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CRYSTAL FIELD STABILISATION ENERGY (CFSE):

The difference in energy of eg and t2Orbitals are called crystal field stabilisation energy (CFSE):
Where m and n = are number of electrons in t2g and eg orbitals respectively and del.oct is crystal field splitting energy in octahedral Complexes.
l = represents the number of extra electron pair formed because of the ligands in comparison to normal degenerate configuration.
P= (Pairing energy) the energy required for electron pairing in a single orbital. The actual configuration of complex adopted is decided by the relative values of delta and P
Case (1): If del.oct is less than P :
We have so called weak field or high spin situation, the fourth electron entered one of the eg orbitals giving configuration (
t2g3 and eg1)
If now 
5th electron is added to a weak field the configuration become  (t2g3 and eg2).
Case (2): If del.oct  is more than P: we have the strong field , low spin situation and pairing will occur in the t2g level with eg level remaining unoccupied in entities of d1 and d6 ions .

Calculation shows that coordination entities with four to seven d electron are more stable for strong field as compared to weak field cases.
(A)For configuration (d0, d1, d2, d3, d8, d9, d10):

SN
METAL ION
EXAMPLE
CONF IN L.F
CFSE(del.oct)
1
d0
Sc3+
t2g 0,0,0  eg 0
=0.0
2
d1
Ti3+
t2g 1,0,0 eg 0
=-0.4
3
d2
V3+
t2g 1,1,0 eg 0
=-0.8
4
d3
Cr3+ , V2+
t2g1,1,1  eg 0
=-1.2
5
d8
Ni2+
t2g 2,2,2  eg1,1
= -2.4 +1.2+ 3P
=-1.2+3P
6
d9
Cu2+
t2g2,2,2  eg 2,1
=-2.4 +1.8+4P
=-0.6+ 4P
7
d10
Zn2+
t2g2,2,2  eg 2,2
=-2.4 +2.4+ 5P
=  5P

Therefore, for the above configurations, there is no effect of the nature of ligand. They may be strong or weak; the formula for CFSE will remain the same.

(A)For configuration (d4, d5, d6, d7):

SN
METAL ION
EXAMPLE
CONF IN L.F
CFSE(del.oct)
1
d4
Cr2+ (S.L.)

Cr2+ (W.L.)
t2g 2,1,1  eg 0,0

t2g 1,1,1  eg 1,0
=-1.6  +1P

=-1.6
2
d5
Mn2+,Fe3+(S.L.)

                 (W.L)
t2g 2,2,1  eg 0,0

t2g 1,1,1  eg 1,1
=-2.0+2P

=0.0
3
d6
Co3+,Fe2+(S.L.)

                (W.L.)
t2g 2,2,2 eg 0,0

t2g 2,1,1 eg 1,1
=-2.4 +3P

=-0.4+ 1P
4
d7
Co2+ (S.L.)

         (W.L.)
t2g2,2,2  eg 1,0

t2g2,2,1  eg 1,1
=-1.8+3 P

-0.8+2P


Crystal field stabilisation energy (CFSE) in Tetrahedral:
The difference in energy of eg and t2g Orbitals are called crystal field stabilisation energy (CFSE) in tetrahedral complexes:

Where m and n = are number of electrons in t2g and eg orbitals respectively and del.oct is crystal field splitting energy in octahedral Complexes.
l = represents the number of extra electron pair formed because of the ligands in comparison to normal degenerate configuration.
P= (Pairing energy) the energy required for electron pairing in a single orbital. The actual configuration of complex adopted is decided by the relative values of delta and P

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