Molecular orbital studies on the structures and properties of dicoordinated phosphenium cations featuring a SPN bond

The geometrical and electronic structures of the recently synthesized dicoordinated phosphenium cations featuring a SPN bond and their corresponding three-coordinated chlorophosphine precursors are investigated theoretically by using the MNDO method. The calculated results show that the chemical bonds in the dicoordinated phosphenium cations are different from those in the corresponding three-coordinated phosphorus systems: in the former the conjugated systems containing the phosphorus atom are formed, which results in many special characteristics. To elucidate the ³¹P NMR chemical shifts of the dicoordinated phosphenium cations, a simple relationship, which includes a term containing not only the net atomic charge at phosphorus, but also the net atomic charges at nitrogen and sulfur, is introduced in this paper. It is shown that the relationship is quite satisfactory for predicting the ³¹P NMR chemical shifts of this class of phosphenium cation. Finally, the reliability of the results obtained from the MNDO calculations is tested and compared with the results obtained from ab initio RHF calculations with a 6-31G^** basis set.