The Stable Pentamethylcyclopentadienyl Cation This work was supported by the U.S. National Science Foundation (Grant No. CHE-0091162). We thank Charlotte L. Stern for performing the crystal-structure analysis, Yuyang Wu for assistance in obtaining solid-state NMR data, Min Zhao and Stoyan Smoukov for providing ESR data, Alice L. Rodriguez for molecular modeling graphics, and John A. Pople and Mark A. Ratner for important discussions.