Dr. S. M. Condren Chapter 10 Bonding and Molecular Structure: Orbital Hybridization and Molecular Orbitals.

Dr. S. M. Condren

Chapter 10

Bonding and Molecular Structure:

Orbital Hybridization and Molecular Orbitals

Dr. S. M. Condren

Advanced Theories of Chemical Bonding

Atomic OrbitalsAtomic Orbitals MoleculesMolecules

Dr. S. M. Condren

• MOLECULAR ORBITAL THEORY — —

• Robert Mullikan (1896-1986)Robert Mullikan (1896-1986)

• valence electrons are valence electrons are delocalizeddelocalized

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

Two Theories of Bonding

Dr. S. M. Condren

Two Theories of Bonding

• VALENCE BOND THEORY — Linus — Linus PaulingPauling

• 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.

Dr. S. M. Condren

Sigma Bond Formation by Orbital Overlap

Two s orbitals Two s orbitals overlapoverlap

Dr. S. M. Condren

Two s Two s orbitals orbitals overlapoverlap

Two p Two p orbitals orbitals overlapoverlap

Sigma Bond Formation

One s One s and one p and one p orbitalorbitaloverlapoverlap

Dr. S. M. Condren

Using VB Theory

Bonding in BFBonding in BF33

planar triangleplanar triangleangle = 120angle = 120oo

F

F F

Boron configuration

2p2s1s•• ••

••••

••

•• ••

••••

B

Dr. S. M. Condren

• How to account for 3 bonds 120How to account for 3 bonds 120oo apart using a apart using a spherical s orbital and p orbitals that are 90spherical 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 HYBRIDIZATION• — — mix available orbitals to form a new set mix available orbitals to form a new set

of orbitals — of orbitals — HYBRID ORBITALS — that — that will give the maximum overlap, in the will give the maximum overlap, in the

correct geometry.correct geometry.

Bonding in BF3

Dr. S. M. Condren

Bonding in BF3

rearrange electronshydridize orbs.

unused porbital

three sp2 hybrid orbitals

2p2s

Dr. S. M. Condren

• The three hybrid orbitals are made from 1 The three hybrid orbitals are made from 1 s orbital and 2 p orbitals 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.

Bonding in BF3

Dr. S. M. Condren

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.

B

F

F

F

B

F

F

F

B

F

F

F

B

F

F

F

Bonding in BF3

Dr. S. M. Condren

BF3, Planar Trigonal

Dr. S. M. Condren

Bonding in CH4

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 in hybrid orbitals pointing in

the correct direction.the correct direction.

109o109o

Dr. S. M. Condren

Bonding in CH4

Dr. S. M. Condren

Bonding in a Tetrahedron — Formation of Hybrid Atomic Orbitals

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

Dr. S. M. Condren

Dr. S. M. Condren

O

CO H

H

H

NH

Hsp3

sp3

sp3

sp2

••

••••C

Bonding in Glycine

Dr. S. M. Condren

BONDSBONDS SHAPESHAPE HYBRID HYBRID

REMAINREMAIN

22 linearlinear sp sp 2 p’s2 p’s

33 trigonaltrigonal sp sp22 1 p1 pplanarplanar

44 tetrahedral sptetrahedral sp33 nonenone

Orbital Hybridization

Dr. S. M. Condren

C

H

H

H

H

sp2120Þ C

Multiple Bonds

Consider ethylene, CConsider ethylene, C22HH44

Dr. S. M. Condren

C

H

H

H

H

sp2120Þ C

Sigma Bonds in C2H4

Dr. S. M. Condren

π π Bonding in C2H4

The unused p orbital on each The unused p orbital on each C atom contains an 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 šbond

3 sp 2

hybrid orbitals

2p2s

Dr. S. M. Condren

π Bonding in C2H4

The unused p orbital on each C atom contains an The unused p orbital on each C atom contains an electron and this p orbital overlaps the p orbital electron and this p orbital overlaps the p orbital

on the neighboring atom to form the π bond.on the neighboring atom to form the π bond.

Dr. S. M. Condren

Multiple Bondingin C2H4

Dr. S. M. Condren

and π Bonding in C2H4

Dr. S. M. Condren

and π Bonding in CH2O

Dr. S. M. Condren

and π Bonding in C2H2

Dr. S. M. Condren

and π Bonding in C2H2

Dr. S. M. Condren

Consequences of Multiple BondingThere is restricted rotation around C=C bond.There is restricted rotation around C=C bond.

Dr. S. M. Condren

Consequences of Multiple Bonding

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

Dr. S. M. Condren

Diatomic MoleculesAO MO AO

H H2 H

1s

ENERGY

1s

B - AB 2 - 0B.O. = ------------ = -------- = 1 2 2all electrons are paired, thus diamagnetic

B => bonding electrons

AB => antibonding electrons

Dr. S. M. Condren

Diatomic MoleculesAO MO AO

He He2 He

1s

ENERGY

1s

B - AB 2 - 2B.O. = ------------ = -------- = 0 2 2

Zero bond order means it does not exist

Dr. S. M. Condren

Diatomic MoleculesAO MO AO

He He2 He+

1s

ENERGY

1s

B - AB 2 - 1B.O. = ------------ = -------- = 1/2 2 2

One unpaired electron, thus, paramagnetic

+

Dr. S. M. Condren

1s 1snonbonding

AOB

AOB

MOB2

2s 2s

2p 2p

Dr. S. M. Condren

Molecular Orbital Diagram

side-by-side

end-to-end

bonding MO

bonding MO

antibonding MO

antibonding MO

Dr. S. M. Condren

1s 1snonbonding

AOC

AOC

MOC2

2s 2s

2p 2p

Dr. S. M. Condren

1s 1snonbonding

AOO

AOO

MOO2

2s 2s

2p 2p

Dr. S. M. Condren

Ene

rgy