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The synthesis and properties of two soluble, weakly coordinating derivatives of the tetrakis(perfluoroaryl)borate anion B(4-C6F4TBS)4 - and B(4-C6F4TIPS)4 - (TBS = tert-butyldimethylsilyl and TIPS = triisopropylsilyl) are reported. Reaction of the trityl salts of the above anions with a variety of zirconium and thorium L2MMe2 complexes in benzene or toluene affords the cationic ion-paired methyl complexes L2MMe+X- or the corresponding hydrido complexes L2MH+X- (L2 = bis(cyclopentadienyl)- or cyclopentadienylamido-type ligand) when the reaction is carried out under dihydrogen. The solid state structure of the complex (Me5Cp)2ThMe+B(C6F5)4 - has been characterized by X-ray diffraction. The B(C6F5)4 --based zirconocenium methyl complexes L2MMe+ are unstable at room temperature with respect to, among other factors, intramolecular C−H activation of the ligand framework. In general, the thermal stabilities of the B(C6F4TBS)4 -- and B(C6F4TIPS)4 --derived complexes are greater than those of the corresponding B(C6F5)4 -- and MeB(C6F5)3 --derived analogues. The relative coordinative tendencies of MeB(C6F5)3 -, B(C6F5)4 -, B(C6F4TBS)4 -, and B(C6F4TIPS)4 - are estimated from the solution spectroscopic information and the structural dynamics of the ion-pairs and follow the order MeB(C6F5)3 - > B(C6F4TBS)4 - ≈ B(C6F4TIPS)4 - > B(C6F5)4 -. The coordination of the neutral metallocene precursors to the cationic metallocenes is found to compete with counteranion coordination. Arene solvent coordination to the zirconium constrained geometry cation [(Me4Cp)SiMe2(NtBu)]ZrMe+ is also observed when B(C6F5)4 - is the counteranion. (1,2-Me2Cp)2ZrMe+B(C6F4TBS)4 - undergoes slow decomposition under an inert atmosphere to afford [(1,2-Me2Cp)2ZrF]2(μ-F)+B(C6F4TBS)4 -, which has been characterized by X-ray diffraction. The olefin polymerization activity and thermal stability of the zirconocene catalysts reaches a maximum when B(C6F4TBS)4 - and B(C6F4TIPS)4 - are used as counteranions. The polymerization activity of the zirconium constrained geometry complex also reaches a maximum in aromatic solvents when B(C6F5)4 - is used as the counteranion, apparently due to solvent coordination.

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