Coordination Chemistry3 Ligand Field Theory

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

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

• ?
• ? ?

2sA ? ? 2sB
coefficients!!! c1 c2
2
Octahedral Case Ligands along ?-bonding
Molecular Orbital Theory (LCAO-MO) applied to
Coordination Compounds Metal Atom Orbitals?
(ligands along x, y, z axes) (

) Ligands
y
y
NH3
x
NH3
x
Metal dx2-y2
Metal dxy
So, (
) 6 interact to form
bonds
do NOT
3
Octahedral Correlation and Molecular Orbital
Energy Diagram ? - bonding
Ligand
Molecule
Metal
?p
???
?s
4p
?d
eg c4 c3
4s
3d
dxy, dxz, dyz
t2g
6 Ligands NH3 sp3 orbitals
6 coordinate covalent bonds
?oct
6? c4 c1, c2, c3
6?
antibonding orbital!!!
non-bonding d orbitals
Molecular orbital ? - bonding model gives same
2-3 splitting pattern as Crystal Field Model
4
Extent of splitting from ? - bonding Weak and
Strong Field ligands Consider Cl- (weak) and CN-
(strong)
? - bonding as before Now ? - bonding between
Cl
M
sp hybridized for ?-bonding, left over
orbitals make orbitals
? - bonding as before Now ? - bonding between No
? - bonding with
M
5
? - bonding p orbitals give Weak Field Ligands
(Cl- example)
Ligand
Molecule
Metal
Cl
?p
E (Cl- p) lt E (M d) !!!
?s
4p
?d
eg
4s
t2g
3d
12 Cl- p orbitals
6 Ligands Cl- sp orbitals
Decrease ?oct ? Weak Field Ligand
?? ?? ?? ?? ?? ??
6?
6
? - bonding ? orbitals give Strong Field Ligands
Ligand
Molecule
Metal
12 CN- ? orbitals
?p
?s
4p
E (CN- ? ) gt E (M d) !!!
?d
eg
4s
3d
6 Ligands CN- sp orbitals
Increase ?oct ? Strong Field Ligand
?-bonding orbitals
?? ?? ?? ?? ?? ??
6?
7

• Molecular Orbital Theory Explains Field Strength
  of Ligands
• Ligand p orbitals cause ? - bonding that raises
  t2g energies
• Field Ligands
• 2) Ligand ? orbitals cause ? - bonding that
  lowers t2g energies
• Field Ligands
• 3) sp3 hybridized ligands do not change t2g
  orbitals very much
• Field Ligands
  

?d
eg
t2g
12 Cl- p orbitals
12 CN- ? orbitals
?d
eg
t2g
8
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