mixtures thereof, in which A is a halogen, X is a substituted or unsubstituted hydrocarbon radical having from 1 to 3 aliphatic double bonds which is bonded to the platinum atom via a PtC-sigma bond and a Pt-olefin-Pi bond, X' is a hydrocarbon radical which is substituted with an alkoxy or acyloxy group and has from 1 to 3 double bonds and is bonded to the platinum atom via a PtC-sigma bond and a