The new mixed-alkyl metallocene complexes (SBI)M(Me)CH2SiMe3 (M = Zr, Hf) are accessible by the successive treatment of (SBI)MCl2 with Me3SiCH2MgCl and MeMgCl in toluene (SBI = rac-Me2Si(1-Ind)(2)). Reaction with B(C6F5)(3) or CPh3+[B(C6F5)(4)](-) in toluene or toluene/difluorobenzene affords (SBI)M delta+(CH2SiMe3)(mu-Me)B-delta-(C6F5)(3) and the ion pairs [(SBI)MCH2SiMe3+center dot center dot center dot B(C6F5)(4)(-)], respectively. Both types of compounds are thermally stable in aromatic solvents at ambient temperature. Whereas in the MeB(C6F5)(3)(-) complexes the alkyl ligand points away from the metal and tight anion coordination forms the familiar inner-sphere ion pair, in the B(C6F5)(4)(-) salts the alkyl ligand adopts a conformation that enables agostic bonding to a gamma-CH3 group. Here, and by implication in M-polymeryl species of similar steric requirements, agostic interactions are preferred over anion coordination, leading to an outer-sphere ion pair structure. This alkyl bonding mode retards the -SiMe3 rotation, which for M = Hf is slow on the NMR time scale at -20 degrees C (at 300 MHz), while in the zirconium analogue cooling to below -60 degrees C is required. It was shown that chain swinging involves a 180 degrees rotation of the alkyl ligand about the Zr-C bond. Measurements of diffusion coefficients by pulsed field gradient spin-echo (PGSE) techniques suggest that while (SBI)Zr(CH2SiMe3)(mu-Me)B(C6F5)(3) exists in solution as mononuclear zwitterions as expected, [(SBI)ZrCH2SiMe3+center dot center dot center dot B(C6F5)(4)(-)] forms ion quadruples ([Zr] approximate to 2 mM), rising to hextuples at higher concentration. The relative positions of cations and anions depend on the ion pair concentration; higher aggregates make it difficult to assign specific anion positions. The rate of ion pair symmetrization ("anion exchange" k(ex)), as determined by variable-temperature NMR spectroscopy, decreases with decreasing metallocene concentration. For [(SBI)ZrCH2SiMe3+center dot center dot center dot B(C6F5)(4)(-)] at 25 degrees C and [Zr] = 2 mM, k(ex) = 500 +/- 170 s(-1); this value represents the upper limit of anion mobility expected under catalytic conditions where concentrations are typically 100 times lower. Ion pair symmetrization rates are therefore at least 1 order of magnitude slower than the growth of the number-average molecular weight of polypropene chains (k(p)[M] approximate to 10(4) s(-1) at [M] = 0.59 mol L-1) generated with tetraarylborate-based (SBI)Zr and other high-activity catalysts at identical temperatures. It is suggested that while for slower, inner-sphere ion pair catalysts the rate of 1-alkene consumption is commensurate with k(ex) ("continuous" chain propagation mechanism), high-activity catalysts may operate by a mechanism where the anion does not bind to the metal center and so does not limit the rate of monomer enchainment. In such a situation, agostic metal-alkyl interactions form the caalyst resting states in preference to anion coordination.