First, the picture below shows the structure of the Ferrocene:
From the picture above, we have a question: What makes this compound an aromatic compound?
Huckel Approximation tells us that the aromatic compound must have 4n + 2 pi electrons. From the structure above, there's only 5 pi electrons for each cyclopentadiene so what makes it?
The answer is simple.
This Iron is in Fe2+ form, which is mean we have to stabilize the positive charge.
In other way, we have to say that each cyclopentadiene has 1 more electron so that it forms an anion. Which is mean now, the diene has 6 pi electrons.
Crystal of Ferrocene