The equilibrium geometries of 5-Methylthio-3-N-phenyl- 2,3-2(H)-1,3,4,2-Thiadiazaphosphenium Cation and other two related organophosphorus molecules have been optimized at the level of ab initio molecular orbital calculation with STO-3G basis set by use of the analytical energy-gradient technique. The optimized geometries show that the chemical bonds in the phosphenium cations are different from those in the corresponding three-coordinated phosphorus systims. From the three-coordinated phosphorus to the phosphenium cation, because of the formation of the new conjugated system containing the phosphorus atom, the length of the single bonds become shorter and the length of the double bond become longer. The numerical results obtained from the ab initio CI calculations at the equilibrium geometries show that the relative values for the positive charges of the phosphorus atoms are in good agreement with the relative values for the chemical shift of ^3^1P NMR and for the molar conductivity .

Key words: STRUCTURAL ANALYSIS, CHEMICAL BONDS, AB INITIO CALCULATION, EQUILIBRIUM STATE, NET CHARGE