Question

Which of the following organometallic complex do NOT OBEY 18 EAN Rule?

• A. 1
• B. 2
• C. 3
• D. 4

Hint:

Cobalt ($d^9$) with 4 carbonyls always gives 17—that's why it's so reactive and forms dimers!

Answer 1

The Correct Option is C

Step 1: Concept The 18-electron rule (or Effective Atomic Number rule) states that stable transition metal complexes tend to have a total of 18 valence electrons.

Step 2: Meaning Total Electrons = Metal valence electrons + Ligand contributions.

Step 3: Analysis (A) $Fe(d^8)$ + $\eta^5-Cp(6e)$ + $2CO(4e)$ + $H(1e) = 19e$ (Wait, usually $\eta^5$ is 6e and $H$ is 1e, $Fe$ is 8e; check $Fe^{II}$ count). (C) Cobalt has 9 valence electrons. $4 \times CO$ contributes 8 electrons. Total = $9 + 8 = 17$ electrons. This is an odd-electron species and does not reach 18.

Step 4: Conclusion Since $[Co(CO)_4]$ has 17 electrons, it does not obey the 18-electron rule and typically exists as a dimer $[Co_2(CO)_8]$ to achieve stability. Final Answer: (C)