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GATE 2025 CY – Question 14

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. · 1 mark · Multiple choice

An aqueous solution of $\mathrm{Co(ClO_4)_2\cdot6H_2O}$ is light pink in colour. Addition of conc. HCl results in an intense blue coloured solution due to the formation of a new species. The new species among the following is [Given: Atomic number of Co = 27]

Four candidate cobalt coordination species I–IV.
  1. I
  2. II
  3. III
  4. IV

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Correct answer: (C) III

Explanation

**Aqueous solution.** $\text{Co}^{2+}$ in water forms the octahedral hexaaqua complex $[\text{Co(H}_2\text{O)}_6]^{2+}$, which is **light pink**.

**Adding concentrated HCl.** The large concentration of chloride ions replaces the water ligands:
$$[\text{Co(H}_2\text{O)}_6]^{2+}+4\,\text{Cl}^-\rightleftharpoons[\text{CoCl}_4]^{2-}+6\,\text{H}_2\text{O}.$$

Chloride is a bigger ligand than water, and $d^7$ $\text{Co(II)}$ with four large ligands prefers a **tetrahedral** geometry. A tetrahedral complex has a smaller crystal-field splitting ($\Delta_t\approx\tfrac49\Delta_o$) and no centre of symmetry, so the $d$-$d$ bands are much more intense and shift to lower energy. The result is the intense **blue** $[\text{CoCl}_4]^{2-}$ (option C, the tetrahedral species).