CRYSTAL FIELD STABILISATION ENERGY (CFSE):

The difference in energy of eg and t₂g Orbitals 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