Read the passage carefully and answer.
The crystal field theory (CFT) is an electrostatic model which considers the metal-ligand bond to be ionic arising purely from electrostatic interactions between the metal ion and the ligand. Ligands are treated as point charges in case of anions or dipoles in case of neutral molecules. The five d orbitals in an isolated gaseous metal atom/ion have same energy, i.e., they are degenerate. This degeneracy is maintained if a spherically symmetrical field of negative charges surrounds the metal atom/ion. However, when this negative field is due to ligands (either anions or the negative ends of dipolar molecules like \( NH_3 \) and \( H_2O \)) in a complex, it becomes asymmetrical and the degeneracy of the d orbitals is lifted. It results in splitting of the d orbitals. The pattern of splitting depends upon the nature of the crystal field.
In tetrahedral coordination complexes, low spin configurations are rare because
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(Δ_t\) is not sufficiently large to cause pairing.
Whether a complex is high spin or low spin depends on the comparison of the crystal field splitting with the pairing energy P. Electrons pair in the lower set only when the splitting is larger than P.
In a tetrahedral complex the splitting is only about four-ninths of the octahedral splitting, \( \Delta_t = \frac{4}{9}\Delta_o \), so it is almost always smaller than P and the electrons stay unpaired.
Why the other options are incorrect
Option (1) d orbital splitting in tetrahedral complexes causes more distortion as compared to octahedral complexes. — Incorrect. Spin pairing is decided by the size of the splitting compared with P; distortion does not come into it.
Option (2) d orbital splitting is inverted as compared to octahedral complexes. — Incorrect. The splitting is indeed inverted (e below t2), but this only changes which set is lower; it is the small size of the splitting that prevents pairing.
Option (3) \(Δ_t = (9/4) Δ_0\) — Incorrect. The ratio is upside down: \( \Delta_t = \frac{4}{9}\Delta_o \). A splitting larger than \( \Delta_o \) would in fact favour low spin complexes.
NCERT (2026-27 edition):
Crystal Field Splitting in Tetrahedral Coordination Entities
