The bis(1,2,3,4,5-pentamethyl-1,3-cyclopentadien-1-yl)phos-also been characterized by single-crystal X-ray diffraction studies. The molecular structure of 11a is governed by steric phanes 3Ϫ7 are formed in good yields by the reaction of the halogenophosphanes 2a or 2b with the appropriate nucle-congestion, which typically would lead to a parallel arrangement of the two pentamethylcyclopentadienyl ligands at the ophile. Following another route, the dialkylaminobis(pentamethylcyclopentadienyl)phosphanes 11aϪc have been syn-phosphorus atom. However, surprisingly, the crystal structure of the (dimethylamino)phosphane 11a, exhibits a tilted, ra-thesized by the treatment of dichloro(dialkylamino)phosphanes with two equivalents of (pentamethylcyclopentadienyl)li-ther than a parallel, conformation of the pentamethylcyclopentadienyl groups [tilt angle: 130.6(1)°]. thium. The compounds 3Ϫ11 have been characterized by multinuclear NMR spectroscopy and 3, 5, 6, and 11a have Previous comprehensive investigations of the main-group phanes 2a and 2b. Both compounds are obtained as yellow, thermally stable solids, that can be distilled in vacuo. cyclopentadienyl (Cp) compounds [1] [2] [3] have revealed a variety of unusual structures and bonding modes analogous Scheme 1. Synthesis of new bis(Cp*)phosphanes by nucleophilic