Syntheses, crystal structures, and polymerization data for new isospecific metallocenes (heterocenes) having cyclopentenyl ligands b-fused to substituted thiophenes (Tp) and pyrroles (Pyr) are reported. The C-2-and C-1-symmetric heterocenes are dimethylsilyl bridged, have methyl groups adjacent to the bridgehead carbon atoms, and have aryl substituents protruding in the front. rac-Me2Si(2,5-Me-2-3-Ph-6-Cp[b]Tp)(2)ZrCl2/MAO (MAO = methyl alumoxanes) is the most active metallocene catalyst for polypropylene reported to date. rac-Me2Si(2,5-Me-2-3-Ph-6-Cp[b]Tp)(2)ZrCl2 and rac-Me2Si(2,5-Me-2-1-Ph-4-Cp[b]Pyr)(2)ZrCl2 have the same structure, and the former is 6 times more active, produces half the total enantiofacial errors, and is 3.5 times less regiospecific in propylene polymerizations at the same conditions. rac-Me2Si(2-Me-4-Ph-1-Ind)(2)ZrCl2/ MAO is 3.5 times lower in activity than rac-Me2Si(2,5-Me-2-3-Ph-6-Cp[b]Tp)(2)ZrCl2 catalyst. and while the former is the more stereospecific and the less regiospecific, the sum of these two enantioface errors is the same for both species. Fine-tuning the heterocene sterics by changing selected hydrogen atoms on the ligands to methyl groups influenced their catalyst activities, stereospecificites, regiospecificites, and isotactic polypropylene (IPP) M-w. Thus, both substituting a hydrogen atom adjacent to the phenyl ring with a methyl group on an azapentalenyl ligand system and replacing one and then two hydrogens on the phenyl ring with methyls on thiopentalenyl ligands provided increased polymer T, and M, with increasing ligand bulk. Polymer molecular weights are sensitive to and inversely proportional to MAO concentration, and the catalyst activities increase when hydrogen is added for molecular weight control. The polymer T, values with the thiopentalenyls as TIBAL/[Ph3C][B(C6F5)(4)] systems were higher than with MAO as catalyst activator. A racemic C-1, pseudomeso complex with a hybrid dimethylsilyl-bridged 2-Me-4-Ph-1-Ind/2,5-Me-2-4-Ph-1-Cp[b]Pyr ligand produced the first sample of IPP with all the steric pentad intensities fitting the enantiomorphic site control model. Speculative mechanistic considerations are offered regarding electronic effects of the heteroatoms and steric effects of the ligand structures, the preferred phenyl torsion angles, and anion effects.