1

Valence Bond Theory vs. MO TheoryVB Theory begins with two steps:

1) hybridization AOs on atoms participating in bonding

2) Combination of hybrid orbitals to make bonds.

Key differences between MO and VB theory:1) MO theory: has electrons distributed over whole molecule. (molecule

centered)

VB theory: localizes an electron pair between two atoms. (bond centered)

2) MO theory: combines AOs between different atoms to make MOs (LCAO) VB theory: combines AOs on the same atom to make hybridized atomic orbitals (hybridization)

3) MO theory: the symmetry must be retained in each orbital. VB theory: all orbitals must be viewed simultaneously to see retention of the molecule’s symmetry.

VB Theory of BeH2

Two hybrid atomic orbitals are made to fit the shape of the molecule, in this case linear, using atomic orbitals of the Be atom!

unhybridzed2

2( )

p

sp

2

2

p

s

Be 2 H

sp hybrids1s 1s

2 Be-H bonds

2 Be-H "antibonds"

2px 2py 2px 2py

Unused AO are left behind as unhybridized atomic orbitals

The energy of the hybrid atomic orbitals are intermediate between those of the original constituent AO’sThe hybrid orbitals combine with other orbitals, atomic or hybrid, creating both bonding and anti-bonding molecular orbitals, which are localized molecular orbitals

VB Theory of BH3 unhybridzed

2

2

2( )

p

sp

2

2

p

s

B 3 H

sp2 hybrids1s 1s 1s

3 B-H bonds

3 B-H "antibonds"

2py 2py

BH3 is trigonal planar with three equal B—H bonds

To get this shape the 2s with two 2p AO’s to generate three equivalent hybrid atomic orbitalsCombination with the H 1s leads to bonding and anti-bonding molecular orbitals, which are localized molecular orbitals pointing to the corners of a triangle

VB Theory of CH4 no unhybridzed orbitals in 2nd shell

3

2( )sp

2

2

p

s

C 4 H

sp3 hybrids1s 1s 1s 1s

4 C-H bonds

4 C-H "antibonds"

CH4 is tetrahedral with 4 equal C-H bondsTo get this shape, we need to combine all the n=2 AO’s to generate four equivalent hybrid atomic orbitals

In combination with the H 1s leads to bonding and anti-bonding molecular orbitals localized and pointing to the corners of a tetrahedron

5

Valence Bond Theory Summary

Atoms orbitals are hybridized only if it’s necessary to attain the observed geometry and bond lengths.

Terminal atoms are not typically hybridized.

Geometry determines hybridization:

i) Linear (180º) = sp (s+p with two leftover p orbitals)

ii) Trigonal planar (120º) = sp2 (s+p+p with one leftover p orbital)

iii) Tetrahedral (109.5º) = sp3 (s+p+p+p with no leftover p orbitals)

Hybrid orbitals combine with each other to make bonds in which two electrons are localized between two atoms.

bonds are made by combining the unused -symmetric p orbitals.

Ethane

C C

H

H

H

H

H

H

= H 1s = C sp3

VSEPR theory requires both carbon atoms to be tetrahedral

The shape of the molecule, requires that contacts be minimized between the atoms – this is known as the staggered conformationBonding is explained by using sp3 hybrid orbitals on each C.

The H atoms bond using their 1s atomic orbitals

In all there are 14 electrons or 7 electron pair bonds in the molecule

1 C-C sp3-sp3 single bond and 6 C-H sp3-s single bonds are formed

Double bonds: ethene

= H 1s = C sp2

2px 2px

The sigma skeleton of ethene

The pi manifold of ethene

Change perspective to show the π bond!

If we treat ethane by the VSEPR theory, we find that both carbon atoms are trigonal planarThe molecule is planar. Why ?sp2 hybrid orbitals on each carbon atom, which leaves one atomic p orbital unused on each C atom, while H atoms use their 1s atomic orbitals

There are 6 e’ pair bonds in the molecule, 5 in σ orbitals, 1 in the π orbital

1 sp2-sp2 C-C bond, 1 px-px C-C bond , and 4 sp2-s C-H bonds

VB Theory of H2Ono unhybridzed orbitals in 2nd shell

3

2( )sp

2

2

p

s

H2O is bent and belongs to the tetrahedral family with 2 BP and 2 LPThe s and p orbitals combine sp3 hybrids

The 6 e’s from O, singly occupy 2 sp3 orbitals and doubly occupy the remaining 2 as LP’s

O 2 H

sp3 hybrids1s 1s

2 C-H

2 LP’s

The 2 sp3 orbitals combine with 2 1s orbital to form 2 C-H bonds

9

Triple Bonds: EthyneThe Lewis structure for ethyne (C2H2)

VSEPR theory: each carbon atom is linear hence sp hybridized

i) C–H bonds: combination of an sp orbital from C and a 1s orbital from H.

ii) C–C bonds: combination of sp orbitals from each C.iii) C–C bonds: combination of 2p orbitals from each C.

10

FormaldehydeThe Lewis structure for formaldehyde (CH2O) .

VSEPR theory: Carbon atom is trigonal planar hence sp2 hybridized:

i) C–H bonds: combination of an sp2 orbital from C and a 1s orbital from H.ii) C–O bond: combination of an sp2 orbital from C and a sp2 orbital from O. iii) C–O bond: combination of 2p z orbitals from C and O.iv) Lone pairs: remaining sp2 hybrid AOs of O.

11

VB Theory of HCNThe Lewis structure for hydrogen cyanide (HCN) is.

VSEPR theory: C and N atoms are linear hence are sp hybridized.

i) C–H bonds: combination of an sp orbital from C and a 1s orbital from H.ii) C–N bond: combination of an sp orbital from C and a 2p orbital from sp ybrid orbital on N. iii) C–N bonds: combination of 2px and 2py orbitals from C and N.iv) Lone pair: remaining sp hybrid atomic orbital (2s).

Bonding in large molecules

sp3

sp3

sp3

sp3sp3

sp2

sp2

sp3-sp3 O-C bond

s-sp3 H -O bond

2 s-sp3 H-C bonds

sp3-sp3 C-N bond s-sp3 H-C bonds

2 s-sp3 N-H bonds 1 sp3 LP sp3-sp2 C-C bond sp3-sp3 C-C bond

sp2-sp2 O-C bond p-p O-C bond

sp2-sp3 C-O bond

s-sp3 H-O bond

2 sp2 LP’s