CH2. Molecules and covalent bonding

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CH2. Molecules and covalent bonding Lewis
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
• ltH-C-H 109.5,
• ltH-N-H 107.3,
• ltH-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
• lt ClPCl 103.3 due to repulsion by multiple
  bond

note that in PCl3 the ltClPCl 100.3, the lone
pair is more repulsive towards other ligands than
the 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
• lt FeqXeFeq 87
• lt FaxXeFeq 78

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Fajans rule

• bond polarization is towards ligands with
  higher c, decreasing repulsive effect. Lone pairs
  are the most repulsive.
• ex NH3 vs NF3
• lt HNH 107.3
• lt 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.
• Even low temp NMR studies cannot resolve two F
  environments

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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 MOs
  / LCAOs, describing the geometric affect on
  relative MO energies.

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H2
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Some rules

• The number of AOs and MOs 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 MOs are symmetric about bond axis, p
  MOs 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 MOs relative to filled AOs.

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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
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Li2 F2 MOs
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Some diatomic bond data
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Spectroscopic data for MOs
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HF
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Ketalaar triangle
HF
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Hybridization

• Linear combinations of AOs 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 AOs 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 lt HMH Be 180 B 131 C
136 N 103 O 105
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Bonding MOs in H2O
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NH3
Use triangular H3 MOs 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