What will be the configuration for a d4 octahedral complex with a strong field ligand?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → t2g4 eg0
By crystal field theory, the ligands of an octahedral complex split the five d orbitals into a lower set of three, t2g, and a higher set of two, eg, separated by the energy Δo.
For a d4 ion, the fourth electron either pairs in t2g or enters eg, depending on whether Δo is larger or smaller than the pairing energy P. A strong field ligand gives Δo > P, so the configuration is t2g4 eg0, with two unpaired electrons (low spin).
Why the other options are incorrect
Option (1) t2g3 eg1 — Incorrect. This is the configuration with a weak field ligand (high spin).
Option (3) e1 t23 — Incorrect. The labels e and t2 (without g) belong to tetrahedral complexes, where e is the lower set. Here e1 t23 would put three electrons in the higher t2 set while an orbital of the lower e set is still empty, which does not happen.
Option (4) e3 t21 — Incorrect. Again a tetrahedral notation. A tetrahedral d4 complex is e2 t22, since Δt is too small to force pairing.
NCERT (2026-27 edition):
Crystal Field Theory – “… For d4 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, Δo and the pairing energy, P (P represents the energy required for electron pairing in a single orbital). The two options are:
(i) If Δo < P, the fourth electron enters one of the eg orbitals giving the configuration t2g3 eg1. Ligands for which Δo < P are known as weak field ligands and form high spin complexes.
(ii) If Δo > P, it becomes more energetically favourable for the fourth electron to occupy a t2g orbital with configuration t2g4eg0. Ligands which produce this effect are known as strong field ligands and form low spin complexes.”