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Shri Sachhidanand Shikshan Sanstha’s
Department of Chemistry
Seminar
On
Crystal Field Theory & its Application to Octahedral Complexes
Saturday, 24th December 2011
Crystal Field Theory
The above topic covered following points
Introduction & Historical Development
Assumptions of CFT
Application to Octahedral Complex
Factors Affecting CFSE
Colour & Magnetic Properties of Complex
Introduction & Historical Development
In 1704 first metal complex
prussian blue (Artist’s Colour) was
discovered by Berlin Colour maker.
In 1799 Tassaert discovered Cobalt
Ammine Coplexes.
In 1893 Werner gave Co-ordination
theory based on primary and
secondary valency.
In 1927 Sidwick introduced concept
of co-ordinate bond and EAN.
Alfred Werner
Modern Theories
Bonding:
of
Metal
Ligand
VBT given by Pauling & Slater in 1935
CFT given by Brethe in
1929
& further developed
by Van Vleck in 1932
LFT given by Van Vleck in 1935
Assumptions of CFT:
The central Metal cation is surrounded by ligand which contain one or
more lone pair of electrons.
The ionic ligand (F-, Cl- etc.) are regarded as point charges and neutral
molecules (H2O, NH3 etc.) as point dipoles.
The electrons of ligand does not enter metal orbital. Thus there is no
orbital overlap takes place.
The bonding between metal and ligand is purely electrostatic
Application of CFT to the formation of Octahedral
complex:
z
L
y
L
L
M = Central Metal Ion
n= Oxidation State of Metal Ion
L= Ligand
n+
M
L
L
L
x
Interaction of ligand with d – orbitals of metal ion
g
g
Hypothetical
Situation of
d - orbital
Splitting of d – orbitals
eg orbitals
t2g orbitals
Factors Affecting on CFSE
1)Nature of metal ion:
a)
Same metal ion with different charge
e.g. [Co(H2O)6]3+
[Co(H2O)6]2+
Co3+
Co2+
Do=18,200 cm-1 >
Do=9,300 cm-1
b) Different metal ion with same charge
e.g. [Co(H2O)6]2+
[Ni(H2O)6]2+
Co2+ (d7)
Ni2+ (d8)
Do=9,300 cm-1
>
Do=8,500 cm-1.
c)
Different metal ion with different charge but same number of d –
electrons
e.g. [Cr (H2O)6]3+
[V(H2O)6]2+
Cr3+ (d3)
V2+ (d3)
Do= 17,400 cm-1 >
Do= 12,400 cm-1
d) Different metal ion with same charge but different principal
quantum number.
e.g. [Ir (NH3)6]3+
[Rh(NH3)6]3+
[Co(NH3)6]3+
Ir3+ (5d6)
Rh3+ (4d6)
Co3+ (3d6)
n=5
n=4
n=3
Do= 41,000 cm-1 > Do= 34,000 cm-1 > Do= 23,000 cm-1
Factors Affecting on CFSE
2)Nature of ligand
a) When the ligands are
strong the energy gap
between t2g and eg is more
the distribution of electron
does not takes place
according to Hund’s rule.
These are Low spin
Complexes .
b) When ligands are weak
CFSE is relatively small
hence five d- orbitals are
Strong field
suppose to be degenerate
and therefore distribution Ligands (violet, low spin)
of electrons takes place
according to Hund’s rule.
These are High spin
Complexes .
Weak field
Ligands (red, high
spin)
Factors Affecting on CFSE
2)Nature of ligand :c) Distribution of electron in High spin and Low spin
Complexes
Strong field
Weak field
Strong field
Weak field
d1
d2
d5
d4
1 u.e.
5 u.e.
2 u.e.
1 u.e.
Weak field
d3
d6
0 u.e.
4 u.e.
d7
1 u.e.
d9
d8
2 u.e.
Strong field
1 u.e.
3 u.e.
d10
0 u.e.
0 u.e.
Factors Affecting on CFSE
2)Nature of ligand :
When the common ligand are arranged in the
order of their increasing splitting power the
series is obtained called Spectrochemical
series.
Application of CFT
1) Colour of complexes :The transition metal complexes whose central
metal ion contain partially filled d – orbitals are usually coloured in their
solid and solution form.
d – d transition of electron
e.g. [ Ti (H2O)6]3+ complex absorb green radiation at 5000 A0 , hence
transmitted the radiation of purple colour due to d – d transition of
electron
h =239kJ/mole
2) Magnetic Properties :
a) in d1, d2, d3, d8, d9 complexes have same spin state and all are paramagnetic.
b) The low spin d6 and d10 complexes are diamagnetic.
c) In d4, d5, d6 and d7 the number of unpaired electron are different in high spin and
low spin octahedral complexes .
Strong field
Weak field
Strong field
Weak field
d1
d2
d5
d4
1 u.e.
5 u.e.
2 u.e.
1 u.e.
Weak field
d3
d6
0 u.e.
4 u.e.
d7
1 u.e.
d9
d8
2 u.e.
Strong field
1 u.e.
3 u.e.
d10
0 u.e.
0 u.e.
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