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Chapter 9

B di II:Bonding II:Molecular Geometry and Bonding Theories

Dr. A. Al-Saadi 1

Molecular Orbital Theory

Molecular orbital theory : Atomic orbitals (AO)

Chapter 9 Section 6

combine to form new molecular orbitals (MO) which are spread out over the entire molecule.

Molecular orbitals (MO) describe the properties of the entire molecule, and not the properties of individual atoms.

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Molecular orbitals (wave functions) result from adding and/or subtracting atomic orbitals (wave functions).

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Molecular Orbital Theory

Molecular orbitals:

Chapter 9 Section 6

have specific shapes and energies.

can have a maximum of two electrons with opposite spins.

are equal to the number of atomic orbitals they have been composed from “the number

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y pof orbitals is conserved”.

Our study about MO theory is restricted to diatomic molecules.

Molecular Orbitals from s Atomic Orbitals

Chapter 9 Section 6

H + H Hydrogen l l

H-H 2

1s1 1s1

bonding orbital

antibonding orbital

*s1difference

molecule

ergy

1s2

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g

s11s 1s

H atomic orbitals H2 molecular orbitals

sum

En

e

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σ1s Molecular Orbital

Chapter 9 Section 6

Hydrogen l l

“Bonding” molecular orbital.

molecule

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g

Constructive combination of “ns” AOs.

High electron density “high probability” between the two nuclei.

Lower energy and more stable than the AOs that were added.

σ*1s Molecular Orbital

Chapter 9 Section 6

Hydrogen l l

“Antibonding” molecular orbital.

Destructive combination of “ns” AOs

molecule

node

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Destructive combination of ns AOs.

Very low electron density “low probability” between the two nuclei. The electron density pulls the two nuclei in opposite directions.

Higher energy and less stable than the AOs that were subtracted.

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σ1s and σ*1s Molecular Orbitals

Chapter 9 Section 6

Hydrogen l l

Showing all the MOs in a molecule can result in a

molecule

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gcomplicated picture.

Energy diagrams / energy levels are often used to represent MOs in the molecule.

MOs in the Hydrogen Molecule

Chapter 9 Section 6

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Energy diagrams / energy levels are often used to represent MOs in the molecule.

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Bond Order

Chapter 9 Section 6

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bond order = 1/2 (2 – 0) = 1single bond

bond order = 1/2 (2 – 2) = 0no bond

Bond Order

The higher the value of the bond order, the more stable the molecule and the stronger the bond.

Chapter 9 Section 6

g

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bond order = 1/2 (2 – 0) = 1single bond

bond order = 1/2 (2 – 2) = 0no bond

MO theory predicts Li2 to be a stable molecule whereas Be2 to be not.

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Molecular Orbitals from p Atomic Orbitals

Chapter 9 Section 6

Some molecules, such as B2, combine their pAOs to generate new MOs. (B: 1s22s22p1).g ( p )

Recall the shape of the 2p orbitals.

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Molecular Orbitals from p Atomic Orbitals

Chapter 9 Section 6

Two pairs of parallel p orbitals can overlap sideways, and the third pair can overlap head-on.y , p p

σ2p

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π2p

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Head-On Overlap for p MOs

Chapter 9 Section 6

σ*2

node

Th bi l h l h d d MO

σ2p

σ 2p

Ene

rgy

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The two p orbitals that overlap head-on produce two σ MOs:

one bonding, σ2p (constructive combination), and

one antibonding, σ*2p, (destructive combination).

Head-On Overlap for p MOs

Chapter 9 Section 6

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Both bonding σ2p and antibonding σ*2p MOs can be shown

together in one diagram. They would look kind of complicated.

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Sideway Overlap for p MOs

Chapter 9 Section 6

node

π*2p

π2p

2p

Ene

rgy

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Two p orbitals that lie parallel overlap to produce two π MOs:

one bonding , π2p (constructive combination), and

one antibonding, π*2p (destructive combination).

Sideway Overlap for p MOs

Chapter 9 Section 6

The Bonding π2p and antibonding, π*2p MOs can be shown

together in one diagram They would look kind of

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together in one diagram. They would look kind of complicated.

Remember that you have two Bonding π2p MOs (along the yand z-axes) and two antibonding, π*

2p MOs (also along the yand z-axes), giving four π MOs in total.

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Molecular Orbital Diagram

Chapter 9 Section 6

MOs generated from the combination of pthe combination of pAOs are always higher in energy than MOs generated form the combination of sAOs.

Antibonding MOs

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Antibonding MOs are higher in energy than bonding MOs.

Molecular Orbital Diagram

Chapter 9 Section 6

The order of MO energies assumes noenergies assumes no interaction taking place between p and sorbitals.

In atoms of smaller nuclear charges (Li, B C and N) the s

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B, C and N) the sorbitals are held less tightly by the nucleus and some s-pinteraction takes place. O2, F2, Ne2 Li2, B2, C2 , N2

s and p orbitals do not interact

s and p orbitals do interact

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Molecular Orbital Diagram of O2

Chapter 9 Section 6

When filling the MO levels, you have to:y

Count the number of valence electrons,

Start with the lower energy orbitals first,

Follow Hund’s rule, and

P t t th t

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Put not more than two electrons in one MO.

Molecular Orbital Diagram of N2

Chapter 9 Section 6

The order of these two MOs are switched

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Paramagnetism and Diamagnetism

Paramagnetism causes the substance to be attracted to a

Chapter 9 Section 6

magnetic field.

Diamagnetism causes the substance to be repelled from a magnetic field.

Paramagnetism is

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Paramagnetism is associated with unpairedelectrons.

Diamagnetism is associated with paired electrons.

Liquid oxygen, O2(l), is attracted to the poles of a

magnet because O2 is paramagnetic.

Molecular Orbital Diagram

Molecular orbital theory helps

Chapter 9 Section 6

Molecular orbital theory helps you predict several important properties of the substance.

Bond order (bond length and bond strength).

Bond enthalpy (bond energy)

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Magnetic properties.

MO diagram of O2

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Molecular Orbital Diagrams

Chapter 9 Section 6

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Exercises

Chapter 9 Section 6

Predict the bond order and magnetism of Ne2.

Bond order

= (8 – 8) / 2 = 0

According to the MO theory, Ne2 doesn’t exist.

Ne2

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Exercises

Chapter 9 Section 6

Give the electron configurations and bond orders for O2, O2

+, and O2–.

O2 O2+ O2

–

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Exercises

Chapter 9 Section 6

Predict the bond order and magnetism of P2.

Bond order

= (8 – 2) / 2 = 3

According to the MO theory, P2 exists and it is diamagnetic.

P2

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Bonding in Heteronuclear Diatomic Molecules

Chapter 9 Section 6

MO model can be expanded to diatomic pmolecules with two different nuclei whose electronic natures are not so different.

Carbon monoxide (CO) is expected from

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MO theory to be diamagnetic and to have a bond order of:

(8 – 2) / 2 = 3The MO energy-level

diagram of the CO molecule

Descriptions of Molecules with Delocalized Bonding

Localized electron model assumes that the bonding electron pair is being shared between two atoms (localized).

Chapter 9 Section 7

p g ( )

In several cases, such as O3 and NO3–, the π-electron

bonding pairs are delocalized and are not present at a specific location in the molecule.

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Descriptions of Molecules with Delocalized Bonding

In molecules with resonance structures,

Chapter 9 Section 7

σ bonds can be viewed to be localized, and

π bonds can be considered to be delocalized.

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Each C atom has 3electron domains

Example: Benzene (C6H6)

Chapter 9 Section 7

electron domains

Hybridization: sp2

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