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.