Relative Importance of Initial- and Transition-state Effects

The first inorganic reactions for which solvent effects on reactivities were analysed into initial-state and transition-state contributions were those of tetra-alkyltin compounds with mercury(ii) halides. There has been some subsequent disagreement over activation parameters for such reactions of tetramethyltin, and these have now been redetermined. For transfer from methanol to acetonitrile, the decreases in AG and A// are mostly due to increases in the initial-state values. For transfer from methanol to DMSO the decreases in AG and IsH are dominated by very large decreases in transition-state values. 

The necessity of introducing extrathermodynamic assumptions in order to estimate transfer parameters for individual ions introduces a degree of uncertainty into initial state-transition state analyses for reactions involving ionic species. This has been illustrated for the reaction of the [Fe(phen)a] + cation with hydroxide in methanol-water mixtures. However, despite the uncertainties it is quite clear that in this system, and for the same reaction in acetone-water mixtures, solvation of the [Fe(phen)3] + reactant or moiety is the dominant factor, at any rate in water-rich solvent mixtures. The pattern for this reaction, and for analogous reactions of related low-spin iron(ii) complexes, closely resembles that for base 

Abraham, J. Andonian-Haftvan, and M. R. Sedaghat-Herati, J. Organomet. Chem., 1979, 172, 31. 

The type of analysis discussed in the preceding paragraph with particular respect to c/ iron(ii) complexes has also been attempted for c/ palladium(ii) and gold(iii) substitutions. In the case of the reactions of [Pd(fo)L], where Hgfo is the formazan (5) and L=ammonia or pyridine, with thiocyanate, thiourea, or triphenylphosphine, a kinetic-thermodynamic analysis shows Gibbs free 

The large increase in rate and decrease in activation energy for base hydrolysis of the tetraphenylphosphonium cation as the proportion of the organic component increases in THF-water mixtures is attributed to a combination of initial-state desolvation and transition-state stabilization. It is claimed that something like 13 water molecules have to be removed from the solvation shell in going from the initial to the transition state. The results from this study are discussed in relation to those from earlier studies involving the co-solvents acetone, dioxan, and DMSO. 

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