What is the crystal field stabilisation energy? How does the magnitude of `Delta_(0)` decide the actual configuration of d-orbital in a coordination entiity?

Explanation

When ligands approach a transition metal in a definite geometry, the degenerate d-orbitals splits into two sets, one with lower energy and the other with higher energy in presence of electrical field of ligands. The difference of energy between the two sets of orbitals is called crystal field splitting energy. It is denoted by `Delta_(0)`,
(for octahedral complexes) and `Delta_(0)`
(for tetrahedral complexes).
`Delta_(0) lt P` (P is the energy required for pairing of electrons) the fourth electron enters one of the `e_(g)` orbitals giving the configuration `t_(2g)^(3) e_(g)^(1)`. It means first four orbitals are singly occupied and no pairing will take place, forming high spin complexes. Such ligands for which `Delta_(0) lt P` are called weak field ligands.
If `Delta_(0) gt P`, the 4th electron pairs up in one of the `t_(2g)` orbitals giving the configuration `t_(2g)^(4)e_(g)^(0)` thereby forming low spin complexes. Such ligands for which `Delta_(0) gt P` are called strong field ligands.