A Study of the Bonding and Geometry in Phosphacumulene Ylides

A series of ab initio molecular orbital calculations have been carried out on phosphacumulene ylides of the type H3P = C = C = X where X = S, O, CNH, and (OH)2. The variation in the P-C-C bond angle was examined and compared to the experimentally determined structures of analogs. The dominant factor in setting the P-C-C angle was found to be the relative π-acceptor capability of the C = X fragment in the plane of the bending motion. Quantitative PMO-arguments were constructed for the X = S, O and (OH)2 cases along this line. It is also shown, that P-C σ* character mixes into the highest occupied molecular orbital. The magnitude of this effect is dictated by the amount of bending and this strongly influences the 1JP-C nuclear spin coupling constants. Finally, the bending in H3P = C = PH3 was also investigated in a similar manner.