The PP bond length in 1,2-diphenyl-1,2-diphospholane-1,2-disulphide compared with that in other compounds

The x-ray structure of 1,2-diphenyl-1,2-diphospholane-1,2-disulphide has been determined (PP 2.253 Å and PS 1.944 and 1.943 Å). In this compound the two phosphorus atoms are linked by three methylene groups into a five-membered heterocyclic ring, which prevents free rotation about the PP bond, and results in a non planar configuration for the SPPS part of the molecule. The PP bond length is very similar to that in the diphosphine disulphides which all have a trans-planar arrangement of phosphorus and sulphur atoms, and the phosphorus sulphides which are based on a tetrahedron of phosphorus atoms. This indicates that the bond length is unaffected by rotation, and hence does not involve any π interaction between phosphorus atoms. This is confirmed by the similarity of the PS bond lengths with those in a number of compounds, showing that the π electron on phosphorus is localised in the PS bond.