Ab initio quantum chemical calculations have been used to study the observed preference for the pentagonal bipyramid (PB) geometry in main group heptafluorides (e.g., TeF7-, IF7, and XeF7+) and main group and transition metal oxofluorides MOF(6)(-) (M = I, Re) while the capped octahedron (CO) or capped trigonal prism (CTP) geometry is preferred by the analogous transition metal counterparts (e.g., MoF7- and WF7-). An explanation of these trends is provided by a molecular orbital (MO) model which describes the main group heptafluorides in terms of three non- or antibonding MO’s localized largely upon the ligand atoms. These MO’s are nonbonding for the PB geometry but slightly antibonding for the CO and CTP geometries because of the lower symmetry of these stereochemistries; thus the PB geometry is predicted for these main group molecules. For transition metal heptafluorides, this MO model predicts that two MO’s will not be involved in M-L bond formation as they are localized on the metal atom. Thus for the PB geometry they are nonbonding and slightly antibonding for the CO and CTP geometries. A consequence of the antibonding nature of these orbitals is the slight stabilization of the bonding orbitals and a preference for the CO and CTP geometries. Ab initio calculations of MF(7)(-) (M = Mo, W) molecules predict that the CO and CTP have approximately the same energy and are lower than the PB by approximately 1-4 kcal mol(-1). Similar MO arguments may be applied to ReOF6- for which the PB geometry was calculated to be lower in energy than the CO and CTP geometries by about 28 kcal mol(-1). Total electron densities (rho) of main group and transition metal fluorides and oxofluorides were compared, and strong ionic character was found in both M-F and M-O bonds. Charge concentration maxima in the core regions of the central atoms were found through analysis of the Laplacian of the charge density (del(2) rho) showing that the central atom is distorted by the ligand atoms.