Computational insights into the acceptor chemistry of phosphenium cations

Phosphines are traditionally considered as Lewis bases or ligands in transition metal and main group complexes. Despite their electron-rich (lone pair-bearing) nature, an extensive coordination chemistry for Lewis acidic phosphorus centers is being developed; such chemistry provides a new synthetic approach for phosphorus-element bond formation, leading to new types of structures and modes of bonding. Complexes of Ph2P+ with a variety of donor elements (P, N, C) give experimentally short donor-acceptor bond lengths, when compared to other cationic phosphorus Lewis acid complexes. We have calculated that the energy of the lowest unoccupied molecular orbital (LUMO) in Ph2P+ is lower than that of (Me2N)2P+, which partially explains the greater exothermicity of reactions of donors with the diaryl acceptor. Furthermore, the energies required to distort the diphenylphosphenium cation from its ground-state geometry are significantly smaller than those of the diamido cations and, thus, enhance the exothermicity of donor coordination. These computational data, in conjunction with evidence from experimental solid-state structures, indicate that Ph2P+ is a significantly better Lewis acid relative to the more common diaminophosphenium analogues (R2N)2P+ and are used to elucidate the nature of the bonding in donor-phosphenium complexes.