Bonding and Molecular Structure

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Bonding and Molecular Structure
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Valence Bond Theory

• In covalent bonding, orbitals overlap
• Most primitive overlap between 2 s-orbitals
• sigma (s) bond
• Electron density greatest along bond axis

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VB Theory Tenants

• Orbitals overlap to form a bond between 2 atoms
• 2 electrons, of opposite spin, can be
  accommodated in the overlapping orbitals
• Higher probability of electrons being found in
  space influenced by both nuclei

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What about more complex VBing?

• HF still forms sigma bond, but skewed
• Why?
• Which valence electrons of F are involved in
  sigma bond? (Hint whats Fs electron
  configuraiton?)
• s-orbital bonds with p-orbital of F to form sigma
  bond

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What about VBing in orbitals above 1s?

• Orbital hybridization
• Mixing of s-, p-, and d-orbitals within atom to
  form molecule
• Two rules
• 1) hybrid orbitals atomic orbitals mixed
• 2) hybrid orbitals more directed from central
  atom toward terminal atoms than unhybridized
  atomic orbitals ? better orbital overlap
  stronger bond between central terminal atoms
• Mixed hybridized orbitals yield electron-pair
  geometry (VSEPR)

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Hybridized Orbitals

• 2 electron pairs
• s-orbital and 1 p-orbital mix ? sp (linear)
• 3 electron pairs
• s-orbital and 2 p-orbitals mix ? sp2 (trigonal
  planar)
• 4 electron pairs
• s-orbital and 3 p-orbitals mix ? sp3
  (tetrahedral)
• 5 electron pairs
• s-orbital, 3 p-orbitals, and 1 d-orbital mix ?
  sp3d (trigonal bipyramidal)
• 6 electron pairs
• s-orbital, 3 p-orbitals, and 2 d-orbitals mix ?
  sp3d2 (octahedral)

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Hybridized Orbitals sp3
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What about electron geometries?

• For instance, NH3, H2O?
• Lone pairs fill hybridized orbitals
• So, NH3 sp3
• And, H2O sp3

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Practice

• Give the hybridization of and energy diagram for
• BeCl2
• BBr3

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Double bonds

• 1 unhybridized p-orbital (on each atom) allows
  for overlap
• Electron densities above and below sigma bonds
  (but not on axis like sigma bonds)
• pi-bond
• Can only exist if all atoms lie on same plane
• Ethylene

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Ethene
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Triple bonds

• 2 unhybridized p-orbitals (on each atom) allow
  for overlap
• 2 pi-bonds formed
• Perpendicular to one another

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Ethyne
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Recapitulate

• Double bond consists of sigma- and pi-bonds
• Triple bond consists of sigma- and 2 pi-bonds
• Pi-bond if unhybridized p-orbitals remain on
  bonding atoms
• Multiple bonds entail either sp or sp2
  hybridization