Structural and electronic properties of neutral phosphoniobenzo[c]phospholides

The neutral monophosphoniobenzo[c]phospholides 2 and 3 were selectively pre pared by reduction of bis(triphenylphosphonio)benzophospholide chloride 1[C l]. Compound 3 was further converted into the borane adduct 6 and the thiox o-phosphorane 7. All products were characterised by spectroscopic methods a nd X-ray diffraction. The most notable features of the molecular structures are the significantly different distances of the two P-C bonds adjacent to the two-coordinate phosphorus atom (P2-C1 175.0-176.6, P2-C3 171.7-173.5 p m), and a short exocyclic C-P(phosphonio) bond (C1-P1 172.5-173,7 pm) relat ive to cationic derivatives such as 1. Structure comparisons revealed furth er variations in the bond lengths, which can be related to the changes in t he inductive electron-withdrawing power of the substituents in the 3-positi on of the fused ring system. Computational studies of monophosphoniobenzoph ospholides, as well as the corresponding anions with no phosphonio substitu ents, and cations with two phosphonio substituents, allowed for the interpr etation of the special properties of the neutral species in terms of a part ial pi -bond localisation. This, in terms of a VB picture, is equivalent to the prominence of a single resonance structure that can best be described as an ylide-substituted cyclic phosphaalkene. The analysis of these results allows important predictions for the chemical properties of these compound s.