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CH2. Molecules and covalent

bondingLewis Structures

VSEPR

MO Theory

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Lewis structure H3PO4

• Skeleton is:

• Count total valence electrons:1 P = 5

3 H = 3

4 O = 24

Total = 32 e- or 16 valence e- pairs.

• 7 e- pairs needed to form s skeleton.

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Lewis structure H3PO4• Add remaining e- pairs:

• Left has a formal charge of +1 on P and -1 on one O, right has 5 e- pairs around P (hypervalence)

• Analysis of phosphoric acid shows purely Td phosphate groups, which requires something beyond either simple Lewis model.

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Resonance in NO3-

experimental

data - nitrate is

planar with 3

equivalent N-O

bonds

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VSEPR model

• Count e- pairs about the central atom (draw Lewis structure if needed). Include non-bonding pairs, but not multiple bonds.

• Geometry maximizes separation:

# e pairs geometry example

2 linear HF2-

3 equilateral triangular BF3

4 tetrahedral (Td) CF4

5 trigonal bipyramidal (TBP) PF5

6 octahedral (Oh) SF6

7 pentagonal bipyramidal IF7

8 square antiprismatic TaF83-

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Drawing Oh and Td molecules

It's often useful to draw octahedra and

tetrahedra with a cubic framework

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Deviations from ideal geometries:

unshared pairs and multiple bonds require larger bite

ex: CH4, NH3, H2O

<H-C-H = 109.5°,

<H-N-H = 107.3,

<H-O-H = 104.5

ex: ICl4-

6 e pairs around I, 2 lone pairs and 4 e pair bonds to Cl

Oh coordination, and geometry is square planar (lone pairs are trans, not cis)

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POCl3

based on Td geometry

< ClPCl = 103.3° due to

repulsion by multiple bond

note that in :PCl3the <ClPCl = 100.3,

the lone pair is

more repulsive

towards other

ligands than the

multiple bond !

Ligands move away from multiple bond

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XeF5+

5 Xe-F bonds and 1 lone

pair on Xe

geometry based on Oh

coordination

lone pair repulsion gives

< FeqXeFeq = 87°

< FaxXeFeq = 78°

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Fajan’s rule

bond polarization is towards

ligands with higher c,

decreasing repulsive effect.

Lone pairs are the most

repulsive.

ex: NH3 vs NF3

< HNH = 107.3°

< FNF = 102.1°

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Inert pair effect

• VSEPR geometries require

hybridization (valence bond term) or

linear combinations (MO term) of

central atom orbitals. For example,

Td angles require sp3 hybrid orbitals.

More on this in MO theory section.

• Period 5 and 6 p-block central atoms

often show little hybridization (ex:

they form bond with orbitals oriented

at 90° as in purely p orbitals). This

can be ascribed to the weaker

bonding of larger atoms to ligands.

In Sn Sb Te

Tl Pb Bi

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Inert pair effect - evidence• Bond angles near 90°:

NH3 107.2 H2O 104.5

AsH3 91.8 H2Se 91

SbH3 91.3 H2Te 89.5

• Increased stability of lower oxidation statesex: Si, and Ge are generally 4+, but Sn and Pb are common as 2+ ions (as in stannous fluoride SnF2)

ex: In and Tl both form monochlorides, B, Al, Ga form trichlorides.

• Vacant coordination sites where the lone pair resides

ex: PbO

PbO unit cell

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Fluxionality

• PF5 if TBP has 2 types of F ligands (equatorial and axial).

• 19F NMR spectra at RT show only a single peak (slightly broadened).

• PF5 is fluxional at RT, i.e. the F ligands exchange rapidly, only a single "average" F ligand is seen by NMR.

• Only occurs if ligand exchange is faster than the analytical method. IR and Raman have shorter interaction times and show 2 types of P-F bonding at RT.

• At low T, exchange is slower and we see 2 NMR peaks in expected 3:2 intensity ratio.

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Berry pseudo-rotation

Sequences of the MD-Simulation of PF5 at 750K

(Daul, C., et al, Non-empirical dynamical DFT calculation

of the Berry pseudorotation of PF5, Chem. Phys. Lett.

1996, 262, 74)

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Molecular Orbitals

Use linear combinations of atomic orbitals to derive symmetry-adapted linear combinations (SALCs).

Use symmetry to determine orbital interactions.

Provide a qualitative MO diagram for simple molecules.

Read and analyze an MO diagram by sketching MO’s / LCAO’s, describing the geometric affect on relative MO energies.

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H2

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Some rules

The number of AO’s and MO’s must be equal.

This follows from the mathematics of independent

linear combinations.

More on symmetry labels later, but they come

from the irreducible representations for the point

group. s MO’s are symmetric about bond axis, p

MO’s are not. Subscipt g is gerade (has center of

symmetry), u is ungerade. Antibonding orbitals

are often given a * superscript.

The bond order = ½ (bonding e- - antibonding e-).

The bond energy actually depends on the

energies of the filled MO’s relative to filled AO’s.

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O2

• MO theory predicts 2 unpaired e-,

this is confirmed by experiment.

• Bond order = ½ (8-4) = 2, as in

Lewis structure.

• MO indicates distribution and

relative energies of the MO's, Lewis

structure says only bonding or non-

bonding.

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I and Ea for atoms and diatomics

species I (kJ/mol) Ea

N 1402

O 1314 142

O2 1165 43

NO 893

F 1681

F2 1515

C 1086 123

C2 300

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Li2 – F2 MO’s

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Some diatomic bond data

bond order r0 in pm D0 in kJ/mol

O2 2 121 494

O2- 1 ½ 126

O22- 1 149

F2 1 142 155

O2+ 2 ½ 112

NO 2 ½ 115

NO+ 3 106

N2 3 110 942

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Spectroscopic data for MO’s

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HF

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Ketalaar triangle

HF

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Hybridization

• Linear combinations of AO’s from same atom

makes hybrid orbitals.

• Hybridization can be included in the MO

diagram.

• In MO theory, any proportion of s and p can

be mixed (the coefficients of the AO’s are

variable). sp and sp3 hybrids are specific

examples.

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H3+

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BeH2

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Correlation diagram for MH2

M < HMH

Be 180°

B 131

C 136

N 103

O 105

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Bonding MO’s in H2O

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NH3

Use triangular H3 MO’s from

above as SALC's of the H

ligand orbitals. Must relabel

to conform with lower

symmetry pt group C3v. They

become a1 and e.

Combine with N valence

orbitals with same symmetry.

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NH3 --calculated MO diagram

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SF6

See textbook Resource Section 5 for SALCs