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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