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Central Tenants of Crystal Field Theory
• The metals (Lewis acids) have d orbitals that are partially filled with electrons.
• Ligands, that are Lewis bases with lone pairs, come in and form a covalent bond.
• Electrostatic repulsion between the ligand lone pairs with the
d-orbital subshell leads to higher energies
• Each metal orbital will either be stabilized or destabilizeddepending on the amount of orbital overlap with the ligand.
CFT (Octahedron)
(dz2, dx2-y2)
Orbitals point
directly at ligands
stronger repulsion
higher energy
(dxy, dyz, dxz)
Orbitals point
between ligands
weaker repulsion
lower energy
dz2 dx2y2
eg
dxy dyz dxz
t2g
E
Energy Levels of d-Orbitals in an Octahedron
• crystal field splitting energy =
• The size of is determined by;
metal (oxidation state; row of metal, for 5d > 4d >> 3d)
ligand (spectrochemical series)
Spectrochemical Series
t2g
eg
t2g
eg
I < Br < Cl < F < OH < H2O < NH3 < en < NO2 < CN < CO
[Cr(H2O)6]3+
Greater
Small Δ
Weak Field
Weak M-L interactions
Large Δ
Strong Field
Strong M-L interactions
How Do the Electrons Go In?
• Hund’s rule: one electron each in lowest energy orbitals first
• what happens from d4 to d7?
d6
Crystal Field
SplittingSpin Pairing
Δ Pvs.
High Spin vs. Low Spin
High Spin Low Spin
Co3+ (d6)
PΔ < PΔ >
High Spin vs. Low Spin Configurations
high spin low spin
d4
d5
d6
d7
High Spin vs. Low Spin Configurations
strong field
ligand
weak field
ligand
4d & 5d
always
3d metal 3d metal
linear (CN = 2)
tetrahedral (CN = 4)
square planar (CN = 4)
What About Other Geometries?
x
y
z
x
y
z
x
y
z
how do the electrons on the d orbitals
interact with the ligands in these cases?
Tetrahedral vs. Octahedral
Spherical
crystal field
Octahedral
crystal field
Energ
y Δoct
Δtet
dxy dyz dxz
dz2 dx2-y2
Tetrahedral
crystal field
Δtetrahedron ≈ (4/9) Δoctahedron
Consequently, tetrahedral complexes are always high spin.
dz2 dx2-y2
dxy dyz dxz
dxy dyz dxz dz2 dx2-y2
Square Planar d-level Splitting
xy
z
x
y
z
octahedral
(CN = 6)
square planar
(CN = 4)
All z-levels lower in
energy due to less
e− repulsion
dz2 more stable so
lower in energy than
dxy orbital
d8
Common for
4d & 5d metals
Low Spin
High Spin or Low Spin?
1) What is the
Coordination Geometry ?
Tetrahedral
(CN = 4)
High Spin
Square Planar
(CN = 4)
Low SpinOctahedral
(CN = 6)
2) Is it a 1st (3d), 2nd (4d) or
3rd (5d) row Transition Metal ?
If 1st Row
Depends on Ligand & Metal
F, Cl, Br, I = High Spin
CN, CO = Low Spin
2nd or 3rd Row
Low Spin
Orbital Splitting vs. Geometry
Electron Configurations of Complexes
[Rh(CN)2(en)2]+
[MnCl6]4
coord. # geom. d-e− config.
Electron Configurations of Complexes
[NiCl4]2
[Pt(NH3)4]2+
coord. # geom. d-e− config.
[FeCl4]2– is tetrahedral. How many unpaired
electrons will [FeCl4]2– have?
Magnetism
• paramagnetism– compounds with one or more
unpaired electrons are attracted by a magnetic field
– The more unpaired electronsthe greater the attractive force
• diamagnetism– compounds with no
unpaired electrons repel a magnetic field
– much weaker effect
Octahedral Crystal Field
d6 High Spin (i.e. Co3+)
t2g
eg
d6 Low Spin (i.e. Co3+)
t2g
eg
Color of Transition Metal Complexes
The color of these compounds comes from the absorption of light that causes an excitation of an electron from one
d-orbital to a different d-orbital on the same metal cation.
CuSO4NiSO4CoSO4FeSO4MnSO4 ZnSO4
• Spectrochemical series
– relative strength of metal ion/ligand interactions
– independent of metal
octahedral cobalt(III) complex ions
a. CN– b. NO2– c. phen d. en e. NH3 f. gly g. H2O h. ox i. CO3
2–
1,10-phenanthroline glycine
Color of Transition Metal Complexes
Absorption Of Light
• compounds absorb light if the light has the correct energy to cause an electron to move from a lower energy state to a higher energy state
octahedral d3 (i.e. Cr3+)
t2g
eg
E
oct
Light (hv)
(photon)
t2g
eg
oct
The size of oct dictates the wavelength of light needed,
and therefore, the color of the compound.
“GROUND STATE” “EXCITED STATE”
Absorption Dependence on d-electrons
Different numbers of transitions will occur for
different d-electron configurations
t2g
eg
E
d10 = colorless
t2g
eg
d0 = colorless
d1-d9 = various visible transitions