Assertion (A): Low spin configurations are rarely observed in tetrahedral coordination entities.
Reason (R): For the same metal, the same ligands and metal-ligand distances, the d orbital splitting in a tetrahedral field is smaller than in an octahedral field, \( \Delta_t = \frac{4}{9}\Delta_o \).
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
In a crystal field the d orbitals split into two sets. Electrons pair up in the lower set only when the splitting energy is larger than the pairing energy; otherwise they go singly into the upper set and the complex is high spin.
Assertion (A): Low spin configurations are rarely observed in tetrahedral coordination entities. — True. Tetrahedral entities are nearly always high spin: the electrons enter the upper set singly before they pair.
Reason (R): For the same metal, the same ligands and metal-ligand distances, the d orbital splitting in a tetrahedral field is smaller than in an octahedral field, \( \Delta_t = \frac{4}{9}\Delta_o \). — True. The tetrahedral splitting is less than half of the octahedral splitting for the same metal and ligands.
Why R explains A?
Pairing in the lower set takes place only when the splitting is larger than the pairing energy.
Because \( \Delta_t \) is so small, it is not large enough to force pairing, so low spin tetrahedral entities are rare; R gives the cause of A.
NCERT (2026-27 edition):
5.5.4 Crystal Field Theory – “In tetrahedral coordination entity formation, the d orbital splitting … is inverted and is smaller as compared to the octahedral field splitting. For the same metal, the same ligands and metal-ligand distances, it can be shown that Δt = (4/9) Δ0. Consequently, the orbital splitting energies are not sufficiently large for forcing pairing and, therefore, low spin configurations are rarely observed.”