Bonding in Molecules

Lecture 3

Molecular shapes

Bent8OH2, SH2, NH2-, FH2

+

Bent7NH2, PH2, CH2-, OH2

+

Bent6CH2, SiH2, BH2-, NH2

+

Bent5BH2, AlH2, CH2+

Linear4BeH2, BH2+

Linear3LiH2, BeH2+

Bent2 LiH2+

ShapeNo. of valence electronsMolecular species

Known shape of some AH2 molecules

118FH2+

92.1SH2

104.5OH2

103NH2

93SiH2

110CH2

131BH2

angleMolecular species

D∞h C2v

Walsh diagram

better overlap of H 1s orbitals

poorer overlapof H1s with 2px

remains non-bonding

First order effects

Walsh diagram

mixing can occurbetween 2 orbitalsbecause they both transform as a1 in the lower symmetry

A second order effect(often called a secondorder Jahn Teller distortion)

1

E∝

∆

Walsh diagram

Bent8OH2, SH2, NH2-, FH2

+

Bent7NH2, PH2, CH2-, OH2

+

Bent6CH2, SiH2, BH2-, NH2

+

Bent5BH2, AlH2, CH2+

Linear4BeH2, BH2+

Linear3LiH2, BeH2+

Bent2 LiH2+

ShapeNo. of valence

electrons

Molecular species

Known shape of some AH2 molecules

Recall: a molecule adopts the structure that best stabilises the HOMO. If the HOMO is unperturbed by the structural change under consideration, then the occupied MO lying closest to it governs the geometric preference.

Walsh diagram

90TeH2

90.6SeH291AsH2

92.1SH292PH293SiH2119AlH2

104NH2-99CH2-102BH2-

118FH2+110.5OH2

+115-120NH2+

104.5OH2103NH2110CH2131BH2

2a12 1b1

22a12 1b1

12a122a1

11a12 1b2

2 +

OH2 vs SH2 vs SeH2?

VSEPR?

Decreased s/p hybridisation?(note bonding with pure p orbitals implies 90°angles)

D∞h C2v

2 x ‘sp’ hybrids + 2 x p lone pairs 3 x ‘sp2’ hybrids + 1 x p lone pair

more p character in bonding orbitals

Strength of 2nd order JT effect proportional to 1/∆E

2 factors to consider:

Decrease in electronegativity destabilises πu

3/4s orbitals of S/Se are lower than 2s of O (increased penetration) and alsooverlap more weakly with H 1s because they are diffuse. Both factors stabilise σg

both make 2nd order JT effect stronger in the heavier elements

2nd order JT effect mixes s character (originally in 2σg) into the lone pair (1πu) so more p character accumulates in the bonding orbitals.

PES revisited: origin of fine structure

PES revisited: origin of fine structure

PES revisited: HF

(0,0) band most intense

higher band most intense narrow band

PES revisited: H2O

(0,0) band most intense

The shapes of H3+ and H3

-: 1st order JT effects, 3-centre-4-electron and 3-centre-2-electron bonds

Consider MOs for the 2 limiting shapes for a 3-atom system: linear (D

∞h) and triangular (D3h)

(using the SALCS given in the handout)

D3h

The shapes of H3+ and H3

-: 1st order JT effects, 3-centre-4-electron and 3-centre-2-electron bonds

Consider MOs for the 2 limiting shapes for a 3-atom system: linear (D

∞h) and triangular (D3h)

D∞h

Walsh diagram

increasing constructive overlap of terminal H 1s

increasing destructive overlap of terminal H 1s

Walsh diagram

H3+

(a 3-centre-2-electron bond)c.f. electron deficient species

such as boranes

H3-

(a 3-centre-4-electron bond)c.f. hypervalent species

such as HF2-, XeF2

orbitally degeneratetherefore a first-orderJT distortion

(1st order) Jahn-Teller theoremIf a non-linear molecule is in an orbitally degenerate state, it will

distort to remove the degeneracy

Bonding in Hypervalent Molecules

XeF2 (‘hypervalent’ = 10 electrons on Xe)

Possible explanations?

Ionic resonance forms?

sp3d hybridisation?

Bond order 0.5

Symmetry analysis (D∞h)