The conjugative ability of the λ 5 -PdC bond has been compared to its λ 3 -PdC 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 λ 3 -PdC bond show similar delocalization energy as those with CdC units. As for λ 5 -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 λ 3 -PdC and CdC bonds a two-way interaction, for the λ 5 -PdC system a one-way interaction is operational. Comparing cyclic systems containing λ 3 -and λ 5 -PdC 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 (1λ 3 ,3λ 3 -and 1λ 5 ,3λ 5 -diphosphacyclobutadienes), however, show completely different behavior, since the λ 5 -P derivative does not show destabilization upon ring formation and has equal bond lengths, while λ 3 -P rings are clearly antiaromatic. Rationalization has been given in terms of the differences in the one-way and two-way conjugative interactions.

The He I and He II photoelectron spectra of two proazaphosphatranes were recorded and interpreted by correlated ab initio quantum chemical calculations. The first ionization energy of the compounds investigated is among the lowest reported hitherto for phosphorus compounds. Geometry optimization of the ionic states of the parent proazaphosphatrane (at the HF and MP2 levels) revealed that there are two minima on the potential energy surface at different P−Nax distances representing two bond-stretch isomers. The structure with the long P−Nax distance (3.3 Å) is similar to that of the neutral molecule and thus has a favorable Frank−Condon factor for the ionization process. The other structure, with a short P−Nax distance (2.07 Å), is as much as 1 eV lower in energy than the other. Due to the unfavorable Frank−Condon factor, however, the corresponding adiabatic ionization energy cannot be seen in the spectrum. The short P−Nax distance in the more stable bond-stretch isomer is characteristic for azaphosphatranes. Its stability can thus be related to the formation of the dative P ← N bond. The unobserved adiabatic ionization energy should be about 1 eV lower than that of the observed one (ca. 5.6 eV), in full agreement with the strong basicity of proazaphosphatranes.