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Explain why [Cu(NH(3))(4)]^(2+) is colou...

Explain why `[Cu(NH_(3))_(4)]^(2+)` is coloured but `[Cu(CN)_(4)]^(3-)` is colourless.

Text Solution

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A compound which have unpaired electron can impart colour due to `d-d` transition. In `[Cu(NH_(3))_(4)]^(2+), Cu^(2+)` has unpaired electron `(3d^(9)4s^(0))`. Hence, it can undergo `d-d` transition. Thus `[Cu(NH_(3))_(4)]^(2+)` ion is coloured. Whereas in `[Cu(CN)_(4)]^(3-), Cu^(+)` does not have unpaired electron `(3d^(10)4s^(0))`. Hence `d-d` transition is not possible in `[Cu(CN)_(4)]^(3-)` and it is a colourless compound .
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