Question

Which complex compound does not obey $18-$electron rule ?

• A. $\left[V\right(CO{)}_5]$
• B. $\left[Fe\right(N{H}_3{)}_6{]}^{2+}$
• C. $\left[Ni\right(CO{)}_6]$
• D. $\left[Mn\right(H_{2}O{)}_6{]}^{2+}$

Answer 1

Answer: (A)

$\left[V\right(CO{)}_5]$

Sidgwick Rule:

Sidgwick suggested that the central metal in a coordination compound will continue accepting electron pairs from the ligands till the total number of electrons surrounding the metal including those donated by the ligands is equal to the next noble gas.

In Option A, $[$$V(CO{)}_6$$]$

V is the Central atom and the Oxidation state of it is Zero.

The number of electrons gained by ligand ---> metal coordination is 2x6 = 12

So 18 electron rule is

EAN = X-Y+Z

Where X = Atomic no of the central atom

Y = Oxidation state of the central atom

Z = Number of electrons gained by ligand ---> metal coordination

EAN = 23-0+12=35

It is not nearest to any noble gas.

So Option A doesn't obey the 18 electron rule