摘要:
The enthalpies of reaction of (BDA)Fe(CO)(3) (BDA = (C6H5)CH=CHO(CH3), benzylideneacetone) with a series of sterically demanding phosphine ligands, 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 7 kcal/mol. The overall relative order of stability established is as follows (PR(3); -Delta H, kcal/mol): PPh(3) < PCy(2)Ph < PCyPh(2) < PCy(3) < (PPr3)-Pr-i < PPh(2)Et < PBz(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 ofreaction in this system. Results of a single-crystal diffraction study for the complex diaxial-(PPh(2)Cy)(2)Fe(CO)(3) (2) show the molecule to be monoclinic, P2(1)/n, with a = 12.393(5) Angstrom, b = 15.811(6) Angstrom, c 18.029(7) Angstrom, alpha = gamma = 90 degrees, beta = 108.00(2)degrees, V = 43360(5) Angstrom(3), Z = 4, d(calcd) = 1.337 g cm(-1), n(obsd) = 3130, and R = 0.052. Electronic effects are overwhelmingly important in this system, yet a steric threshold of 135 degrees can be extracted from the QALE treatment which shows at which point steric factors begin to influence and contribute to the measured enthalpy of ligand substitution.