VESCF-MO studies of molecules containing atoms from the second row of the Periodic Table. III. Properties of the 'trigonal-bipyramidal' molecules, phosphorus pentafluoride, sulphur tetrafluoride, and chlorine trifluoride, for a minimal basis set including 3d-orbitals

Abstract

Summary Molecular-orbital calculations on PFs, SF4, and ClF3 are reported. The VESCF procedure, using the NDDO approximation and treating only the a-electrons, was employed to determine how far the 3d-orbitals of the heavy atom should be regarded as being involved in the bonding. It is concluded that the 3d-orbitals play only a minor role, so that, to a first approximation, the bonding is best described as involving only the 3s- and 3p-orbitals of the heavy atom, as proposed particularly by Rundle. However, the effectiveness of the 3d-orbitals depends critically on the value of the orbital exponent, and our conclusions may need revision if our estimate of this is astray. Our calculations imply that the 3d-orbitals are no more significant than the 4s-orbitals in contributing to bonding, judging by orbital occupation numbers. Calculated dipole moments are particularly sensitive to 3d-components in the molecular wave function.