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The d-electron configurations of Cr²⁺, Mn²⁺, Fe²⁺ and Ni²⁺ are 3d⁴, 3d⁵, 3d⁶ and 3d⁸ respectively. Which one of the following aqua complexes will exhibit the minimum paramagnetic behaviour?

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

Correct answer: [Ni(H₂O)₆]²⁺

Solution

Paramagnetic behavior depends on the number of unpaired electrons. [Ni(H₂O)₆]²⁺ has a 3d⁸ configuration, resulting in 2 unpaired electrons, which is the least among the given options.

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