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Co2+ → [Ar]3d7 4s0, here, Br− is a weak field ligand so will not cause pairing of d-electrons in Co2+.

1. [CoBr4]2− will exhibit paramagnetic behaviour due to unpaired electrons.
2. [CoBr4]2− will exhibit diamagnetic behaviour due to paired electrons.
3. [CoBr4]2− will exhibit paramagnetic behaviour due to paired electrons.
4. [CoBr4]2− will exhibit diamagnetic behaviour due to unpaired electrons.

Correct answer: [CoBr4]2− will exhibit paramagnetic behaviour due to unpaired electrons.

Solution

Br− is a weak field ligand, so it does not cause pairing of d-electrons in Co2+. The electronic configuration of Co2+ remains [Ar]3d7, with three unpaired electrons, making [CoBr4]2− paramagnetic.

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