CORE DISTORTIONS AND GEOMETRIES OF THE DIFLUORIDES AND DIHYDRIDES OF CA, SR, AND BA

rho Studies of the Laplacian of the calculated electron density (del(2) rho) for the fluorides of Mg, Ca, Sr, and Ba, for the hydrides of Ca, Sr, and Ba, and for Ca(CH3)(2) were undertaken in order to understand why these molecules with the exception of MgF2 have a bent rather than a linear geometry. A linear geometry is expected on the basis of the simple ionic model or the VSEPR model and is observed for BeF2 and MgF2. The Laplacian distribution for the metal atom in each of the remaining molecules shows that the outer shell of its core is perturbed by the polarizing field of the ligands and is distorted in a manner corresponding to the presence of four approximately tetrahedrally localized concentrations of electronic charge. The angular shapes of these molecules can be understood in terms of the interaction between the distorted core and the ligands. In contrast, the distortion of the magnesium core in MgF2 exhibits only two charge concentrations, one associated with each Mg-F bond, so that no deviation from the linear geometry is expected or found. This paper helps to establish the generality of the formation of localized concentrations of electronic charge in positions that are spatially opposed to the attached ligands, in the outer shell of the core of an element with accessible (n - 1)d orbitals, where n is the valence shell. The distortions of the cores of the heavier alkaline earth elements that are revealed by the Laplacian of the electron density have the same form as that proposed some years ago in an extension of the VSEPR model to account for the angular shape of these molecules.