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

Well, you do that by having two ${\text{C"_5"H}}_{5}$ molecules:
2"C"_5"H"_5 + "Fe" -> "Fe"("C"_5"H"_5)_2
But I don't think a neutral ${\text{C"_5"H}}_{5}$ exists or is readily found. I would have done it the following way instead. This is actually a rather interesting reaction: