Question

Which of the following has an aromatic ring in complex?
(A) DMG in dimethyl glyoximate.
(B) Cyclopentadienyl anion in ferrocene.
(C) Both (A) and (B).
(D) None of these.

Answer 1

We know that a compound is said to be aromatic if it has $\left( 4n+2 \right)\pi$ electrons which are delocalized in the structure and the structure is planar. Also remember that neutral structures are always more stable than charged structures.

Complete answer:
First we should start from the Aromaticity of these two structures. To be known as an aromatic compound any structure should fulfill following conditions:
The molecule should be cyclic
These π electrons should be delocalized in the ring or structure
The structure should be planar
It should follow Huckel’s rule or the $\left( 4n+2 \right)$ rule.
According to the Huckel’s rule (also known as $\left( 4n+2 \right)$ rule any compound having number of π electrons equal to $\left( 4n+2 \right)$ where n is zero or any other whole natural number $\left( 1,\text{ }2,\text{ }3\text{ }\ldots . \right)$ is said to be an aromatic compound. For benzene: it has $6\pi$ electrons which is equal to $\left( 4n+2 \right)\pi$ electrons, where $n\text{ }=\text{ }1.$ So, we can say that benzene follows Huckel’s rule. For Cyclopentadienyl anion: it also has 6π electrons which is equal to $\left( 4n+2 \right)\pi$ electrons, where $n\text{ }=\text{ }1.$ So, we can say that this also follows Huckel’s rule.
Now let us come to the point of stability. We already know that neutral structures are more stable than charged structures. So, we can say that benzene is more stable than Cyclopentadienyl anion. Another reason for stability of benzene is less angle strain in its structure. Hence from the above discussion we have come to a conclusion that: Cyclopentadienyl anion and benzene both are aromatic and benzene is more stable among both of them.
Cyclopentadienyl anion in ferrocene is aromatic as it is planar, cyclic and has $\left( 4n+2 \right)\pi$ electrons.
Therefore, the correct answer is option B.

Note:
Remember that major mistake that we can make here is while finding a compound’s Aromaticity. Always make sure you declare any compound aromatic only if it fulfills one single condition it cannot be called aromatic. So, we can say that benzene is more stable than Cyclopentadienyl anion. Another reason for stability of benzene is less angle strain in its structure.