TRIGONAL BIPYRAMIDAL AND RELATED MOLECULES OF THE MAIN-GROUP ELEMENTS - INVESTIGATION OF APPARENT EXCEPTIONS TO THE VSEPR MODEL THROUGH THE ANALYSIS OF THE LAPLACIAN OF THE ELECTRON-DENSITY

Topological analyses of the charge density (rho) and the Laplacian of the charge density (del2rho) for molecules of the types AF4E (A = P, S, Cl; E = a lone pair of electrons) and YSF4 (Y = O, NH, CH2) are presented. Examination of the relative sizes and shapes of bonding and nonbonding valence shell charge concentrations (VSCC's), which are made evident from the study of del2rho, enables the geometrical features of these molecules to be understood and shows that they are in agreement with the concepts of the VSEPR model of molecular geometry. The ellipticity of a region of charge concentration is introduced as a measure of its shape, and its utility is demonstrated by the relationship between the geometry of the AF4E molecules and the size and shape of the lone pair. Similarly, the shape of the double-bond domain, i.e., its ellipticity, in the YSF4 molecules is shown to be an important factor in the determination of their geometry. These molecules have been cited as exceptions to the VSEPR model because of the unexpected increase in the Y-S-F(eq) angle in the series OSF4, HNSF4, and H2CSF4. Examination of the shape of Y-S bond electron density shows, however, that the observed bond angles are consistent with an increased spreading of the double-bond domain in the equatorial plane of the molecule. Examination of the electron density distribution of the Y-S bond gives useful information about its nature in these molecules, and the representation of Y-S bonds by Lewis structures is discussed.