The solid state conformation of oxo stabilised ylides: X-ray structures of four new polyoxo phosphorus ylides

The first X-ray structure determinations of a trioxo ylide 7, a tetraoxo ylide 8, a tetraoxo bis(ylide) 10 and a hexaoxo bis(ylide) 9 show these compounds to exhibit unexpected effects as regards the relative alignment of the 4-8 contiguous C=X units present. Crystal-packing forces are sufficient to over-ride the normal preference for the P=C-C=O unit in such stabilised ylides to have the syn configuration. A survey of all previous X-ray structures of acyclic oxo-stabilised phosphorus and arsenic ylides is also presented.