Racemic-Meso Interconversion for ansa-Scandocene and ansa-Yttrocene derivatives.: Molecular Structures of rac-{Me2Si[η5-C5H2-2,4-(CHMe2)2]2}ScCl•LiCl(THF)2, [meso-{Me2Si[η5-C5H2-2,4-(CHMe2)2]2}Y(μ2Cl)]2, and meso-{Me2Si[η5-C5H2-2,4-(CHMe2)2]2}Zr(NMe2)2

The dilithium bis(cyclopentadienide) species Li-2{Me2Si[C5H2-2,4-(CHMe2)(2)](2)} (Li(2)Ip) reacts with ScCl3(THF)(3) to afford rac-IpYCl . LiCl(THF)(2) (I) and [meso-IpSc(mu(2)-Cl)](2) (2) and with YCl3(THF)(3.5) to afford rac-IpYCl . LiCl(THF)(2) (3) and [meso-IpY(mu(2)-Cl)](2) (4). Met;alation with both scandium and yttrium chlorides yields the metallocene chlorides in approximately 3:1 racemic:meso ratios. Reaction of IpH(2) with Zr(NMe2)(4) yields exclusively meso-IpZr(NMe2)(2) (9). Treatment of 1 or 2 with allylmagnesium bromide affords the allyl complexes rac-IpSc(eta(3)-C3H5) (5) and meso-IpSc(eta(3)-C3H5) (6) and with crotylmagnesium chloride affords rac-IpSc(eta(3)-C3H4Me) (7) and meso-IpSc(eta(3)-C3H4Me) (8). Diastereomerically pure rac dichlorometalate compounds (1 or 3) or pure meso chloro dimers (2 or 4) undergo spontaneous isomerization upon dissolution in THF-d(8) with reversion back to a 3:1 racemic:meso ratio. Isomerization of 5 and 6 is observed in THF-d(8), above 55 degrees C, affording an equilibrium ratio of similar to 2:1 racemic:meso isomers. While spontaneous isomerization of 5, 6, 7, or 8 is very slow at room temperature, various salts and Grignard reagents promote isomerization even at; room temperature; the isomerizations are not accelerated by light. The proposed mechanism far racemic-meso isomerization involves heterolytic dissociation of one cyclopentadienide ligand from the metal, rotation around that Si-Cp- band, and recoordination on tl le opposite face, effecting net epimerization. X-ray diffraction studies have been performed on rac-IpScCl . LiCl(THF)(2) (1), [meso-IpY(mu(2)-Cl)](2) (4), and meso-IpZr(NMe2)(2) (9).