GEOMETRY OF THE FLUORIDES, OXOFLUORIDES, HYDRIDES, AND METHANIDES OF VANADIUM(V), CHROMIUM(VI), AND MOLYBDENUM(VI) - UNDERSTANDING THE GEOMETRY OF NON-VSEPR MOLECULES IN TERMS OF CORE DISTORTION

This paper describes a study of the topology of the electron density a nd its Laplacian for the molecules VF5, VMe(5), VH5, CrF6, CrMe(6), Cr OF4, MoOF4, CrO2F2, CrO2F42- and CrOF5- all of which, except VF5, CrF6 , and CrOF5- have a non-VSEPR geometry. It is shown that in each case the interaction of the ligands with the metal atom core causes it to d istort to a nonspherical shape. In particular, the Laplacian of the el ectron density reveals the formation of local concentrations of electr on density in the outer shell of the core, which have a definite geome trical arrangement such as four in a tetrahedral. arrangement or five in a square pyramidal or trigonal bipyramidal and six in an octahedral arrangement. Ligands that are predominately covalently bonded are fou nd opposite regions of charge depletion between these core charge conc entrations. In VH5, VMe(5), CrOF4, and MoOF4, these core charge concen trations have a square pyramidal arrangement, and the regions of charg e depletions have the corresponding inverse square pyramidal arrangeme nt so that these molecules have a square pyramidal geometry rather tha n a trigonal prism geometry. In CrMe(6), there are five core charge co ncentrations with a trigonal bipyramidal arrangement so that the regio ns of charge depletion have a trigonal prismatic arrangement and the m olecule has the corresponding trigonal prism geometry rather than an o ctahedral geometry. In contrast, molecules in which the only ligand is the more ionically bound fluorine are less affected by core distortio n and have VSEPR-predicted structures. The unexpected bond angles in C rO2F2 and the preference of CrO2F42- for a cis structure are also disc ussed in terms of the pattern of core charge concentrations.