Color of [Cu(NH₃)₄]²⁺:
AColorless
BPale blue
CDEEP BLUE (intense d-d after ligand exchange)
DYellow
Answer & Solution
Correct answer: C. DEEP BLUE (intense d-d after ligand exchange)
Cu²⁺(aq) is pale blue. Adding NH₃ gives deep royal blue [Cu(NH₃)₄]²⁺ (NH₃ stronger field than H₂O, larger Δ, shifts absorption).
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