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Tetragonal distortion from octahedral symmetry
)(
)(
)(4
2
426
h
v
h DTrans
C Cis
L MX O ML
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Reduction of symmetry: consequences
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How a Term of Oh symmetry splits when the symmetry decreases, is given in
the following correlation table.
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Tetragonal distortion from octahedral symmetry
Tetragonal distortion from octahedral symmetry in ML6 type complex ??
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Jahn - Teller Distortion
Jahn-Teller theorem: For a nonlinear molecule in an electronically degenerate state,
distortion must occur to lower the symmetry, remove the degeneracy, and lower the
energy.
No J-T distortion:d3, d5(HS), d6(LS), d8
Degeneracy of orbitals & Terms
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Jahn - Teller Distortion: Orbital picture
J-T gives NO inform at ion:
-Magnitude of splitting
- whether elongation or
compression?
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Jahn - Teller Distortion: Examples
Example: d1 system, [TiCl6]3-
B2g
Eg
B1g
A1g
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Pronounced Jahn - Teller Distortion
Dynamic J-T:
Elongated:
Undisturbed:Compressed:
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Jahn - Teller Distortion in Chelated compounds(Conflict b/w stabilization from chelate & J-T distortion)
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CHARGE TRANSFER TRANSITION
Classifications:
1) Transition b/w levels primarily located on the metal, e.g. d → d transition
2) Transition … ligands, e.g. π→π* transition
3) Transition b/w levels of metal-to-ligand or vice versa (CT Transition)
MLCT, LMCT, LLCT, MMCT
Charge Transfer transition may be regarded as an internal redox process, whichmakes it possible to use the concept of HOMO and LUMO
Koopman’s Theorm (Frozen orbital): IP = - HOMO energy, EA = - LUMO energy
Atoms/ions with Low IP
- Readily oxidisable- Filled orbitals of relative high energy
Atoms/ions with High EA
- Readily reducible- Low laying empty orbitals
Ideal combination for CT Transition: Metals with high IP & Ligands with High EA
(minimum gap b/w HOMO of metal & LUMO of ligands)
Note: If the gap is too small ( < 10000 cm-1 ), complete electron transfer may occur with
oxidation of metal and reduction of ligands, resulting break down of the complex, e.g.Co(H 2 O)6 ]
3+, FeI 3
CT transition do not involve the complete transfer of one electron from one atom to
another; rather in a molecular orbital sense, they represent the transition of an
electron from a MO primarily located on one atom, to a MO primarily located in
another atom.
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Charge Transfer Spectra in TM cpmplexes
Ideal Conditions:
Low IP of metal (readily oxidizable); High EA of ligand (readily reducible)
To see transition in visible range: oxidizable Ligands + reducible metal
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Ligand-to-Metal Charge Transfer Spectra
For MnO4-
Metal-Ligand combination:Metal with high IE, low energy empty orbitals
e.g. transition metal with high oxidtion state
Ligand with low EA, high energy filled orbitals
e.g. chalcogens/heavy halides
Increase in energy of CT band
(easy of oxidation)Iodide < Bromide < Chlodide < Floride
- Presence of electron donating ligand: lower
- More reducible the metal (M-L ionic bond):
Lower
- Independent of oxidation state of metal
(M-L covalent)
- Increases with coordination no.
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Metal-to-Ligand Charge Transfer Spectra
Metal-Ligand combination:
Ligands with low energy empty orbitalse.g. Pi* orbital: CO, pyridine, pyrazine
Occupied metal centered filled orbitals
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MAGNETIC PROPERTIES OF COMPLEXES
Depends on:• Oxidation state of metal
• Electron configuration
• Coordination number of metal
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Diamagnetic materials tend
to repel flux lines weakly,
Examples: water, protein, fat
“magnetic” materials tend to
concentrate flux lines.
Examples: iron, cobalt
MATERIAL IN A MAGNETIC FIELD(a) Diamagnetic material:
in the presence of a field, dipoles are induced and
aligned opposite to the field direction.
(b) Paramagnetic material:
Diamagnetism is a universal property.
Paramagnetism is much larger than diamagnetismand decreases with temperature.
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The response of a material to a Magnetic Field H is called Magnetic
Induction B
The relationship between B and H is a property of the material
In some materials and in free space B is a linear function of H but in
general it is much more complicated and sometimes it is not even
single valued
0 H B H ][Tesla B
d
material of Densityd
litySusceptibimassSpecific
)(
ion Magnetizat of Intensity I I 4
4/41/ 00 H I H B
volumeunit per litySusceptibi Magnetic H I 0/
MW litySusceptibi Molar M .
MATERIAL IN A MAGNETIC FIELD
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Nuclear spin
Orbital motion of electrons
Origin of Magnetism Spin of electrons
A moving electric charge, macroscopically or “microscopically” is
responsible for Magnetism
Weak effect
Unpaired electrons required
for net Magnetic Moment
Magnetic Moment resultant from the spin of a single unpaired electron
→ Bohr Magneton = 9.273 x 10
24 A/m2
ORIGIN OF MAGNETISM IN A MATERIAL
M i M M
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Magnetic Moment Measurement
(a) Magnet (of field H) off
(b) Paramagnetic (HB > H)
(c) Diamagnetic (HB < H)
By a magnetic balance.If a substance has unpaired electrons, it is paramagnetic,
and attracted to a magnetic field.
Faraday Method Gouy MethodSample size: mg gGives: molar sus. (χ) Volume sus. (κ) r r
r
m
f
m
f
])([
10644.1
4
135
SCN Co Hg for
mol cmr
M ti t f l Th
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Magnetic moment of complexes: Theory
Magnetic moment incorporating all the three types of coupling, viz., spin-spin,orbital- orbital, and spin-orbital:
μ is the magnetic moment, J is the total angular momentum quantum number and
g is the Landé splitting factor for the electron, L is the orbital-angular momentum
and S is the spin-angular momentum.
When the spin-orbit coupling is negligible or absent:
Spin-only value:
(spin-orbit)
Substituting, S = n/2, where ‘n’ is the number of unpaired electrons, gives
When orbital contribution is negligible or absent:
RT
N M
3
22
M
M T N
RT
84.2
32
Theory
M ti ti f L l
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Magnetic properties of Ln-complexes
M ti ti f TM l
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Calculated values are spin-only
Magnetic properties of 1st-Row TM complexes
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Co-existance of different spin statesMany transition metal ions are able to form high-spin and low-spin complexesdepending up on the strength of the ligand field. When the ligand is ofintermediate field strength, both high-spin and low-spin complexes can coexist
in equilibrium.
kT P
Example: Fe2+ (d6) system
[Fe(H2O)6]2+ (∆ - P < 0), S = 2
[Fe(CN)6]4+ (∆ - P > 0), S = 0
Variation in magnetic moment of [Fe(Phen)2(NCS)2]
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Temperature DependenceFor magnetically dilute paramagnetic substances
Materials that are not magnetically dilute gives “magnetic exchange”.
TC: Curie Temperature, TN: Neel Temperature
Comparison of magnetic behavior
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Spin-alignment
(below Tc & TN )
vs.
spin-randomness
(above Tc & TN )
Comparison of magnetic behavior