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.
If the fourth electron in d subshell enters one of the \( e_g \) orbitals in octahedral complexes, it indicates that
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → the ligand is weak, and forms high spin complexes.
In an octahedral field the d orbitals split into a lower \( t_{2g} \) set and a higher \( e_g \) set, separated by \( \Delta_o \). The first three electrons go singly into the \( t_{2g} \) orbitals. For the fourth electron there are two choices, decided by \( \Delta_o \) and the pairing energy P.
Here the fourth electron enters an \( e_g \) orbital, so \( \Delta_o \lt P \): the ligand is weak and the complex is high spin.
Why the other options are incorrect
Option (1) the ligand is weak, and forms low spin complexes. — Incorrect. A weak field ligand leaves the electrons unpaired, which is high spin, not low spin.
Option (2) the ligand is strong, and forms low spin complexes. — Incorrect. This is the other case: with a strong ligand the fourth electron pairs in \( t_{2g} \) and does not enter \( e_g \).
Option (3) the ligand is strong, and forms high spin complexes. — Incorrect. A strong field ligand forces pairing and so gives low spin complexes.
NCERT (2026-27 edition):
Crystal Field Theory – …
