`[Cr(NH_(3))_(6)]^(3+)` is paramagnetic while `[Ni(CN)_(4)]^(2-)` is diamagnetic. Explain, why ?

Explanation

`[Cr(NH_(3))_(6)]^(3+)` is paramagnetic.
The oxidation state of Cr in this complex is +3. Hence, we have
Electronic configuration of `Cr^(3+)`
`d^(2)sp^(3)` hybridisation

As NH3 is a weak ligand, pairing of electrons does not take place. Hence, `d^(2)sp^(3)` hybridisation takes place forming inner complex with octahedral shape. As there are three unpaired electrons, the compound is paramagnetic.
`[Ni(CN)_(4)]^(2-)` is diamagnetic due to the absence of unpaired electrons.