lecture 3 molecular shapes - university of...
TRANSCRIPT
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)