Download Crystal Field Theory of Coordination Complexes Octahedral Splitting

Crystal Field Theory of Coordination Complexes
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•
•
•
•
historically developed for solid state crystal lattices
adapted for molecular complexes (later versions: ligand field theory)
purely ionic model, ligands treated as point charges
M-L interaction limited to electrostatic repulsion and splitting of M d-orbitals
can explain spectroscopy, magnetism, and reactivity trends of d-block compounds
Field Splitting of Orbitals
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•
• Mn+ •
•
•
n+
n+
n+
M
M
M
E
pointing at ligands:
destabilized relative to spherical field
"eg"
"t2g"
free
metal ion
spherical
charge field
6 localized
point charges:
octahedral
field
pointing between ligands:
stabilized relative to spherical field
Octahedral Splitting and Crystal Field Stabilization Energy
–
#e
1
d
d2
d3
d4
d5
d6
d7
d8
d9
d10
low spin
(strong field)
config CFSE
1
0
t2g eg
t2g2eg0
t2g3eg0
t2g4eg0
t2g5eg0
t2g6eg0
t2g6eg1
t2g6eg2
t2g6eg3
t2g6eg4
–4Dq
–8Dq
–12Dq
–16Dq
–20Dq
–24Dq
–18Dq
–12Dq
–6Dq
0Dq
high spin
(weak field)
config CFSE
eg
dz2 dx2–y2
+ 6Dq
Octahedral Field
t2g3eg1
t2g3eg2
t2g4eg2
t2g5eg2
10Dq = Δoct
–6Dq
0Dq
–4Dq
–8Dq
dxy
dxz
dyz
t2g
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Tetrahedral Splitting
Factors that Affect ∆
An alternate view of a tetrahedron: a cube with half the corners missing
1) Number of ligands and geometry (see previous)
Three orbitals point at ligands
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•
2) Oxidation state: ∆↑ as n+ ↑ (Note: n is oxidation state, not principal QN)
•
z
MLn
[Cr(H2O)6]2+
[Cr(H2O)6]3+
y
•
x
dxz dyz dxy
dxy
• •
dxz
•
• •
•
•
•
•
4.45Dq = ∆tet
4) Ligands: Spectrochemical Series
dz2 dx2–y2
– 2.67Dq
"e"
∆tet splitting is inverse of ∆oct: two below three rather than three below two
∆tet splitting ≈ ½∆oct: tetrahedral compounds are almost always HS
dx2–y2
+12.28
dz2
dz2
+7.07
E
dxy
+1.78
+2.28
dxz dyz
+1.14
0 Dq
dz2 dx2–y2
–2.67
dz2
dxz dyz
tetrahedral
–4.28
–5.14
square
planar
–0.82
dxy dx2–y2
–2.72
dxz dyz
–6.28
dxy dx2–y2
linear
–1
–1
∆o (cm )
MLn
3–
[CoF6]
13100
[Co(H2O)6]3+ 18200
[Co(NH3)6]3+ 22900
[Co(CN)6]3+ 33500
∆o (cm )
15000
17500
21500
26700
MLn
[FeCl6]3–
[Fe(H2O)6]3+
[Fe(NH3)6]3+
[Fe(CN)6]3–
–1
∆o (cm )
11000
13700
17500
32800
+10.28
dz2 dx2–y2
+6.00
dxy dxz dyz
MLn
[CrF6]3–
[Cr(H2O)6]3+
[Cr(NH3)6]3+
[Cr(CN)6]3–
Spectrochemical Series: Ligand Effect on ∆
I– < Br– < Cl– < ONO– < F– < OH– < H2O < MeCN < py < NH3 < en < bpy < phen < NO2– < PR3 < CN– < CO
←→
weaker field, smaller splitting
stronger field, larger splitting
Crystal Field Splitting for Common Geometries (Dq units)
–4.00
–1
∆o (cm )
MLn
3+
[Co(NH3)6]
22900
[Rh(NH3)6]3+ 24000
[Ir(NH3)6]3+ 41200
y
dx2–y2
octahedral
9400
13700
+ 1.78Dq
x
dxy dxz dyz
–1
∆o (cm )
3) Period: ∆↑ down a column
dyz
Tetrahedral Field
• •
dz2
MLn
[Fe(H2O)6]2+
[Fe(H2O)6]3+
14100
17400
"t2"
Two orbitals point between ligands
•
–1
∆o (cm )
trigonal
bipyramidal
Licensed by WSM under a Creative Commons Attribution-NonCommercial-ShareAlike 2.5 Canada Licence.
Some rules of thumb about the magnitude of ∆:
• Tetrahedral complexes tend to be high spin
• Octahedral complexes will be high spin only if
• first row transition metal (3d), AND
• either weak field ligand or low oxidation state
An aside:
cm–1 = wavenumbers, a unit of energy favoured by certain breeds of spectroscopist
ν = =
so E = hν =
= hcν
1000 cm–1 ≈ 12 kJ/mol
Licensed by WSM under a Creative Commons Attribution-NonCommercial-ShareAlike 2.5 Canada Licence.