Reaction of the protonic ether compound [H(OEt2)2][Al(ORF)4] (RF = C(CF3)3) with AlCp3 leads to the formation of the [AlCp2]+-cation, which is stabilized by the weakly coordinating [Al(ORF)4]– ion. Besides [AlCp2]+, the ion [AlCp2·2Et2O]+, which is stabilized by two ether molecules, is formed in an equilibrium reaction. The so far unknown molecular structure of [AlCp2]+ and the molecular structure of [AlCp2·2Et2O]+ are presented in this work. To get insight in the formation and the equilibrium between these two cations, quantum mechanical calculations were performed. Moreover, the influence of counterions like [Al(ORF)4]– and [MeB(C6F5)3]– on the activity of [AlCp2]+ to act as a polymerization initiating agent for isobutene was investigated.