Conversion of ferrocene carboxylic acid into ferrocene carbonyl chloride is expected to follow the normal procedure done in converting R-COOH into R-CO-Cl. One of the best reagents that can do that is liquid thionyl chloride, SOCl2, but some chemists may use phosphorus trichloride, PCl3 . When I was MSc student, I preferred the former over the latter in converting dicarboxylic acids into diacid chlorides since the reaction proceeds as the equation indicates:

RCOOH + SOCl2 → R-CO-Cl + SO2(g) + HCl(g)

Equimolar amounts of the reactants are placed in a r.b. flask & refluxed in the hood for ~ 20 minutes. {Reflux is when the condenser is attached to the flask vertically}. The mouth of the condenser is left open. The success of the reaction is proven when a moist blue litmus paper turns into red when placed on the condenser's mouth.

A positive aspect of this procedure is that there will be no side product left in the r.b.flask after the reaction is over. Note that R-CO-Cl is highly reactive & water-sensitive so it has to be used immediately and it has to be stored in a dry place.

Nizar Matar thanks for you responds on synthesis of ferrocene carbonyl chloride .it will be so good if you add any refrence to corresponding procedure.