[(Diphenoxyphosphinyl)methylidene]triphenylphosphorane—the double P+-stabilised carbanion: a crystallographic, computational and solution NMR comparative study on the ylidic bonding

The crystal and molecular structure of the title compound () was established by an X-ray diffraction analysis. Some geometrical parameters including a slightly pyramidal shape around its ylidic Cβ-atom were determined (trans-bent type conformation), providing evidence for a strong electron delocalisation in the PαCβPγOδ backbone. The charge density redistribution within this molecular unit and its other fully optimised geometries was evaluated in ab initio MO calculations using both the HF and DFT (B3LYP) formalism, which supported such a concept. As a result, the double zwitterionic form (structure ) was suggested as the best description of . The absence of an experimentally NMR observable 2JPα–Cβ–Hβ coupling was tentatively rationalised in terms of fast pyramidalisation of the Cβ-anionic site. Crystallographic and solution NMR data for ylide were compared with those reported for the other mesomerically stabilised Wittig-type reagents and structurally related anionic species. It was concluded that all aforementioned systems have almost identical P-ylidic bonding most likely governed mainly by very strong electrostatic interactions, with a small contribution of negative hyperconjugation.