Models for the likely active catalysts in homogeneous Ziegler-Natta systems have been studied using ab initio quantum chemical methods. We investigated the geometries of the isoelectronic model complexes, X(2)M-R where X = Cl or Cp = (eta(5)-C5H5); where M = Sc and Ti+ (and also Ti); and where R = H, CH3, or SiH3. The general trend is that the M = Sc compounds strongly prefer a planar configuration, whereas the M = Ti+ cases generally prefer pyramidal geometries. This difference in geometry can be related to the differing ground-state electronic configurations for the metals : Sc is (4s)(2)(3d)(1), whereas Ti+ is (4s)(1)(3d)(2). The nonplanar geometry for [Cp(2)Ti-R](+) suggests an explanation for the origin of stereospecificity in the syndiotactic polymerization by unsymmetric metallocene catalysts. These results suggest that {(eta(5)-C5H4)CMe(2)(eta(5)-fluorenyl)}Sc-R would not catalyze syndiotactic polymerization under these conditions.