GATE 2026 CY – Question 52

Inorganic Chemistry · Transition Metals: Coordination chemistry – structure and isomerism, theories of bonding (VBT, CFT, and MOT). Metal-metal multiple bonds. Energy level diagrams in various crystal fields, CFSE, applications of CFT, Jahn-Teller distortion. Electronic spectra of transition metal complexes: spectroscopic term symbols, selection rules, Orgel and Tanabe-Sugano diagrams, nephelauxetic effect and Racah parameter, charge-transfer spectra. Magnetic properties of transition metal complexes. Ray-Dutt and Bailar twists, Reaction mechanisms: kinetic and thermodynamic stability, associative, dissociative substitution and redox reactions. · 2 marks · Multiple select

Consider the given pairs of complex ions. The correct pair(s) in which each complex ion exhibits three spin-allowed d-d transitions (excluding those arising due to distortions) is(are)

[V(H2O)6]3+ and [Ni(H2O)6]2+
[Cr(H2O)6]3+ and [NiCl4]2−
[Ti(H2O)6]3+ and [Mn(H2O)6]2+
[FeF6]3− and [FeCl4]2−

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Correct answer: (A) [V(H2O)6]3+ and [Ni(H2O)6]2+; (B) [Cr(H2O)6]3+ and [NiCl4]2−

Explanation

The number of **spin-allowed d-d transitions** depends on the electron configuration (the free-ion terms).

$d^1$ (such as Ti(III)): one transition.
$d^2$ (V(III), octahedral): three transitions.
$d^3$ (Cr(III), octahedral): three transitions.
$d^8$ (Ni(II), octahedral or tetrahedral): three transitions.
High-spin $d^5$ (Mn(II), Fe(III)): **no** spin-allowed transitions (all excited states have a different spin multiplicity).
Tetrahedral Fe(II) $d^6$: only one spin-allowed transition.

Check each pair:
- **A. $[\text{V(H}_2\text{O)}_6]^{3+}$ ($d^2$) and $[\text{Ni(H}_2\text{O)}_6]^{2+}$ ($d^8$):** three each. ✓
- **B. $[\text{Cr(H}_2\text{O)}_6]^{3+}$ ($d^3$) and $[\text{NiCl}_4]^{2-}$ (tetrahedral $d^8$):** three each. ✓
- **C. $[\text{Ti(H}_2\text{O)}_6]^{3+}$ ($d^1$) and $[\text{Mn(H}_2\text{O)}_6]^{2+}$ (high-spin $d^5$):** one and none. ✗
- **D. $[\text{FeF}_6]^{3-}$ (high-spin $d^5$) and $[\text{FeCl}_4]^{2-}$ (tetrahedral $d^6$):** none and one. ✗

Answer **A and B**.