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The structures of the tricyclo[2.2.1.0 2,6 ]heptaphosphide trisanion, tricyclo[2.2.1.0 2,6 ]heptaphosphane triradical and their carbocyclic analogues have been investigated theoretically at the density functional theory (DFT) (B3LYP/6-31 + G*) level. The existence of negative hyperconjugation in these molecules could be established by natural bond orbital analysis. The weakening of the  σ  bonds combined with the possibility of the valence-shell expansion at the anionic charge carrying phosphorus atoms in the tricyclo[2.2.1.0 2,6 ]heptaphosphide trisanion induces a degenerate [2,2]sigmatropic rearrangement with a low energy barrier making all the phosphorus atoms equivalent, as detected by  31 P NMR experimentally. This energy barrier is enhanced in the presence of counterions. Its carbocyclic analogue trisanion fails to undergo a [2,2]sigmatropic rearrangement due to the inability of the anionic carbon centres to expand their valence shell. The tricyclo[2.2.1.0 2,6 ]heptaphosphane triradical and its carbocyclic analogue undergo a [2,2]sigmatropic rearrangement, as valence-shell expansion at the carbon atom is not required in this case. A lower activation energy barrier for the [2,2]sigmatropic rearrangement of the tricyclo[2.2.1.0 2,6 ]heptane triradical as compared to that for its phospha-analogue can be rationalised on the basis of the higher ring strain of cyclopropane than triphosphirane.

Fluxional behaviour of tricyclo[2.2.1.0 2,6 ]heptaphosphide trisanion: a DFT investigation