Although many synthesis methods of ferrocene are discussed in literature, I want to know about the black precipitation which occurs when $\ce{FeCl2・4H2O}$ in $\ce{DMSO}$ solution reacts with cyclopentadienyl anion in ether solution. What’s this?
At first, crushed $\ce{KOH}$ ($\pu{20 g}$) and diethyl ether $\pu{100 mL}$ are added into a $\pu{300 mL}$ erlenmeyer flask, then the solution is stirred in $10$ minutes. In addition, cracked cyclopentadiene (monomer) $\pu{5.5 mL}$ is added into this solution. Crushed $\ce{FeCl2・2H2O}$ ($\pu{6.5 g}$) and $\ce{DMSO}$ $\pu{25 mL}$ are added into a $\pu{100 mL}$ beaker, then the solution is stirred. This solution is added drop by drop into the flask. This is the time when the black precipitation forms.
