N-Pyrrolyl Phosphines:  Enhanced π-Acceptor Character via Carboalkoxy Substitution

A series of 3,4-dicarboethoxy- N -pyrrolyl phosphine ligands (R 2 P−NC 4 H 2 (CO 2 Et) 2 ) is described. A number of techniques, including reaction calorimetry, infrared spectroscopy, and X-ray crystallography, demonstrate that these ligands are potent π-acceptors and that the electronic substituent parameter χ i for the 3,4-(EtO 2 C) 2 C 4 H 2 N moiety is ca . 16, placing it in a position between fluorine and chlorine.