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

Abstract

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-XC 6 H 4 ) 3 and PR 3 . 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 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.

The stereoelectronic parameters of phosphites. The quantitative analysis of ligand effects (QALE)

“…The next step is to get an estimate of the π parameters for P(OMe) 3 and P(OCH 2 ) 3 CEt. From the analysis of ∆SЊ and ∆HЊ for the η-Cp(CO)(L)Fe(COMe) ϩ/0 couple we know that the aryl effect in ∆GЊ vanishes at 320 K (the E ar isoequilibrium temperature [83][84][85][86] ). Thus, we calculated ∆GЊ (320 K) and analyzed the data for the phosphines in terms of χ d and θ only (entry 6, Table 3).…”

Multi-mechanism linear free energy relationships and isoequilibrium or isokinetic temperatures

“…It is, therefore, of interest to find sets of data that can be used both to derive values for the various coefficients contained in equations such as eqn ( 1)-( 3), and to investigate their temperature dependence. Suitable data have been reported by Fernandez et al 30,31,45 E1 values for the reduction reactions shown in eqn (14) were measured to an excellent precision, estimated to be AE0.7 mV for a large selection of P-donor ligands, L, with reference to the reduction of [(Z 5 -C 5 H 5 )(Z 5 -C 5 H 4 COMe)Fe] + .…”

“…It can then be shown 30,38 that the LFER can be expressed as in eqn (8), where x = À2.303R, so that a, b and g are given by eqn ( 9), (10) and (11), respectively.…”

“…One plot shows quite a sharp IET and the other shows that some members of a group of phosphite ligands do not fit the trend shown by the others. (The data for these plots were based on the relative equilibrium constants derived from the reduction potentials for the reaction shown below in eqn (14) 30,31 and these data will be considered in greater detail later. )…”

A Thermodynamically Based and Definitive Demonstration of the Inadequacy of the ECW Model for Phosphorus(III) Ligands

“… a Δ H ° values are in kJ mol -1 , and Δ S ° values are in J K -1 mol -1 . The Δ H ° and Δ S ° values are relative to the reduction of acetylferrocene (see ref ). b These values refer to the (η-Cp)(CO)(L)Fe(COMe) +/0 redox couple relative to the (η-Cp)(η-C 5 H 4 COMe)Fe +/0 redox couple.…”