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For convenience, let us assume that the six ligands are positioned symmetrically along the Cartesian axes, with the metal atom at the origin. As the ligands approach, first there is an increase in the energy of d orbitals to that of the free ion just as would be the case in a spherical filed. Next, the orbitals lying along the axes (dz2and dx2-y2 d) get repelled more strongly than dxy, dyz and dxz orbitals, which have lobes directed between the axes. The dxy , dyz , dxz orbitals are lowered in energy relative to the average energy in the spherical crystalfiled. Thus, the degenerate set of d orbitals get split into two sets: the lower energy orbitals set, t2g and the higher energy, eg set. The energy separation is denoted by del.oct (the subscript o is for octahedral.

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 crystalfield 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
We have so called weak field or high spin situation, the fourth electron entered one of the eg orbitals giving configuration (t2g3 and eg1If 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 .

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