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

Abstract

Isoequilibrium behavior arising from the temperature dependence of ΔG° of families of complexes containing phosphorus(III) ligands provides a diagnostic method for evaluating a model (used to describe ligand effects) and the associated set of stereoelectronic parameters. This evaluation is based not on the quality of a linear free energy relationship, but rather on the results of a thermodynamic argument. Studies of the temperature dependence of E°/T values for the η-Cp(CO)(L)Fe(COMe)+/η-Cp(CO)(L)Fe(COMe)0 couple, where L is a phosphorus(III) ligand, provide isoequilibrium data to test the ECW model for analysis of ligand effects. The ECW model fails this test. Therefore, we conclude that the ECW model is, in general, inadequate to describe the stereoelectronic properties of phosphorus(III) ligands.

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).…”

Electronic reorganization: Origin of sigma trans promotion effect

“…We further imply that destabilization of d increases energies of the many‐electron states in which this orbital is occupied, the so‐called ligand field excited states 129–131. Although this picture can be made more rigorous, and even though more sophisticated measures of σ‐donicity have been introduced,132–138 the above shall suffice for the purposes of the TPE model.…”

Assessing the stereoelectronic properties of pyrrolyl phosphines and related ligands. The quantitative analysis of ligand effects (QALE)

“…The plot of EЊ/T versus 1/T for these complexes shows a fan shaped array of lines that intersect at a point-this is isoequilibrium behavior. 56, 57 We have defined a 'family' as a set of ligands that exhibit isoequilibrium behavior. 12 Isoequilibrium behavior requires that a family of ligands respond to variation in an 'effectively single' parameter.…”