Exploring Ligand Effects through Isoequilibrium Phenomena: The Quantitative Analysis of Ligand Effects

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Article - Merrimack Access Only

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American Chemical Society

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Abstract/ Summary

Analysis of the E° values of the η-Cp(CO)(L)Fe(COMe)+/o couple (measured via cyclic voltammetry at 229, 252, 264, 273, and 293 K in acetonitrile/0.1 M TBAH) has revealed different isoequilibrium behavior for P(p-XC6H4)3 and PR3. We classify ligands as a family when they exhibit a single isoequilibrium point. A family of ligands affects a property via variation of a single, or effectively single, parameter. Thus, through isoequilibrium behavior, we have an unambiguous way to identify families, which in quantitative analysis of ligand effects (QALE), must be treated separately in the preliminary graphical analysis of data that leads to the full regression analysis. The QALE model predicts that there is an isoequilibrium temperature associated with each stereoelectronic parameter; i.e., for each parameter, there is a specific temperature where the effect disappears. The fan-shaped arrays of lines that result when E°/T is plotted versus either 1/T or the stereoelectronic parameter χ gives insight into the nature of and the number of stereoelectronic parameters necessary to describe the system and the partitioning of steric effects between enthalpy and entropy. In this particular system, steric effects are almost entirely an entropic phenomenon.