Download Transition Metals and Coordination Chemistry - pp. 92-114

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 destabilized
depending on the amount of orbital overlap with the ligand.
CFT (Octahedron)
(dz2, dx2-y2)
eg
E
stronger repulsion
higher energy
dx2y2
dz2
Orbitals point
directly at ligands
(dxy, dyz, dxz)
t2g
dxy
dyz
dxz
Orbitals point
between ligands
weaker repulsion
lower energy
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
eg
eg


t2g
t2g
[Cr(H2O)6]3+
Greater 
I < Br < Cl < F < OH < H2O < NH3 < en < NO2 < CN < CO
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?
Crystal Field
Splitting
Δ
d6
Spin Pairing
vs.
P
High Spin vs. Low Spin
Co3+ (d6)
High Spin
Low Spin
Δ< P
Δ> P
High Spin vs. Low Spin Configurations
High Spin vs. Low Spin Configurations
high spin
3d metal
weak field
ligand
low spin
d4
d5
3d metal
strong field
ligand
d6
4d & 5d
always
d7
What About Other Geometries?
linear (CN = 2)
z
y
x
tetrahedral (CN = 4)
z
y
x
square planar (CN = 4)
z
y
x
how do the electrons on the d orbitals
interact with the ligands in these cases?
Tetrahedral vs. Octahedral
dxy dyz dxz
dz2
Energy
Δtet
dx2-y2
Δoct
dxy dyz dxz dz2 dx2-y2
dz2 dx2-y2
Spherical
crystal field
Tetrahedral
crystal field
dxy dyz dxz
Δtetrahedron ≈ (4/9) Δoctahedron
Octahedral
crystal field
Consequently, tetrahedral complexes are always high spin.
Square Planar d-level Splitting
z
z
y
x
y
All z-levels lower in
energy due to less
e− repulsion
x
dz2 more stable so
lower in energy than
dxy orbital
d8
Common for
4d & 5d metals
octahedral square planar
(CN = 4)
(CN = 6)
Low Spin
High Spin or Low Spin?
1) What is the
Coordination Geometry ?
Tetrahedral
(CN = 4)
Square Planar
(CN = 4)
Octahedral
(CN = 6)
High Spin
2)
Low Spin
Is it a 1st (3d), 2nd (4d) or
3rd (5d) row Transition Metal ?
If 1st Row
2nd or 3rd Row
Depends on Ligand & Metal
F, Cl, Br, I = High Spin
CN, CO = Low Spin
Low Spin
Orbital Splitting vs. Geometry
Electron Configurations of Complexes
coord. #
[Rh(CN)2(en)2]+
[MnCl6]4
geom.
d-e− config.
Electron Configurations of Complexes
coord. #
[NiCl4]2
[Pt(NH3)4]2+
geom.
d-e− config.
[FeCl4]2– is tetrahedral. How many unpaired
electrons will [FeCl4]2– have?
Magnetism
• paramagnetism
• diamagnetism
– compounds with one or more
unpaired electrons are
attracted by a magnetic field
– The more unpaired electrons
the greater the attractive force
– compounds with no
unpaired electrons repel a
magnetic field
– much weaker effect
Octahedral Crystal Field
eg
eg
t2g
d6 High Spin (i.e. Co3+)
t2g
d6 Low Spin (i.e. Co3+)
Color of Transition Metal Complexes
MnSO4
FeSO4
CoSO4
NiSO4
CuSO4
ZnSO4
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.
Color of Transition Metal Complexes
• 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. CO32–
1,10-phenanthroline
glycine
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+)
“EXCITED STATE”
“GROUND STATE”
E
eg
eg
Light (hv)
(photon)
oct
t2g
oct
t2g
The size of oct dictates the wavelength of light needed,
and therefore, the color of the compound.
Absorption Dependence on d-electrons
Different numbers of transitions will occur for
different d-electron configurations
eg
E
eg
t2g
d10 = colorless
t2g
d0 = colorless
d1-d9 = various visible transitions