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Question
[Cr(NH3)6]3+ is paramagnetic while [Ni(CN)4]2− is diamagnetic. Explain why?
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Solution
Formation of [Cr(NH3)6]3+: The oxidation state of chromium in [Cr(NH3)6]3+ ion is +3. The electronic configuration of chromium is [Ar] 3d54s1. The hybridisation is shown in the following diagram:

The Cr3+ ion provides six vacant orbitals to accommodate six electron pairs from six ammonia molecules. As a result, the complex [Cr(NH3)6]3+ has d2sp3 hybridisation and is octahedral. The presence of three unpaired electrons in the complex explains its paramagnetic property.
Formation of [Ni(CN)4]2−: In [Ni(CN)4]2− the oxidation state of Ni is +2 and its electronic configuration is 3d8. Hybridisation can be explained as follows:

Each hybridised orbital accepts a pair of electrons from the cyanide ion. The absence of unpaired electrons confirms the diamagnetic behaviour of [Ni(CN)4]2−.
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