Theory of Chemical Bonding

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Theories of chemical bonding 1

Theories of Chemical Bonding

Theories of bonding: explanations for chemical bond, Lewis dotstructures 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


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The Valence-bond Method

Valence bondmethod considers the covalent bond as a result ofoverlap 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 aboutatomic orbitalshybridization.

How does valence-bond approach explain

the formation of chemical bonds?


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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

formnew set of orbitalsthis method is called hybridization.

During the lecture, these hybridized orbitals will be explained:

sp 2sp hybrid orbitals from mixing of a s and ap orbital

sp 2 3sp2 hybrid orbitals from mixing of a s and 2p orbital

sp3 fill in you explanation please

sp3d 5 sp3d hybrid orbitals from mixing of a s and 3p and ad orbital

sp3d 2 ____________

Provide a description for hybrid orbitals sp, sp2, sp3, sp3d, and sp3d2


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The spHybrid Orbitals

The sp hybrid orbitals: formation of two sp hybrid orbitals+ + + - = + -

+ + - = - +

hybridization of s andporbitals = 2 sphybrid orbitals

_ _ __ __

__ __ __ Two sp hybrid orbitlas =>

Two states of Be


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Bonds with spHybrid Orbitals

Formations of bonds in these molecules are discussed during thelecture. Be prepared to do the same by yourself.

ClBeCl HCCH HCN : O=C=O

Double and triple bonds involve pi pbonding, and the the application

of valence bond method to pbonds will be discussed.

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


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A pBond

Overlap of 2 2porbitals forthe formation of pbond

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 - lessoverlap.

Nodal plane

Bonding of C2H4

C

2s 2p 2p2p

sp2 sp2sp22p

How are pi bonds formed?


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Triple Bonds in H-CC-H

H-C-C-H: threes

bonds due to overlapping of 1sHspC; spCspC;and spC1sH.

Two pbonds in HCCH and HCN triple bonds are due to

overlapping ofporbitals results.

Draw and describe how atomic orbitals overlap

to form all bonds in acetylene, HCCH

pyover lap

pxover lap

H H

sp hybrid orbitals

Two nodal planesof p

bonds are perpendicular

to each other.

in pbond

in pbond

C

2s2p2p2p

spsp2p2p


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Two pBonds in HCCH

A triple bond consists of a sigma and two pi bonds. Overlaps

of two sets ofporbitals form of two pbonds.


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Bonding of CO2For CO2, the C atom forms a sbond and a pbond with each of two O

atoms. The two nodal planes of the two pbonds are also perpendicular.

During the lecture, I draw diagrams and explain the two stwo pbonds

in CO2. You are expected to be able to do the same, in a test.

pyover lap in pbond

Overlap pp in sbonds

pxover lap in pbond

O=C=O or H2C=C=CH2Discuss the bonding of allene H2C=C=CH2

See extra problems B17 in the handout

Resonance structures

: OC O :

: O CO :. .


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09_174

O C O

sigma bond(1 pair of electrons) pi bond

(1 pair ofelectrons)

pi bond(1 pair ofelectrons)

(a)

O C O

Bonding in CO2another view

Compare with H2C=C=CH2


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The sp2Hybrid Orbitals

Ground state and excited stateelectronic configuration of B

_ _ _ __

_ __ __

The hybridization of a sand two

porbitals led to 3 sp2hybrid

orbitalsfor bonding.

Compounds involving sp2 hybridorbitals: BF3, CO3

2, H2CO,

H2C=CH2, NO3, etc

Nov. 25


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An example of using sp2hybrid orbitals

__ orbitals for bonding?

Dipole moment = ____?


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Bonding of H2C=CH2molecules

Utilizing the sp2hybrid orbitals, each C atom form

two HC sbonds for a total of 4 sHC bonds.

The CC sbond is common to both C atoms.

A CC pbond is formed due to overlap ofp

orbitals from each of the C atoms.

Hybrid orbitals (sp2) for

HC and CC sbond

Overlap ofporbital

for CC pbond

C

2s 2p 2p2p

sp2 sp2sp22p


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The sp3Hybridized Orbitals

Ground state and excited state

electronic configuration of C

_ _ _ _

_ _ __

The hybridization of a sand three

porbitals led to 4 sp3hybrid

orbitalsfor bonding.

Compounds involving sp3

hybridorbitals: CF4, CH4, :NH3, H2O::,

SiO44, SO4

2, ClO4, etc


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C

2s 2p 2p2p

sp3 sp3sp3sp3


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The sp3dHybrid OrbitalsHybridization of one s, threep, and a d

orbitals results in 5 sp3dhybrid

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 presentin TeCl4and BrF3?

