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

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.