3d Orbitals and bonds of second-row atoms. Part III. The cyclic phosphonitrilic halides

Abstract

The five 3d orbitals of phosphorus in cyclic phosphonitrilic halides are compared both in relation to overlap with orbitals on neighbouring atoms, and by the effects of the perturbations caused by the molecular environment. Energy-minimised exponents are estimated for the valence shell orbitals of phosphorus in the planar molecules (F₂PN)₃, (F₂PN)₄, and (Cl₂PN)₃; each 3d orbital is treated separately assuming that 3d interactions are involved predominantly in the ring bonds. The main conclusions are that the 3d_xz and 3d_x²–y² orbitals of phosphorus are compact and available for bonding, but that the 3d_z², 3d_yz, and 3d_xy orbitals are diffuse and have high energies. These results are consistent with the model which emphasises cyclic electron delocalisation in two π systems: one symmetric and one antisymmetric to reflection in the molecular plane.