Define crystal field splitting energy. On the basis of crystal field theory, write the electronic configuration for `d^(4)` ion if `Delta_(0)ltP`.

Explanation

In a free transition metal ion, all the five d-orbitals are degenerate but when it is involved in a complex formation, the degeneracy is split into two sets of d-orbitals. This is called crystal field splitting.
The difference of energy between the two sets of d-orbitals is called crystal field splitting energy (CFSE).
The lower energy sets of three d-orbitals is called `t_(2g)` and the higher energy set of two d-orbital is called `e_(g)`. The three electrons first enter into `t_(2g)`, one in each orbital. If `Delta_(0)ltP`, the 4th electron will enter `e_(g)`. Hence, the configuration will be `t_(2g)^(3)e_(g)^(1)`.