The energy levels of the d orbitals in an octahedral complex are written with the subscript ‘g’ (t2g and eg), but this subscript is not used for the energy levels in a tetrahedral complex. Which of the following is the reason?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Tetrahedral complexes have no centre of symmetry.
The subscript ‘g’ is used only for complexes that have a centre of symmetry. Octahedral and square planar complexes have one, so their levels are written t2g and eg. A tetrahedral complex lacks a centre of symmetry, so its two sets of levels are written without ‘g’.
Why the other options are incorrect
Option (1) The d orbital splitting in a tetrahedral field is inverted. — Incorrect. The splitting is indeed inverted compared with the octahedral field, but the order of the levels has nothing to do with the subscript.
Option (2) The splitting in a tetrahedral field is only four-ninths of that in an octahedral field. — Incorrect. Δt = (4/9)Δo is true, but the size of the splitting does not decide the label.
Option (3) Low spin configurations are rarely observed in tetrahedral complexes. — Incorrect. This follows from the small splitting, which cannot force pairing; it is about electron filling, not about the symbol.
NCERT (2026-27 edition):
5.5.4 Crystal Field Theory – “…The ‘g’ subscript is used for the octahedral and square planar complexes which have centre of symmetry. Since tetrahedral complexes lack symmetry, ‘g’ subscript is not used with energy levels.”