1 © 2009 Brooks/Cole - Cengage Advanced Theories of Chemical Bonding Chapter 9 Atomic Orbitals...

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© 2009 Brooks/Cole - Cengage

Advanced Theories of Advanced Theories of Chemical BondingChemical Bonding

Chapter 9Chapter 9

Atomic OrbitalsAtomic Orbitals MoleculesMolecules

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• MOLECULAR MOLECULAR ORBITAL THEORYORBITAL THEORY — — Robert Mullikan (1896-Robert Mullikan (1896-1986)1986)

• valence electrons are valence electrons are delocalizeddelocalized

• valence electrons are valence electrons are in orbitals (called in orbitals (called molecular orbitals) molecular orbitals) spread over entire spread over entire molecule.molecule.

Two Theories of BondingTwo Theories of BondingTwo Theories of BondingTwo Theories of Bonding

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Two Theories of BondingTwo Theories of BondingTwo Theories of BondingTwo Theories of Bonding

• VALENCE BOND THEORYVALENCE BOND THEORY — Linus Pauling— Linus Pauling

• valence electrons are valence electrons are localized between atoms localized between atoms (or are lone pairs).(or are lone pairs).

• half-filled atomic orbitals half-filled atomic orbitals overlap to form bonds.overlap to form bonds.

Linus Pauling, 1901-1994Linus Pauling, 1901-1994Linus Pauling, 1901-1994Linus Pauling, 1901-1994

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Sigma Bond Formation by Sigma Bond Formation by Orbital OverlapOrbital Overlap

Sigma Bond Formation by Sigma Bond Formation by Orbital OverlapOrbital Overlap

Two s orbitals Two s orbitals overlapoverlap

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Sigma Bond FormationSigma Bond FormationSigma Bond FormationSigma Bond Formation

Two s Two s orbitals orbitals overlapoverlap

Two p Two p orbitals orbitals overlapoverlap

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Using VB TheoryUsing VB TheoryUsing VB TheoryUsing VB TheoryBonding in BFBonding in BF33

planar triangleplanar triangleangle = 120angle = 120oo

FFFBoron configuration↑↑↓↑↓2p2s1s•••••••••••• ••••••B

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Bonding in BFBonding in BF33Bonding in BFBonding in BF33

• How to account for 3 bonds 120How to account for 3 bonds 120oo apart using apart using a spherical s orbital and p orbitals that are 90a spherical s orbital and p orbitals that are 90oo apart?apart?

• Pauling said to modify VB approach with Pauling said to modify VB approach with

ORBITAL HYBRIDIZATIONORBITAL HYBRIDIZATION• — — mix available orbitals to form a new mix available orbitals to form a new

set of orbitals — set of orbitals — HYBRID ORBITALSHYBRID ORBITALS — that will give the maximum overlap — that will give the maximum overlap in the correct geometry. in the correct geometry.

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Why Hybridize?Why Hybridize?

Just looking at valence electrons:

Be should form no covalent bonds

B should form one covalent bond

C should form 2 covalent bonds

But…

BeF2, BF3 and CF4

Exist! HOW?

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Hybrid Orbitals: Why?Hybrid Orbitals: Why?

• To explain the bonding in molecules like BeF2, BF3 and CF4, Linus Pauling proposed that orbitals become ‘hybridized’

– Hybrid orbitals are orbitals created by mixing the s, p or d orbitals of an atom.

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Hybrid Orbitals: The RulesHybrid Orbitals: The Rules

1. The number or hybrid orbitals is ALWAYS equal to the number of atomic orbitals that are combined to make the hybrid set

2. Hybrid orbital sets are always built by combining an s orbital with as many p or d orbitals necessary to accommodate the bonding and lone pairs on the central atom (Remember Electron Pair Geometry?)

3. The Hybrid Orbitals are directed TOWARDS the terminal atoms• This results in a better orbital overlap AND stronger

bonds between the central and terminal atoms

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spsp Hybrid Orbitals Hybrid Orbitals

Mix an s orbital with a p orbital to create two sp orbitals

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spsp22 Hybrid Orbitals Hybrid Orbitals

Mix an s orbital with 2 p orbitals to create three sp2 orbitals

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spsp33 Hybrid Orbitals Hybrid Orbitals

Mix an s orbital with 3 p orbitals to create four sp3 orbitals

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spsp33 Hybrid Orbitals: Examples Hybrid Orbitals: Examples

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rearrange electronshydridize orbs.

unused porbital

three sp 2 hybrid orbitals

2p2s

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• The three hybrid orbitals are made The three hybrid orbitals are made from 1 s orbital and 2 p orbitals from 1 s orbital and 2 p orbitals 3 sp 3 sp22 hybrids.hybrids.