What are their shapes?


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The sp3d2Hybrid OrbitalsHybridization of one s, threep, and

two dorbitals results in 6 sp3d2hybrid 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 ofAX3E3andAX2E4are known or recognized,

because they are predicted to have a T shape and linear shape

respectively when the lone pairs of electrons are ignored.


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Molecules with more than one central atomDescribe the structure of CH3NCO.

Draw the skeleton and add all valence electrons

H3CNCO

Which Lewis dot structure is the most important (stable)?

N = C = O

HC

H H

120o109o

180o

What hybridized orbitals are used for bonding in N and C? Why are the

bond angles as indicated? No of sand pbonds = __, __?

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?

NCO

HC

H H

NCO

HC

H H


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Why Molecular Orbital (MO) TheoryLewis dotand valence bondtheories do not always give satisfactory

account for various properties of molecules.

For example, the dot and VB theory does not explain the fact that O2is

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 its human nature to theorize. The theory

is beautiful, and worth learning or teaching.


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The Molecular Orbital (MO) Theory

The two atoms in the H2molecule may be

represented by A and B. Their s orbitals 1sA

and 1sBrespectively, are used for two MOs:

s* = 1sA1sB

s= 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

s

MO

s*

AO AO


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MO for H2type molecules: H2+, H2, H2

, He2+

Generalize the technique of LCAO


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Electronic configuration, s2, for H2molecules



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Electronic Configuration of H2-type Molecules

From the previous theory, we can fill the M Os with electrons for theH2-type molecule:

Molecule e-configuration Bond order bondlength

H2+ 1s(1s1) 106 pm

H2, He22+ 1s2 1 74, ~75H2

, He2+ 1s2 1s* ~106, 108

H22, He2

1s2 1s*2 0 not formed

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


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Sigma MOs Formed UsingpAOs

Sigma MOs (s2ps

2p

* ) can be formed usingpAOs, similar to VB

theory. The gain in bonding orbital s2p(lower energy) is at the

expense of the anti-bonding orbital s2p*(higher energy)



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Pi pMOs frompAOs



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MO Diagrams for O2and F2

A full diagram of the energylevel of molecular orbitals of

O2and F2is shown here.

The relative (approximate)

height of these energies willbe 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.


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The O2+, O2, O2

, & F2+, F2, F2

Molecules

O=O

Paramagnetic ,

bond length

indicates double

bond, electronicconfiguration

agrees

FF

Electronic

configuration agree

with single bond.

For

O=O

_ _

See p. 457 for two MO

energy-level diagrams


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MO Energy Level Diagram for Be2N2Due to close energy levels of 2sand 2p, the MO energy level diagram

for Be2to N2differs from those of O2to F2. Reasons and explanationare 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


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A more realistic energy level diagram for

Be2N2involving spmixing, not in text

_ _ _

2p

_

2s

Atomic

orbital

__ s*2p__ __ *

2p

__ s2p__ __2p

__ s*2s

__ s2s

Molecular orbitals

_ _ _

2p

_

2s

Atomic

orbital

This diagram from my CaCt

website accounts for the spmixingof the AO for the bonding

consideration. This spmixing

effect is more detailed than that

required for freshman chemistry(not to be tested).

The spmixingof AO gives

stronger s2sbond and a weakers 2sbond. Thus, the split from 2sis not even. Effects on other

bonds are also shown, but

qualitatively.


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BenzeneThe benzene structure has fascinated scientists for centuries. Its bonding is

particularly interesting. The C atom utilizes sp2 hybrid AO in the sigmabonds, and the remaining p AO overlap forming a ring of p bonds.

Sigma sbonds are

represented by lines,and the porbitals for

the pbonds are shown

by balloon-shape

blobs. Note the + and

signs of the porbitals.

Thus, we represent it

by

+

+ +

+

++

Ch 120 t d t i


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More About BenzeneChem120 students may ignore

this slide.

The pand p* of C6H6are shown

here; the symmetry is also

interesting.


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


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Joy on structure of benzene a storyKathleen 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.


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MO for Heteronuclear Diatomic Molecules

For heteronuclear

diatomic molecules,

the atomic orbitals

are at different

energy levels. Thus,

the MO shifts aredifferent from those

of homonuclear

diatomic molecules.

The interactions ofAO for MO for HF,

LiF are similar, and

explained in lectures.

Chem120 students may ignore this slide.


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Theory of Chemical Bonding