CO in dichloromethane Cooling (-78°C) of solns. of approx. 5:1 mixt. of alkenyliron complex and (Cp2Fe)(BF4), stirring (20 min, CO, 1 atm), addn. of (Cp2Fe)(BF4) soln. to iron complex by CO-pressure, stirring (45 min), removing from cold bath, stirring (1 h).; Chromy. (Al2O3, hexane and hexane/CH2Cl2) yields first ferrocene, then a 4:1 mixt. of stereoisomers in 84 percent yield, elem. anal.;