• Now we have 3, half-filled HYBRID orbitals Now we have 3, half-filled HYBRID orbitals that can be used to form B-F sigma bonds.that can be used to form B-F sigma bonds.

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An orbital from each F overlaps one of the An orbital from each F overlaps one of the spsp22 hybrids to form a B-F hybrids to form a B-F bond. bond.


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BF3, Planar Trigonal BF3, Planar Trigonal

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Bonding in CHBonding in CH44

How do we account for 4 CHow do we account for 4 C

—H sigma bonds 109—H sigma bonds 109oo

apart? apart?

Need to use 4 atomic Need to use 4 atomic

orbitals — s, porbitals — s, pxx, p, pyy, and , and

ppzz — to form 4 new — to form 4 new

hybrid orbitals pointing hybrid orbitals pointing

in the correct direction.in the correct direction.

109o109o

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4 C atom orbitals 4 C atom orbitals hybridize to form hybridize to form four equivalent spfour equivalent sp33 hybrid atomic hybrid atomic orbitals.orbitals.


Bonding in a Tetrahedron Bonding in a Tetrahedron Formation of Hybrid Atomic OrbitalsFormation of Hybrid Atomic Orbitals

Bonding in a Tetrahedron Bonding in a Tetrahedron Formation of Hybrid Atomic OrbitalsFormation of Hybrid Atomic Orbitals

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are needed to see this picture.

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Bonding in a Tetrahedron — Bonding in a Tetrahedron — Formation of Hybrid Atomic Formation of Hybrid Atomic

OrbitalsOrbitals



QuickTime™ and aVideo decompressor


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Bonding in CHBonding in CH44Bonding in CHBonding in CH44

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OCOHHHNHHsp3sp3

sp3sp2••••••C

Bonding in Bonding in GlycineGlycine


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OCOHHHNHHsp3sp3

sp3sp2••••••C



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OCOHHHNHHsp3sp3

sp3sp2••••••C



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OCOHHHNHHsp3sp3

sp3sp2••••••C



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OCOHHHNHHsp3sp3

sp3sp2••••••C

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Orbital HybridizationOrbital HybridizationOrbital HybridizationOrbital Hybridization

2 e2 e-- clouds clouds

3 e- clouds3 e- clouds




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Multiple BondsMultiple BondsMultiple BondsMultiple BondsConsider ethylene, CConsider ethylene, C22HH44

C

H

H

H

H

sp2120° C

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Sigma Bonds in CSigma Bonds in C22HH44Sigma Bonds in CSigma Bonds in C22HH44

C

H

H

H

H

sp2120° C



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ππ Bonding in C Bonding in C22HH44ππ Bonding in C Bonding in C22HH44

The unused p orbital on The unused p orbital on each C atom contains an each C atom contains an electron and this p orbital electron and this p orbital overlaps the p orbital on overlaps the p orbital on the neighboring atom to the neighboring atom to form the π bond. form the π bond.

p orb.for πbond3 sp2hybrid orbitals↑↑↑↑↑↑↑↓2p2s

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ππ Bonding in C Bonding in C22HH44ππ Bonding in C Bonding in C22HH44

The unused p orbital on each C atom contains The unused p orbital on each C atom contains an electron and this p orbital overlaps the p an electron and this p orbital overlaps the p orbital on the neighboring atom to form the orbital on the neighboring atom to form the π bond.π bond.



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Multiple BondingMultiple Bondingin Cin C22HH44

Multiple BondingMultiple Bondingin Cin C22HH44



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and and ππ Bonding in Bonding in CC22HH44

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and and ππ Bonding in Bonding in CHCH22OO

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Consequences of Multiple Consequences of Multiple BondingBonding


There is restricted rotation around C=C bond.There is restricted rotation around C=C bond.

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Restricted rotation around C=C bond.Restricted rotation around C=C bond.



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Consequences of Multiple BondingConsequences of Multiple BondingConsequences of Multiple BondingConsequences of Multiple Bonding

Formation of IsomersFormation of Isomers

One isomer may have biological One isomer may have biological activity while the other may notactivity while the other may not

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Double Bonds and Double Bonds and VisionVision



1 © 2009 Brooks/Cole - Cengage Advanced Theories of Chemical Bonding Chapter 9 Atomic Orbitals...

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Transcript of 1 © 2009 Brooks/Cole - Cengage Advanced Theories of Chemical Bonding Chapter 9 Atomic Orbitals...