Solution thermochemical study of ligand substitution reaction of novel pyrrolyl-substituted tertiary phosphine ligands in the L(2)Fe(CO)(3) system

The enthalpies of reaction of (BDA)Fe(CO)(3) (BDA = (C6H5)CH=CHC(O)CH3, benzylideneacetone) with a series of novel phosphine ligands, PPh(3-x)(pyrrolyl)(x) (x = 1-3) and P(pyrrolidinyl)(3), leading to the formation of (L)(2)Fe(CO)(3) complexes (L = phosphine) have been measured by solution calorimetry in THF at 50 degrees C. The range of reaction enthalpies spans some 32 kcal mol(-1). The overall relative order of stability established is as follows (PR(3); -Delta H, kcal mol(-1):P(NC4H4)(3) < P(NC4H4)(2)Ph < P(NC4H4)Ph(2) << P(NC4H8)(3). A quantitative analysis of ligand effect of the present and previously obtained data for L(2)Fe(CO)(3) complexes helps clarify the exact steric versus electronic ligand contributions to the enthalpy of reaction in this system. Furthermore, the present enthalpy information helps in the estimation of useful electronic parameters associated with the novel phosphine series.