Correlations of the structural properties of a complete R₂PX Series (X = Hydrogen or Halogen)

The CgP(Se)X series (X = H, F, Cl, Br and I; CgP = 6-phospha-2,4,8-trioxa- 1,3,5,7-tetramethyladamant-6-yl) is generated by the addition of elemental Se to the corresponding CgPX. The crystal structures of CgPX (X = H, F, Cl, Br and I) and CgP(Se)X (X = F, Cl, Br and I) are compared. The J_P,Se values are correlated with the P-Se distance and the electronegativity of X. For both the CgP(Se)X and the CgPX series, the P-C bond lengths correlate with the P-X bond lengths and with the C-P-C bond angle. These trends can be explained in terms of the δ+ charge on the P atom, which has been calculated in the gas phase. Cone-angle measurements suggest that CgPCl, CgPBr and CgPI have essentially the same steric bulk. Analysis of the crystal structures of the halophosphines CgPX (CgP = 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-6- yl) and their Se derivatives CgP(Se)X reveals evidence for: (a) the smaller covalent radius of P^V than P^III, (b) a reduction in P-C distance in response to an increase in the electronegativity of X, (c) the effective steric bulk of the halogen atoms in P-Cl, P-Br and P-I is approximately equal.