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Coordination Compounds

Question
CBSEENCH12007387

[Cu(NH3)4]2+ ion is coloured while [Cu(CN)]3– ion is colourless. Why?

Solution
A compound which have unpaired electron can impart colour due to d-d transition. 
[Cu(NH
3)4]2+ has unpaired electron. Hence, [Cu(NH3)4]2+ ion is coloured whereas [Cu(CN)4]3–does not have unpaired electron. it is a colourless compound.