Crystal field Theory:-

This theory is based upon 2 assumption

1. Central metal and legands  behave  as   point  charges.
2. Bond between central metal and legand is ionic not Co-ordinate.

According to this theory when legand attack toward Central atom due to attraction between opposite charges repulsive between e^-s of metal and  legand  are also observed. Due to these repulsive forces degenerate orbital of metals are divided in more  than one difference energy level. (like t₂g and eg)

If legand is strong means its crystal field energy is greater than pairing energy of metal electron of higher energy level are shifted towards lower energy level to make pairing therefore low spin inner complex is prepared. But if legand is weak pairing is not possible therefore high spin outer complex is prepared.