[Cr(CO)6] obey EAN?
Forecast: Cr is Z=24, oxidation state 0, CN 6.
Verify: EAN =24+12=36= Kr ✔. Obeys → very stable carbonyl. (Pattern: d-block carbonyls of Cr, Fe, Ni all hit 36.)
Each ligand donates a lone pair = 2 electrons through a coordinate bond.
State the EAN rule.
A complex is especially stable when its EAN equals the atomic number of the nearest noble gas (2,10,18,36,54,86).
EAN of [Ni(CO)₄]?
28 − 0 + 2×4 = 36 (Kr). Obeys the rule.
EAN of [Fe(CN)₆]⁴⁻?
Fe is +2: (26−2) + 2×6 = 36 (Kr).
EAN of [Co(NH₃)₆]³⁺?
(27−3) + 12 = 36 (Kr).
Does [Ni(NH₃)₆]²⁺ obey EAN?
No, EAN = (28−2)+12 = 38 ≠ 36. EAN is a guide, not a law.
How is EAN related to the 18-electron rule?
EAN minus noble-gas-core electrons = valence count; EAN = noble gas Z ⇔ 18 valence electrons.
How do you find the metal's oxidation state in a complex?
Overall complex charge minus sum of ligand charges.
Who proposed the EAN rule?
Nevil Sidgwick.
Recall Feynman: explain to a 12-year-old
Imagine the metal is a kid who feels happiest with a full plate of cookies — exactly as many cookies as the "cool kid" next to him (the noble gas). The metal first gives away some cookies because it's charged (that's the oxidation state), then each friend (ligand) hands it 2 cookies (a lone pair). If after all the gifts the metal's plate has exactly the same number of cookies as the cool kid, it's super happy and stable. That happy number is the EAN. Sometimes a kid is content with a slightly different number too — so it's a good guess, not a strict law.
Dekho, EAN (Effective Atomic Number) ka idea simple hai: central metal ke around jitne total electrons "dikhte" hain, unko gino. Metal neutral state me Z electrons rakhta hai, lekin complex banate waqt wo kuch electrons khota hai (oxidation state ke barabar). Phir har ligand ek lone pair yaani 2 electrons donate karta hai coordinate bond ke through. Toh formula ban jata hai: EAN = Z − oxidation state + 2 × coordination number.
EAN rule (Sidgwick ka diya hua) kehta hai — agar yeh total count kisi noble gas ke atomic number ke barabar ho jaye (Kr = 36 sabse common), tab complex extra stable hota hai. Isi wajah se metal carbonyls jaise Ni(CO)4, Cr(CO)6, Fe(CN)64− sab 36 par aate hain aur bahut stable hote hain.
Do common galtiyan se bachna: (1) Har ligand 2 electrons deta hai, 1 nahi — kyunki dative bond ek poora lone pair hota hai. (2) Metal ki oxidation state nikalne ke liye complex ka charge minus ligand charges karo, sirf bracket ka number mat utha lo. Jaise [Fe(CN)6]4− me charge −4 hai par Fe +2 hai.
Aur yaad rakho — EAN ek guideline hai, koi strict law nahi. [Ni(NH3)6]2+ ka EAN 38 aata hai phir bhi wo stable hai. EAN basically 18-electron rule ka hi cousin hai: jab valence electrons 18 ho jaate hain tabhi EAN noble gas ko hit karta hai. Exam me yeh ratio-fatio nahi, bas formula laga ke gino — number 36 (ya nearest noble gas) aaya toh "obeys", warna "violates".
