Coordination Chemistry(3) Ligand Field Theory

1
Coordination Chemistry(3)Ligand Field Theory
Fred J. Grieman

• Molecular orbital theory as before
• 2) ?-bonding gives eg and t2g 2-3 splitting of
  d orbitals
• 3) ?-bonding explains strong and weak field
  ligands

• ? c1(2sA) c2(2sB)
• ? ? c1(2sB) c2(2sA)

2sA ? ? 2sB
coefficients!!! c1 gt c2
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Octahedral Case Ligands along x, y, z
axes ?-bonding Molecular Orbital Theory
(LCAO-MO) applied to
Coordination Compounds
Ligand Field Theory Metal Atom Orbitals?
(ligands along x, y, z axes) (4s,
4px, 4py, 4pz, 3dz2, 3dx2-y2) Ligands
(6 lone pair orbitals)
y
y
NH3
x
NH3
x
Metal dx2-y2
Metal dxy
Right symmetry for ? interaction
Wrong symmetry for ? interaction
So, (4s, 4px, 4py, 4pz, 3dz2, 3dx2-y2) 6
lone pair orbitals interact to form sigma
bonds dxy, dxz, dyz do NOT
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Octahedral Correlation and Molecular Orbital
Energy Diagram ? - bonding
Ligand
Molecule
Metal Ion
?p
???
?s
4p
c43dx2-y2 c43dz2 - c3 NH3 (sp3)
c4 gt c3
?d
eg
4s
3d
dxy, dxz, dyz
t2g
6 Ligands NH3 sp3 orbitals
d e-s
6 coordinate covalent bonds
?oct E(?d) E(3d)
?? ?? ?? ?? ?? ??
6? c14s c24px c24py c24pz
c33dx2-y2 c33dz2
c4NH3 (sp3) c4 gt c1, c2, c3
6?
antibonding orbital!!!
non-bonding d orbitals
Localized Bonding Model d2sp3 hybridized
orbitals on metal form sigma bonds with NH3 sp3
orbitals
Molecular orbital ? - bonding model gives same
2-3 splitting pattern as Crystal Field Model
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Extent of splitting from ? - bonding Weak and
Strong Field ligands Consider Cl- (weak) and CN-
(strong)
? - bonding as before Now ? - bonding between p
dxy, dxz, dyz
Cl
M
sp hybridized for ?-bonding, left over p
orbitals make ? and ? orbitals
? - bonding as before Now ? - bonding between CN-
? dxy, dxz, dyz No ? - bonding with CN- ?
M
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? - bonding p orbitals give Weak Field Ligands
(Cl- example)
Ligand
Molecule
Metal
Cl
Input d e-s
?p
E (Cl- p) lt E (M d) !!!
?s
4p
?d
eg
4s
?-antibonding orbitals
12 Cl- p orbitals
3d
dxy, dxz, dyz
t2g
?? ?? ?? ?? ?? ??
?? ?? ?? ?? ?? ??
6 Ligands Cl- sp orbitals
?-bonding orbitals
Decrease ?oct ? Weak Field Ligand
?? ?? ?? ?? ?? ??
6?
6
? - bonding ? orbitals give Strong Field Ligands
Ligand
Molecule
Metal
?- antibonding orbitals
12 CN- ? orbitals
?p
Input d e-s
?s
4p
E (CN- ? ) gt E (M d) !!!
?d
eg
4s
3d
dxy, dxz, dyz
t2g
6 Ligands CN- sp orbitals
Increase ?oct ? Strong Field Ligand
?-bonding orbitals
?? ?? ?? ?? ?? ??
6?
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• Molecular Orbital Theory Explains Field Strength
  of Ligands
• Ligand p orbitals cause ? - bonding that raises
  t2g energies
• Weak Field Ligands
• 2) Ligand ? orbitals cause ? - bonding that
  lowers t2g energies
• Strong Field Ligands
• 3) sp3 hybridized ligands do not change t2g
  orbitals very much
• Medium Field Ligands

?d
eg
?
12 Cl- p orbitals
dxy, dxz, dyz
t2g
12 CN- ? orbitals
?d
eg
dxy, dxz, dyz
t2g
?
No p or ? orbitals for ?-bonding !!!
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Molecular orbital model for square planer and
tetrahedral similar, but a little more
complicated Square planar is dsp2 hybridized
(4s, 4px, 4py, 3dx2-y2) Tetrahedral is sp3
hybridized Next explain properties
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