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JEE Advanced levelSingle correct optionDifficulty 3 of 3df149

Pink aqueous $[Co (H X_{2} O) X_{6}] X^{2 +}$ turns deep blue on adding concentrated HCl: $[Co (H X_{2} O) X_{6}] X^{2 +} + 4 Cl X^{-} [CoCl X_{4}] X^{2 -} + 6 H X_{2} O$ . The blue species is:

Aoctahedral, with 3 unpaired electrons, and more intensely coloured because $Cl X^{-}$ is a stronger-field ligand
Btetrahedral, with 1 unpaired electron, and more intensely coloured because it lacks a centre of symmetry
Csquare planar, with 3 unpaired electrons, and less intensely coloured than the pink species
Dtetrahedral, with 3 unpaired electrons, and more intensely coloured because it lacks a centre of symmetry

Answer: D

Four chloride ligands give a tetrahedral

[CoCl X_{4}] X^{2 -}

(balanced: charge -2 on both sides). Co(II) is

d^{7}

, and the weak-field tetrahedral

e^{4} t_{2}^{3}

arrangement keeps 3 unpaired electrons. With no inversion centre, d-d bands are more intense than in the centrosymmetric octahedral ion.

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