Question

Metal carbonyls can have formula M(CO)\(_x\) where \(x =\) number of CO units coordinated to metal M. What is the formula of the carbonyl of Fe (26) if EAN of Fe in metal carbonyl is 36?

• A. 2
• B. 3
• C. 4
• D. 5

Hint:

EAN rule (18-electron rule): Most stable carbonyls have total electrons = 36 (for 4th period metals).

Answer 1

The Correct Option is D

Step 1: Concept:
EAN (Effective Atomic Number) = Atomic number - (oxidation state) + 2 \(\times\) (number of ligands). CO is neutral ligand.
Step 2: Explanation:
For Fe(CO)\(_x\) in zero oxidation state: EAN = 26 + 2x = 36 \(\Rightarrow\) 2x = 10 \(\Rightarrow\) x = 5.
Formula = Fe(CO)\(_5\). Iron pentacarbonyl.
Step 3: Conclusion:
Thus, \(x = 5\).