What is the electronic configuration of a d5 ion in terms of t2g and eg in an octahedral field when Δo < P, where P is the energy required for the pairing of electrons in a single orbital?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → t2g3 eg2
In an octahedral field the d orbitals split into three lower t2g orbitals and two higher eg orbitals, separated by Δo. When Δo < P (a weak field), it costs less energy to put an electron in an eg orbital than to pair it in t2g.
For a d5 ion:
Electrons 1 to 3 go singly into the three t2g orbitals: t2g3.
Electrons 4 and 5 enter the two eg orbitals singly instead of pairing: eg2.
The result is t2g3 eg2, a high spin arrangement with five unpaired electrons.
Why the other options are incorrect
Option (1) t2g5 eg0 — Incorrect. This is the low spin arrangement, formed when Δo > P (strong field ligands), where pairing in t2g is favoured.
Option (2) t2g2 eg3 — Incorrect. The two eg orbitals can hold three electrons only if one pairs, while a lower t2g orbital is left empty; electrons fill the lower set first.
Option (4) t2g0 eg5 — Incorrect. The eg set has only two orbitals and cannot hold five electrons; the lower t2g set would also be left empty.
NCERT (2026-27 edition):
5.5.4 Crystal Field Theory – “… 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.”