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Which complex obeys the **EAN rule** (EAN = noble gas count)?
A$[Fe(CN)_6]^{3-}$ (EAN = 35)
B$[Cu(NH_3)_4]^{2+}$ (EAN = 35)
C$Ni(CO)_4$ (EAN = 36 = Kr)
DAll of the above obey the rule
Answer & Solution
Correct answer: C. $Ni(CO)_4$ (EAN = 36 = Kr)
Ni(CO)₄: Z = 28, OS = 0, ligand electrons = 8. EAN = 28 − 0 + 8 = 36 → Kr ✓. $[Fe(CN)_6]^{3-}$: 26 − 3 + 12 = 35 — does NOT obey. $[Cu(NH_3)_4]^{2+}$: 29 − 2 + 8 = 35 — does NOT obey. Only Ni(CO)₄ obeys among the three.
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