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The title compound [(µ-CH 2 )(ZrCp 2 Me) 2 ] (4) was prepared by the reaction of [Cp 2 Zr(Cl)Me] (6) with CH 2 (MgBr) 2 (5) or (CH 2 Mg) n (7). It had been expected that the carbon atom bridging the two zirconium atoms of 4 might be a candidate for a planar tetracoordinate carbon atom, but the X-ray crystal structures of two different modifications (crystals 1 and 2) showed it to be non-planar; however, the widened Zr−C−Zr angle of 132.7(1) and 134.0(3)°, respectively, and an unsymmetrical coordination of the central µ-CH 2 group (average: Zr1−CH 2 2.225 Å , Zr2−CH 2 2.242 Å ) indicates a tendency towards planarity. While 4 is relatively stable in THF solution, it disproportionates in toluene solution to give the 1,3-dizirconacyclobutane (Cp 2 ZrCH 2 ) 2 (8) and [Cp 2 ZrMe 2 ] (9), espe-

Durch eine intramolekulare Ionenpaarwechselwirkung wird die ungewöhnlich verzerrte Koordinationsgeometrie an C1 in 1 stabilisiert. Im Verlauf der NMR‐spektroskopisch verfolgten Topomerisierung 1⇋ent‐1 muß das innere Zr…CH2‐Ionenpaar aufgelöst werden, dabei wird vermutlich eine Spezies mit “normaler” tetraedrischer Koordinationsgeometrie an C1 durchlaufen.

An intramolecular ion‐pair interaction stabilizes the unusual, distorted coordination geometry of C1 in 1. In the course of the topomerization 1⇋ent‐1, which was followed by NMR spectroscopy, the internal Zr ⃛CH2 ion pair must be cleaved. The rearrangement pathway probably proceeds via a species with “normal” tetrahedral coordination geometry at C1.