Question

In the metal carbonyls of general formula $M{\left(CO\right)}_x$ if $M$ is $Ni,Fe$ and $Cr$ the value of $x$ will be respectively:

• A. 6, 5, 6
• B. 4, 5, 6
• C. 4, 4, 5
• D. 4, 6, 6

Answer 1

The correct option is B 4, 5, 6

The EAN rule is as called the 18-Electron Rule. The $CO$ ligand donates two electrons to the metal (usually in the 0 oxidation state): $O\equiv C:⟶$M.

Hence ,
$Ni$ atomic number 28 ($d^8{s}^2$) (10 valence electrons) requires 4 $CO$ ligands: $Ni(CO{)}_4$ (nickel tetracarbonyl:) is an extremely poisonous low-boiling liquid.
$Fe$ atomic number 26 ($d^6{s}^2$) (8 valence electrons) requires 5 $CO$ ligands: $Fe(CO{)}_5$ (iron pentacarbonyl) higher boiling straw-coloured liquid, air stable but also poisonous
$Cr$ atomic number 24($d^4{s}^2$)6 valence electrons (group 6) hence $Cr(CO{)}_6$ (chromium hexacarbonyl) colorless/white air-stable crystals.

Hence 4,5,6 is the correct answer.