The electron distribution in dn coordination complexes depends on magnitude of crystal field splitting, (∆0 ) and pairing energy (P). The condition which favours formation of high spin complexes is :

High spin complex means pairing does not occur. This condition is possible only in presence  of weak field ligands. In octahedral complexes energy of t2g< eg . In presence of weak field  ligand, electrons progressively enter to t2g until each of the three orbitals of t2g are half-filled  then fourth and fifth electrons enter into eg  levels are half filled. This condition is possible  when Δo