Question

How do you balance `C_5H_5 + Fe→ Fe(C_5H_5)_2`?

Answer 1

Well, you do that by having two `"C"_5"H"_5` molecules:

`2"C"_5"H"_5 + "Fe" -> "Fe"("C"_5"H"_5)_2`

But I don't think a neutral `"C"_5"H"_5` exists or is readily found. I would have done it the following way instead. This is actually a rather interesting reaction: