Nature of Bonding in Cyclic Conjugated Ylides

The conjugative ability of the λ5-PC bond has been compared to its λ3-PC counterpart at the MP2/6-31G*//MP2/6-31G* level of theory, using isodesmic reaction energies. Investigating heterobutadienes, it has been observed that compounds containing a λ3-PC bond show similar delocalization energy as those with CC units. As for λ5-phosphabutadienes, however, stabilization is achieved only in the case of C substitution. This behavior has been rationalized by perturbation theory arguments with the conclusion that while for the λ3-PC and CC bonds a two-way interaction, for the λ5-PC system a one-way interaction is operational. Comparing cyclic systems containing λ3- and λ5-PC bonds, similar structures (bond lengths) and stabilizations (as concluded from bond separation and homodesmic reaction energies) were obtained for six-membered rings including benzene and other rings containing one and three (symmetrically arranged) phosphorus atoms. The four-membered rings (1λ3,3λ3- and 1λ5,3λ5-diphosphacyclobutadienes), however, show completely different behavior, since the λ5-P derivative does not show destabilization upon ring formation and has equal bond lengths, while λ3-P rings are clearly antiaromatic. Rationalization has been given in terms of the differences in the one-way and two-way conjugative interactions.