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The d-electron configurations of Cr²⁺, Mn²⁺, Fe²⁺ and Co²⁺ are d⁴, d⁵, d⁶ and d⁷, respectively. Which one of the following will exhibit minimum paramagnetic behaviour?

1. [Mn(H₂O)₆]²⁺
2. [Fe(H₂O)₆]²⁺
3. [Co(H₂O)₆]²⁺
4. [Cr(H₂O)₆]²⁺

Correct answer: [Co(H₂O)₆]²⁺

Solution

Paramagnetic behavior depends on the number of unpaired electrons. Co²⁺ ([Ar] 3d⁷) in an octahedral field has the least unpaired electrons (3), making it the least paramagnetic among the given options.

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