o-, m-, and p-diphosphabenzenes and their P-2(C-H)(4) valence isomers. An ab initio theoretical study

The 24 Valence isomers of diphosphinine, P-2(C-H)(4), have been investigate d at an electron-correlated ab initio level and by density functional theor y. In analogy with benzene, the three planar forms exhibit full aromaticity and are the most stable isomers on the potential surface. The next isomers , by order of stabilities, are the diphosphabenzvalenes, the Dewar diphosph abenzenes, the prismanes, and the diphosphabicyclopropenyls. They are disti nctly less stable than the planar isomers, and their relative energies rang e from 23 all the way to 93 kcal/mol above the absolute minimum. Among the planar isomers, the most stable one is the (ortho) 1,2-diphosphabenzene, in apparent contradiction with the fact that this species has not been synthe sized yet. Some lines of thought to resolve this apparent dilemma are propo sed, and the relative reactivities of planar isomers are discussed in terms of their different diradical character. Strain energies are calculated for the nonplanar isomers and compared to those of the isomers of C6H6 and P-6 . Tentative extrapolations to tri-, tetra-, and pentaphosphinines are propo sed.