Counteranion effects on the rate and stereochemistry of syndiotactic propylene enchainment by the archetypal C-s-symmetric precatalyst [Me2C(Cp)(Flu)]ZrMe2 (1; Cp = C5H4; Flu = C13H8, fluorenyl) are probed using the cocatalysts MAO (2), B(C6F5)(3) (3), B(2-C6F5C6F4)(3) (4), Ph3C+B(C6F5)(4)(-) (5), and Ph3C+-FAI(2-C6F5C6F4)(3)(-) (6), offering greatly different structural and ion pairing characteristics. Reaction of 1 with 3 affords [Me2C(CP)(Flu)]ZrMe+ MeB(C6F5)(3)(-) (7). In the case of 4, this reaction leads to formation the mu-methyl dinuclear diastereomers {([Me2C(Cp)(Flu)]ZrMe)(2)(mu-Me)}(+) MeB(2-C6F5C6F4)(3)(-) (8). A similar reaction with 6 results in diastereomeric [Me2C(Cp)(Flu)]ZrMe+ FAI(2-C6F5C6F4)(3)(-) (10) ion pairs. The molecular structures of 7 and 10 have been determined by single-crystal X-ray diffraction. Reorganization pathways available to these species have been examined using EXSY and dynamic NMR, revealing that the cation-MeB(C6F5)(3)(-) interaction is considerably weaker/more mobile than in the FAI(2-C6F5C6F4)(3)(-)-derived analogue. Polymerizations mediated by 1 in toluene over the temperature range of -10degrees to +60 degreesC and at 1.0-5.0 atm propylene pressure (at 60 degreesC) reveal that activity, product syndiotacticity, m and mm stereodefect generation, and chain transfer processes are highly sensitive to the nature of the ion pairing. Thus, the complexes activated with 4 and 5, having the weakest ion pairing, yield the highest estimated propagation rates, while with 6, having the strongest pairing, yields the lowest. The strongly coordinating, immobile FAI(2-C6F5C6F4)(3)(-) anion produces the highest/least temperature-dependent product syndiotacticity, lowest/least temperature-dependent m stereodefect abundance, and highest product molecular weight. These polypropylene microstructural parameters, and also M, are least sensitive to increased propylene pressure for FAI(2-C6F5C6F4)(3)(-), but highest with MeB(C6F5)(3)(-). In general, mm stereodefect production is only modestly anion-sensitive; [propylene] dependence studies reveal enantiofacial propylene misinsertion to be the prevailing mm-generating process in all systems at 60 degreesC, being most dominant with 6, where trim stereodefect abundance is lowest. For 1,3-dichlorobenzene as the polymerization solvent, product syndiotacticity, as well as m and mm stereodefects, become indistinguishable for all cocatalysts. These observations are consistent with a scenario in which ion pairing modulates the rates of stereodefect generating processes relative to monomer enchainment, hence net enchainment syndioselectivity, and also dictates the rate of termination relative to propagation and the preferred termination pathway.In comparison to 3-6, propylene polymerization mediated by MAO (2) + 1 in toluene reveals an estimated ordering in site epimerization rates as 5 > 4 > 2 > 3 > 6, while product syncliotacticities rank as 6 > 2 > 5 similar to 4 > 3.