In which of the following complexes spin only magnetic moment is independent of nature of ligand (L)?
A. [TiL6]2+
B. [CoL6]3+
C. [CrL6]3+
D. [NiL6]2+
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A, C and D only
In an octahedral complex the d electrons fill the t2g set (three orbitals) and then the eg set (two orbitals). The ligand decides the size of the splitting Δo:
Weak field ligand (Δo < pairing energy): the fourth electron enters eg, giving high spin.
Strong field ligand (Δo > pairing energy): electrons pair in t2g first, giving low spin.
With a neutral ligand L, the charge on each complex is the oxidation state of the metal. A choice exists only for d4, d5, d6 and d7 ions. For d1, d2, d3, d8, d9 and d10 ions both arrangements give the same number of unpaired electrons, so the spin-only magnetic moment does not depend on the ligand.
A. [TiL6]2+ — Correct. Ti (Z = 22) is [Ar]3d24s2; losing the two 4s electrons gives \( \ce{Ti^{2+}} \): 3d2, t2g2, 2 unpaired electrons with any ligand.
B. [CoL6]3+ — Incorrect. Co (Z = 27) is [Ar]3d74s2; losing the two 4s electrons and one 3d electron gives \( \ce{Co^{3+}} \): 3d6, t2g4eg2 (4 unpaired) with a weak field ligand, but t2g6 (0 unpaired) with a strong field ligand.
C. [CrL6]3+ — Correct. Cr (Z = 24) is [Ar]3d54s1; losing the 4s electron and two 3d electrons gives \( \ce{Cr^{3+}} \): 3d3, t2g3, 3 unpaired electrons with any ligand.
D. [NiL6]2+ — Correct. Ni (Z = 28) is [Ar]3d84s2; losing the two 4s electrons gives \( \ce{Ni^{2+}} \): 3d8, t2g6eg2, 2 unpaired electrons with any ligand.