Ion-ion and ion-solvent interactions in transition metal-organic salts
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Description
The elucidation of ion-pair structures of transition metal-organic salts was approached through detailed conductance measurements on tetrahydrofuran (THF) and acetonitrile solutions of bis(triphenyl-phosphine)iminium and alkali salts of carbonylates, hydridocarbonylates, cyclopentadienylcarbonylates, and cyanocarbonylates. The derived conductance parameters were correlated with other physical probes and with chemical reactivity The large bis(triphenylphosphine)iminium cation ({PPN}('+)) was shown to be extensively associated to the above-mentioned anions in THF solution. Ion-pair center-to-center contact distances a were obtained from analysis of conductance-concentration data and compared with known crystallographic parameters. The slightly larger a values were attributed to an equilibrium in solution between contact and solvent-separated ion pairs. The solution geometry of the species {HFe(CO)(,4)}('-) as its {PPN}('+) salt was established from infrared intensity data and demonstrated to be very similar to that found in the solid state structure Interactions of Na('+), {(H(,3)C)(,4)N}('+), and {PPN}('+) with several of the mentioned anions were analyzed. The charge delocalization of the {PPN}('+) cation was shown to be so great that ion-pair interactions produced no perturbation of the anions, as observed via IR, but the sodium ion interacts directly with a carbonyl oxygen or with the cyanide group of the anionic species. Charge-dispersal considerations were used to explain the relative values of (nu)(CO) frequencies. Conductance data indicated all salts to be associated in THF solution and the differences among the cations were apparent. The existence of specific cation interaction in Na('+){HFe(CO)(,4)}('-) was shown to greatly increase the reactivity of the carbonylate towards oxidation (O(,2)) and also towards carbonyl-ligand exchange with gaseous ('13)CO Systems in which the transition metal-organic moiety is cationic were also studied. Conductance measurements established that {((eta)('5)-C(,5)H(,5))Fe((eta)('6)-C(,6)H(,6))}('+) and {((eta)('5)-C(,5)H(,5))Fe((eta)('6)-C(,6)Me(,6))}('+) as salts with several of the anions {BF(,4)}('-), {PF(,6)}('-), {SbF(,6)}('-), and {BPh(,4)}('-), as well as {((eta)('5)-C(,5)H(,5))Fe(CO)(,2)L}('+){BF(,4)}('-), are all associated in THF solutions. Attempts to detect ion-pairing effects on the chemistry of those species, or in locating precise sites of interaction, were unsuccessful The influence of the solvent medium on ion-pairing was addressed in several parts of this work. A closer comparison of the solvent properties of THF and CH(,3)CN, as determined by the actual values of solution conductances and derived parameters, was also made. The dissociation constants in CH(,3)CN were 10('2) to 10('4) larger than those observed in THF, yet the salts were still found to be somewhat associated in the former solvent, despite its much greater polarity. The Fuoss electrostatic expression used to calculate the a values in THF can not be applied to acetonitrile solutions. The approach of Hanna, Pethybridge, and Prue was used instead. However, no unique center-to-center distances could be obtained