[coldest]

from books, it says that ferrocene doesn't react with HCl. however in my expriment, when I put ferrocene powder in HCl solution in atmosphere, the solution turns blue a few days later. So I'm confused. I wish your help, thantks!

[plu]

In the presence of O₂ and in strong acid solution, ferrocene will be oxidized to Fe(C₅H₅)₂⁺.  I believe this compound appears blue in solution.

[coldest]

In the presence of O₂ and in strong acid solution, ferrocene will be oxidized to Fe(C₅H₅)₂⁺.  I believe this compound appears blue in solution.

thank you very much!
by the way, could you suggest some papers about the reaction please

[coldest]

ok, I got it from google. The blue solution is result of dissolving ferrocenium ion. So in air, Fe2+ was oxidated into Fe3+, however if I put NaOH in the solution, a orange-yellow deposit was come out, then I put HCl again, the deposit dissolving and the solution is also blue, can anyone tell me what is it?
thanks!

[formamide]

It is not uncommon to wash certain metallocenes (Cp2Fe, Cp2TiCl2, etc.) with conc aq HCl to remove the assorted cats & dogs that might be present. A few of them are considerably more hydrolytically stable than one would imagine.  Typically, long term acid exposure is to be avoided. Without doing a lit search I would agree that the Fe (3+) species is a good guess. Some metallocene species form oxo bridges on exposure to water. I've never heard of ferrocene doing it though.