| Solution structural and thermodynamic properties of metallocenium ion pairs of general type L2MR+ X- (where L2 = cyclopentadienyl-type ligand; M = typically group 4 low valent metal; R = alkyl group; and X- = perfluoroaryl group 13 anion) having the anion in either the inner coordination sphere (inner sphere ion pairs, ISIPs) or outer coordination sphere (outer sphere ion pairs, OSIPs) have been investigated by a wide array of spectroscopic and physical techniques.; Freezing point depression and pulsed gradient spin echo NMR measurements indicate that ISIPs do not show appreciable aggregation in low permittivity solvents such as benzene and toluene, and the data are consistent with a 1:1 ion pair model. In contrast, OSIPs exhibit strongly concentration-dependent aggregation behavior, although aggregation effects become appreciable only over concentrations well above those typically employed in polymerization reactions.; Data from nuclear Overhauser effect NMR measurements show that the anions of ISIPs are tightly associated with their metallocenium cationic counterparts in solution and that the weak coordination of the anion enforces a rigid anion/cation relative orientation geometry. Similar experiments on OSIPs indicate that even in the absence of a coordinative interaction, the anion remains tightly associated with the metallocenium cation. Furthermore, while the anion can explore a somewhat greater range of orientations relative to the cation, the cation/anion interaction remains highly specific.; Enthalpies of formation of ion pairs derived from group 4 dimethyl metallocenes as determined by isoperibol solution calorimetry show that Al(C6F 5)3 abstracts methide about 8 kcal mol-1 less enthalpically than B(C6F5)3. This conclusion is supported by X-ray crystallographic analysis of C2H4(eta 5-Ind)2ZrCH3+ H3CAl(C 6F5)3-, which indicates the Zr-CH3(bridging) bond length to be longer than that found for analogous B(C6F5)3-derived ion pairs. The thermodynamics parameters of ion pair side exchange as determined by variable temperature NMR are consistent with Al(C6F5)3-abstracted methide being more strongly bonded to the metal center than B(C6F 5)3-abstracted methide. |
