not as strong as sigma - less overlap.

Nodal plane

Bonding of C2H4

C2s 2p 2p 2p

sp2 sp2 sp2 2p

How are pi bonds formed?


Triple Bonds in H-CC-HH-C-C-H: three bonds due to overlapping of 1sH – spC; spC – spC; and spC – 1sH.

Two bonds in HCCH and HCN triple bonds are due to overlapping of p orbitals results.

Draw and describe how atomic orbitals overlap to form all bonds in acetylene, H–CC–H

py over lap

px over lap H H

sp hybrid orbitals

Two nodal planes of bonds are perpendicular to each other.

in bond

in bondC

2s 2p 2p 2psp sp 2p 2p


Two 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 bonds.


Bonding of CO2For CO2, the C atom forms a bond and a bond with each of two O atoms. The two nodal planes of the two bonds are also perpendicular.

During the lecture, I draw diagrams and explain the two two bonds in CO2. You are expected to be able to do the same, in a test.

py over lap in bond

Overlap p–p in bonds

px over lap in bond

O=C=O or H2C=C=CH2

Discuss the bonding of allene H2C=C=CH2

See extra problems B17 in the handout

Resonance structures

: O – C O :

: O C – O :

. .


09_174

O C O

sigma bond(1 pair of electrons) pi bond

(1 pair ofelectrons)

pi bond(1 pair ofelectrons)

(a)

(b)

O C O

Bonding in CO2 – another view

Compare with H2C=C=CH2


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, CO3

2–, H2CO, H2C=CH2, NO3

–, etc

Nov. 25


An example of using sp2 hybrid orbitals

__ orbitals for bonding?

Dipole moment = ____?


Bonding of H2C=CH2 molecules

Utilizing the sp2 hybrid orbitals, each C atom form two H–C bonds for a total of 4 H–C bonds. The C–C bond is common to both C atoms.

A C–C bond is formed due to overlap of p orbitals from each of the C atoms.

Hybrid orbitals (sp2) for H–C and C–C bond

Overlap of p orbital for C–C bond

C2s 2p 2p 2p

sp2 sp2 sp2 2p


The sp3 Hybridized OrbitalsGround 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::, SiO4

4–, SO42–, ClO4

–, etc


C2s 2p 2p 2p

sp3 sp3 sp3 sp3


The sp3d Hybrid OrbitalsHybridization 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?


The sp3d2 Hybrid OrbitalsHybridization 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.


Molecules with more than one central atomDescribe 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 H–C

H H

120o109o

180o

What hybridized orbitals are used for bonding in N and C? Why are the bond angles as indicated? No of and bonds = __, __? Give formal charges to all atoms in all structures.

Take a new look at slide 22 in Bonding Basics

Which structure is more stable, and why?

NC–O

H–C

H H

N–CO

H–C

H H


Why Molecular Orbital (MO) TheoryLewis 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.


The Molecular Orbital (MO) Theory

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:

* = 1sA – 1sB

= 1sA + 1sB

The energy levels of these AO and MO are represented by the diagram here, with the math hidden.

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

1sA 1sB

MO

AO AO


MO for H2–type molecules: H2+, H2, H2

–, He2+

Generalize the technique of LCAO


Electronic configuration, 2, for H2 molecules



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 bondlengthH2

+ 1 (11) ½ 106 pm H2, He2

2+ 12 1 74, ~75H2

–, He2+ 12 1 ½ ~106, 108

H22–, He2

12 12 0 not formed

Describe the relationships of bondlength & bondorder and e-configurations; learn to reason


Sigma MOs Formed Using p AOs

Sigma MOs (2p2p* ) can be formed using p AOs, similar to VB

theory. The gain in bonding orbital 2p (lower energy) is at the expense of the anti-bonding orbital 2p

* (higher energy)



Pi MOs from p AOs



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.


The O2+, O2, O2

– , & F2+, F2, F2

– Molecules

O=O

Paramagnetic , bond length indicates double bond, electronic configuration agrees

F–F

Electronic configuration agree with single bond.

For O=O_ _

See p. 457 for two MO 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,B2

–, C2+,

C2, C2

–, N2+,

N2, N2

–. See p. 457 for two MO energy-level diagrams


A more realistic energy level diagram for Be2 – N2 involving sp mixing, not in text

_ _ _2p

_2s

Atomicorbital

__ *2p

__ __ *2p

__ 2p

__ __2p

__ *2s

__ 2s

Molecular orbitals

_ _ _2p

_2s

Atomicorbital

This diagram from my CaCt 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).

The sp mixing of AO gives stronger 2s bond and a weaker 2s bond. Thus, the split from 2s is not even. Effects on other bonds are also shown, but qualitatively.


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 bonds are represented by lines, and the p orbitals for the bonds are shown by balloon-shape blobs. Note the + and – signs of the p orbitals. Thus, we represent it by

+

–

+ +

+

– ––

++


More About BenzeneChem120 students may ignore this slide.

The and * of C6H6 are shown here; the symmetry is also interesting.


Delocalized electrons in Benzene and OzoneWhen 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.

O

O O

CO32– & have delocalized electrons

http://chem.ufl.edu/~chm2040/cgi-bin/image_eval.cgi?Op=display&Chapter=12&File=theory&Image=c10f35.gif


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.


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.

Chem120 students may ignore this slide.


ReviewExplain the bonding and anti-bonding orbitals, with a picture if possible

Describe a and a bond, picture may be used.

Give the electronic configurations and bond orders for N2, N2+, N2

2+, 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.

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