In
2 and d
3 coordination entities, the d electrons occupy the t2g orbitals
singly in accordance with the Hund’s rule For d
4 ions, two possible
patterns of electron distribution arise: (i) the fourth electron could
either enter the t2g level and pair with an existing electron, or (ii) it
could avoid paying the price of the pairing energy by occupying the
eg level Which of these possibilities occurs, depends on the relative
magnitude of the crystal field splitting, Do and the pairing energy, P
(P represents the energy required for electron pairing in a single
orbital) The two options are:
(i) If Do < P, the fourth electron enters one of the eg orbitals giving the
configuration 
3
1
2g
g
t
e