Study of the electric dipole moments of the (C2H5)n PX3?n and P{EIVB (CH3)3}3 compounds (X = Cl,Br, I; EIVB = C,Si, Sn; n = 0, 1, 2, 3)

The dipole moments of the (C₂H₅)_nPX_3?n (X = Cl, Br, I; n = 0, 1, 2, 3) and P{E^IVB(CH₃)₃}₃ compounds (E^IVB = C, Si, Sn) have been determined by dielectric measurements in benzene at 25°C by Higasi's method. The results are interpreted in terms of an additive vector model from bond moment calculations and a maximum phosphorus lone pair contribution of 1.75 D as calculated for P(C₂H₅)₃. The high dipole moment of the mixed PA₂B-type compounds in comparison with PA₃ moments is mainly ascribed to a loss of C_3v symmetry, characterized by a quite asymmetric orientation of the phosphorus lone pair with respect to the phosphorus bonding orbitals.